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Versions: (draft-hodges-strict-transport-sec) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 RFC 6797

WEBSEC Working Group                                           J. Hodges
Internet-Draft                                                    PayPal
Intended status: Standards Track                              C. Jackson
Expires: December 9, 2012                     Carnegie Mellon University
                                                                A. Barth
                                                            Google, Inc.
                                                             Jun 7, 2012


                 HTTP Strict Transport Security (HSTS)
               draft-ietf-websec-strict-transport-sec-09

Abstract

   This specification defines a mechanism enabling web sites to declare
   themselves accessible only via secure connections, and/or for users
   to be able to direct their user agent(s) to interact with given sites
   only over secure connections.  This overall policy is referred to as
   HTTP Strict Transport Security (HSTS).  The policy is declared by web
   sites via the Strict-Transport-Security HTTP response header field,
   and/or by other means, such as user agent configuration, for example.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 9, 2012.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents



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   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Organization of this specification . . . . . . . . . . . .  5
   2.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     2.1.  Use Cases  . . . . . . . . . . . . . . . . . . . . . . . .  5
     2.2.  HTTP Strict Transport Security Policy Effects  . . . . . .  5
     2.3.  Threat Model . . . . . . . . . . . . . . . . . . . . . . .  6
       2.3.1.  Threats Addressed  . . . . . . . . . . . . . . . . . .  6
         2.3.1.1.  Passive Network Attackers  . . . . . . . . . . . .  6
         2.3.1.2.  Active Network Attackers . . . . . . . . . . . . .  7
         2.3.1.3.  Web Site Development and Deployment Bugs . . . . .  7
       2.3.2.  Threats Not Addressed  . . . . . . . . . . . . . . . .  7
         2.3.2.1.  Phishing . . . . . . . . . . . . . . . . . . . . .  7
         2.3.2.2.  Malware and Browser Vulnerabilities  . . . . . . .  8
     2.4.  Requirements . . . . . . . . . . . . . . . . . . . . . . .  8
       2.4.1.  Overall Requirement  . . . . . . . . . . . . . . . . .  8
         2.4.1.1.  Detailed Core Requirements . . . . . . . . . . . .  8
         2.4.1.2.  Detailed Ancillary Requirements  . . . . . . . . .  9
   3.  Conformance Criteria . . . . . . . . . . . . . . . . . . . . .  9
     3.1.  Document Conventions . . . . . . . . . . . . . . . . . . . 10
   4.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . . 10
   5.  HSTS Mechanism Overview  . . . . . . . . . . . . . . . . . . . 12
   6.  Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     6.1.  Strict-Transport-Security HTTP Response Header Field . . . 13
       6.1.1.  The max-age Directive  . . . . . . . . . . . . . . . . 14
       6.1.2.  The includeSubDomains Directive  . . . . . . . . . . . 14
     6.2.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . 14
   7.  Server Processing Model  . . . . . . . . . . . . . . . . . . . 15
     7.1.  HTTP-over-Secure-Transport Request Type  . . . . . . . . . 15
     7.2.  HTTP Request Type  . . . . . . . . . . . . . . . . . . . . 15
   8.  User Agent Processing Model  . . . . . . . . . . . . . . . . . 16
     8.1.  Strict-Transport-Security Response Header Field
           Processing . . . . . . . . . . . . . . . . . . . . . . . . 16
       8.1.1.  Noting an HSTS Host  . . . . . . . . . . . . . . . . . 17
     8.2.  Known HSTS Host Domain Name Matching . . . . . . . . . . . 18
     8.3.  URI Loading and Port Mapping . . . . . . . . . . . . . . . 19
     8.4.  Errors in Secure Transport Establishment . . . . . . . . . 20
     8.5.  HTTP-Equiv <Meta> Element Attribute  . . . . . . . . . . . 20
     8.6.  Missing Strict-Transport-Security Response Header Field  . 20
   9.  Domain Name IDNA-Canonicalization  . . . . . . . . . . . . . . 20



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   10. Server Implementation and Deployment Advice  . . . . . . . . . 21
     10.1. HSTS Policy expiration time considerations . . . . . . . . 21
     10.2. Using HSTS in conjunction with self-signed public-key
           certificates . . . . . . . . . . . . . . . . . . . . . . . 22
     10.3. Implications of includeSubDomains  . . . . . . . . . . . . 23
   11. User Agent Implementation Advice . . . . . . . . . . . . . . . 23
     11.1. No User Recourse . . . . . . . . . . . . . . . . . . . . . 23
     11.2. User-declared HSTS Policy  . . . . . . . . . . . . . . . . 24
     11.3. HSTS Pre-Loaded List . . . . . . . . . . . . . . . . . . . 24
     11.4. Disallow Mixed Security Context Loads  . . . . . . . . . . 25
     11.5. HSTS Policy Deletion . . . . . . . . . . . . . . . . . . . 25
   12. Constructing an Effective Request URI  . . . . . . . . . . . . 25
     12.1. ERU Fundamental Definitions  . . . . . . . . . . . . . . . 25
     12.2. Determining the Effective Request URI  . . . . . . . . . . 26
       12.2.1. Effective Request URI Examples . . . . . . . . . . . . 27
   13. Internationalized Domain Names for Applications (IDNA):
       Dependency and Migration . . . . . . . . . . . . . . . . . . . 27
   14. Security Considerations  . . . . . . . . . . . . . . . . . . . 27
     14.1. Ramifications of HSTS Policy Establishment only over
           Error-free Secure Transport  . . . . . . . . . . . . . . . 28
     14.2. The Need for includeSubDomains . . . . . . . . . . . . . . 29
     14.3. Denial of Service  . . . . . . . . . . . . . . . . . . . . 30
     14.4. Bootstrap MITM Vulnerability . . . . . . . . . . . . . . . 31
     14.5. Network Time Attacks . . . . . . . . . . . . . . . . . . . 31
     14.6. Bogus Root CA Certificate Phish plus DNS Cache
           Poisoning Attack . . . . . . . . . . . . . . . . . . . . . 31
     14.7. Creative Manipulation of HSTS Policy Store . . . . . . . . 32
     14.8. Internationalized Domain Names . . . . . . . . . . . . . . 32
   15. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 33
   16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 33
     16.1. Normative References . . . . . . . . . . . . . . . . . . . 33
     16.2. Informative References . . . . . . . . . . . . . . . . . . 35
   Appendix A.  Design Decision Notes . . . . . . . . . . . . . . . . 37
   Appendix B.  Differences between HSTS Policy and Same-Origin
                Policy  . . . . . . . . . . . . . . . . . . . . . . . 38
   Appendix C.  Acknowledgments . . . . . . . . . . . . . . . . . . . 39
   Appendix D.  Change Log  . . . . . . . . . . . . . . . . . . . . . 39
     D.1.  For draft-ietf-websec-strict-transport-sec . . . . . . . . 39
     D.2.  For draft-hodges-strict-transport-sec  . . . . . . . . . . 46
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 47











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1.  Introduction

   The HTTP protocol [RFC2616] may be used over various transports,
   typically the Transmission Control Protocol (TCP).  However, TCP does
   not provide channel integrity protection, confidentiality, nor secure
   host identification.  Thus the Secure Sockets Layer (SSL) protocol
   [RFC6101] and its successor Transport Layer Security (TLS) [RFC5246],
   were developed in order to provide channel-oriented security, and are
   typically layered between application protocols and TCP.  [RFC2818]
   specifies how HTTP is layered onto TLS, and defines the Uniform
   Resource Identifier (URI) scheme of "https" (in practice however,
   HTTP user agents (UAs) typically offer their users choices among
   SSL2, SSL3, and TLS for secure transport).

   UAs employ various local security policies with respect to the
   characteristics of their interactions with web resources depending on
   (in part) whether they are communicating with a given web resource's
   host using HTTP or HTTP-over-a-Secure-Transport.  For example,
   cookies ([RFC6265]) may be flagged as Secure.  UAs are to send such
   Secure cookies to their addressed host only over a secure transport.
   This is in contrast to non-Secure cookies, which are returned to the
   host regardless of transport (although subject to other rules).

   UAs typically announce to their users any issues with secure
   connection establishment, such as being unable to validate a TLS
   server certificate trust chain, or if a TLS server certificate is
   expired, or if a TLS host's domain name appears incorrectly in the
   TLS server certificate (see Section 3.1 of [RFC2818]).  Often, UAs
   enable users to elect to continue to interact with a web resource's
   host in the face of such issues.  This behavior is sometimes referred
   to as "click(ing) through" security [GoodDhamijaEtAl05]
   [SunshineEgelmanEtAl09], and thus can be described as "click-through
   insecurity".

   A key vulnerability enabled by click-through insecurity is the
   leaking of any cookies the web resource may be using to manage a
   user's session.  The threat here is that an attacker could obtain the
   cookies and then interact with the legitimate web resource while
   impersonating the user.

   Jackson and Barth proposed an approach, in [ForceHTTPS], to enable
   web resources to declare that any interactions by UAs with the web
   resource must be conducted securely, and that any issues with
   establishing a secure transport session are to be treated as fatal
   and without direct user recourse.  The aim is to prevent click-
   through insecurity, and address other potential threats.

   This specification embodies and refines the approach proposed in



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   [ForceHTTPS].  For example, rather than using a cookie to convey
   policy from a web resource's host to a UA, it defines an HTTP
   response header field for this purpose.  Additionally, a web
   resource's host may declare its policy to apply to the entire domain
   name subtree rooted at its host name.  This enables HSTS to protect
   so-called "domain cookies", which are applied to all subdomains of a
   given web resource's host name.

   This specification also incorporates notions from [JacksonBarth2008]
   in that policy is applied on an "entire-host" basis: it applies to
   HTTP (only) over any TCP port of the issuing host.

   Note that the policy defined by this specification is distinctly
   different than the "same-origin policy" defined in "The Web Origin
   Concept" [RFC6454].  These differences are summarized below in
   Appendix B.

1.1.  Organization of this specification

   This specification begins with an overview of the use cases, policy
   effects, threat models, and requirements for HSTS (in Section 2).
   Then, Section 3 defines conformance requirements.  The HSTS mechanism
   itself is formally specified in Section 4 through Section 15.


2.  Overview

   This section discusses the use cases, summarizes the HTTP Strict
   Transport Security (HSTS) policy, and continues with a discussion of
   the threat model, non-addressed threats, and derived requirements.

2.1.  Use Cases

   The high-level use case is a combination of:

   o  Web browser user wishes to interact with various web sites (some
      arbitrary, some known) in a secure fashion.

   o  Web site deployer wishes to offer their site in an explicitly
      secure fashion for both their own, as well as their users',
      benefit.

2.2.  HTTP Strict Transport Security Policy Effects

   The effects of the HTTP Strict Transport Security (HSTS) Policy, as
   applied by a conformant UA in interactions with a web resource host
   wielding such policy (known as an HSTS Host), are summarized as
   follows:



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   1.  UAs transform insecure URI references to an HSTS Host into secure
       URI references before dereferencing them.

   2.  The UA terminates any secure transport connection attempts upon
       any and all secure transport errors or warnings.

2.3.  Threat Model

   HSTS is concerned with three threat classes: passive network
   attackers, active network attackers, and imperfect web developers.
   However, it is explicitly not a remedy for two other classes of
   threats: phishing and malware.  Addressed and not addressed threats
   are briefly discussed below.  Readers may wish refer to Section 2 of
   [ForceHTTPS] for details as well as relevant citations.

2.3.1.  Threats Addressed

2.3.1.1.  Passive Network Attackers

   When a user browses the web on a local wireless network (e.g., an
   802.11-based wireless local area network) a nearby attacker can
   possibly eavesdrop on the user's unencrypted Internet Protocol-based
   connections, such as HTTP, regardless of whether or not the local
   wireless network itself is secured [BeckTews09].  Freely available
   wireless sniffing toolkits (e.g., [Aircrack-ng]) enable such passive
   eavesdropping attacks, even if the local wireless network is
   operating in a secure fashion.  A passive network attacker using such
   tools can steal session identifiers/cookies and hijack the user's web
   session(s), by obtaining cookies containing authentication
   credentials [ForceHTTPS].  For example, there exist widely-available
   tools, such as Firesheep (a web browser extension) [Firesheep], which
   enable their wielder to obtain other local users' session cookies for
   various web applications.

   To mitigate such threats, some Web sites support, but usually do not
   force, access using end-to-end secure transport -- e.g., signaled
   through URIs constructed with the "https" scheme [RFC2818].  This can
   lead users to believe that accessing such services using secure
   transport protects them from passive network attackers.
   Unfortunately, this is often not the case in real-world deployments
   as session identifiers are often stored in non-Secure cookies to
   permit interoperability with versions of the service offered over
   insecure transport ("Secure cookies" are those cookies containing the
   "Secure" attribute [RFC6265]).  For example, if the session
   identifier for a web site (an email service, say) is stored in a non-
   Secure cookie, it permits an attacker to hijack the user's session if
   the user's UA makes a single insecure HTTP request to the site.




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2.3.1.2.  Active Network Attackers

   A determined attacker can mount an active attack, either by
   impersonating a user's DNS server or, in a wireless network, by
   spoofing network frames or offering a similarly-named evil twin
   access point.  If the user is behind a wireless home router, an
   attacker can attempt to reconfigure the router using default
   passwords and other vulnerabilities.  Some sites, such as banks, rely
   on end-to-end secure transport to protect themselves and their users
   from such active attackers.  Unfortunately, browsers allow their
   users to easily opt-out of these protections in order to be usable
   for sites that incorrectly deploy secure transport, for example by
   generating and self-signing their own certificates (without also
   distributing their certification authority (CA) certificate to their
   users' browsers).

2.3.1.3.  Web Site Development and Deployment Bugs

   The security of an otherwise uniformly secure site (i.e. all of its
   content is materialized via "https" URIs), can be compromised
   completely by an active attacker exploiting a simple mistake, such as
   the loading of a cascading style sheet or a SWF (Shockwave Flash)
   movie over an insecure connection (both cascading style sheets and
   SWF movies can script the embedding page, to the surprise of many web
   developers, plus some browsers do not issue so-called "mixed content
   warnings" when SWF files are embedded via insecure connections).
   Even if the site's developers carefully scrutinize their login page
   for "mixed content", a single insecure embedding anywhere on the
   overall site compromises the security of their login page because an
   attacker can script (i.e., control) the login page by injecting code
   (e.g., a script) into another, insecurely loaded, site page.

   Note:  "Mixed content" as used above (see also Section 5.3 in
          [W3C.REC-wsc-ui-20100812]) refers to the notion termed "mixed
          security context" in this specification, and should not be
          confused with the same "mixed content" term used in the
          context of markup languages such as XML and HTML.

2.3.2.  Threats Not Addressed

2.3.2.1.  Phishing

   Phishing attacks occur when an attacker solicits authentication
   credentials from the user by hosting a fake site located on a
   different domain than the real site, perhaps driving traffic to the
   fake site by sending a link in an email message.  Phishing attacks
   can be very effective because users find it difficult to distinguish
   the real site from a fake site.  HSTS is not a defense against



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   phishing per se; rather, it complements many existing phishing
   defenses by instructing the browser to protect session integrity and
   long-lived authentication tokens [ForceHTTPS].

2.3.2.2.  Malware and Browser Vulnerabilities

   Because HSTS is implemented as a browser security mechanism, it
   relies on the trustworthiness of the user's system to protect the
   session.  Malicious code executing on the user's system can
   compromise a browser session, regardless of whether HSTS is used.

2.4.  Requirements

   This section identifies and enumerates various requirements derived
   from the use cases and the threats discussed above, and lists the
   detailed core requirements HTTP Strict Transport Security addresses,
   as well as ancillary requirements that are not directly addressed.

2.4.1.  Overall Requirement

   o  Minimize the risks to web browser users and web site deployers
      that are derived from passive and active network attackers, web
      site development and deployment bugs, as well as insecure user
      actions.

2.4.1.1.  Detailed Core Requirements

   These core requirements are derived from the overall requirement, and
   are addressed by this specification.

   1.  Web sites need to be able to declare to UAs that they should be
       accessed using a strict security policy.

   2.  Web sites need to be able to instruct UAs that contact them
       insecurely to do so securely.

   3.  UAs need to retain persistent data about web sites that signal
       strict security policy enablement, for time spans declared by the
       web sites.  Additionally, UAs need to cache the "freshest" strict
       security policy information, in order to allow web sites to
       update the information.

   4.  UAs need to re-write all insecure UA "http" URI loads to use the
       "https" secure scheme for those web sites for which secure policy
       is enabled.

   5.  Web site administrators need to be able to signal strict security
       policy application to subdomains of higher-level domains for



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       which strict security policy is enabled, and UAs need to enforce
       such policy.

       For example, both example.com and foo.example.com could set
       policy for bar.foo.example.com.

   6.  UAs need to disallow security policy application to peer domains,
       and/or higher-level domains, by domains for which strict security
       policy is enabled.

       For example, neither bar.foo.example.com nor foo.example.com can
       set policy for example.com, nor can bar.foo.example.com set
       policy for foo.example.com.  Also, foo.example.com cannot set
       policy for sibling.example.com.

   7.  UAs need to prevent users from clicking-through security
       warnings.  Halting connection attempts in the face of secure
       transport exceptions is acceptable.

   Note:  A means for uniformly securely meeting the first core
          requirement above is not specifically addressed by this
          specification (see Section 14.4 "Bootstrap MITM
          Vulnerability").  It may be addressed by a future revision of
          this specification or some other specification.  Note also
          that there are means by which UA implementations may more
          fully meet the first core requirement; see Section 11 "User
          Agent Implementation Advice".

2.4.1.2.  Detailed Ancillary Requirements

   These ancillary requirements are also derived from the overall
   requirement.  They are not normatively addressed in this
   specification, but could be met by UA implementations at their
   implementor's discretion, although meeting these requirements may be
   complex.

   1.  Disallow "mixed security context" loads (see Section 2.3.1.3).

   2.  Facilitate user declaration of web sites for which strict
       security policy is enabled, regardless of whether the sites
       signal HSTS Policy.


3.  Conformance Criteria

   This specification is written for hosts and user agents (UAs).

   [[IESG Note: the next two paragraphs are for readers with a



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   background in W3C specification style, of which we expect many.  RFC
   Editor, please remove this note before publication.]]

   A conformant host is one that implements all the requirements listed
   in this specification that are applicable to hosts.

   A conformant user agent is one that implements all the requirements
   listed in this specification that are applicable to user agents.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

3.1.  Document Conventions

   Note:  This is a note to the reader.  These are points that should be
          expressly kept in mind and/or considered.

   Warning:  This is how a warning is shown.  These are things that can
             have negative downside risks if not heeded.


4.  Terminology

   Terminology is defined in this section.

   ASCII case-insensitive comparison:
                     means comparing two strings exactly, codepoint for
                     codepoint, except that the characters in the range
                     U+0041 ..  U+005A (i.e.  LATIN CAPITAL LETTER A to
                     LATIN CAPITAL LETTER Z) and the corresponding
                     characters in the range U+0061 ..  U+007A (i.e.
                     LATIN SMALL LETTER A to LATIN SMALL LETTER Z) are
                     considered to also match.  See [Unicode] for
                     details.

   codepoint:        is a colloquial contraction of Code Point, which is
                     any value in the Unicode codespace; that is, the
                     range of integers from 0 to 10FFFF(hex) [Unicode].

   domain name:      domain names, also referred to as DNS Names, are
                     defined in [RFC1035] to be represented outside of
                     the DNS protocol itself (and implementations
                     thereof) as a series of labels separated by dots,
                     e.g., "example.com" or "yet.another.example.org".
                     In the context of this specification, domain names
                     appear in that portion of a URI satisfying the reg-
                     name production in "Appendix A.  Collected ABNF for



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                     URI" in [RFC3986], and the host component from the
                     Host HTTP header field production in Section 14.23
                     of [RFC2616].

                     Note:  The domain names appearing in actual URI
                            instances and matching the aforementioned
                            production components may or may not be a
                            fully qualified domain name.

   domain name label:  is that portion of a domain name appearing
                     "between the dots", i.e. consider
                     "foo.example.com": "foo", "example", and "com" are
                     all domain name labels.

   Effective Request URI:
                     is a URI, identifying the target resource, that can
                     be inferred by an HTTP host for any given HTTP
                     request it receives.  Such inference is necessary
                     because HTTP requests often do not contain a
                     complete "absolute" URI identifying the target
                     resource.  See Section 12 "Constructing an
                     Effective Request URI".

   HTTP Strict Transport Security:
                     is the overall name for the combined UA- and
                     server-side security policy defined by this
                     specification.

   HTTP Strict Transport Security Host:
                     is a HTTP host implementing the server aspects of
                     the HSTS Policy.  This means that an HSTS Host
                     returns the "Strict-Transport-Security" HTTP
                     response header field in its HTTP response messages
                     sent over secure transport.

   HTTP Strict Transport Security Policy:
                     is the name of the combined overall UA- and server-
                     side facets of the behavior defined in this
                     specification.

   HSTS:             See HTTP Strict Transport Security.

   HSTS Host:        See HTTP Strict Transport Security Host.








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   HSTS Policy:      See HTTP Strict Transport Security Policy.

   Known HSTS Host:  is an HSTS Host for which the UA has an HSTS Policy
                     in effect.  I.e., the UA has noted this host as a
                     Known HSTS Host.

   Local policy:     is comprised of policy rules deployers specify and
                     which are often manifested as configuration
                     settings.

   MITM:             is an acronym for man-in-the-middle.  See "man-in-
                     the-middle attack" in [RFC4949].

   Request URI:      is the URI used to cause a UA to issue an HTTP
                     request message.

   UA:               is a an acronym for user agent.  For the purposes
                     of this specification, a UA is an HTTP client
                     application typically actively manipulated by a
                     user [RFC2616] .

   unknown HSTS Host:  is an HSTS Host that the user agent in question
                     has not yet noted.


5.  HSTS Mechanism Overview

   This section provides an overview of the mechanism by which an HSTS
   Host conveys its HSTS Policy to UAs, and how UAs process the HSTS
   Policies received from HSTS Hosts.  The mechanism details are
   specified in Section 6 through Section 15.

   An HSTS Host conveys its HSTS Policy to UAs via the Strict-Transport-
   Security HTTP response header field over secure transport (e.g.,
   TLS).  Receipt of this header field signals to UAs to enforce the
   HSTS Policy for all subsequent connections made to the HSTS Host, for
   a specified time duration.  Application of the HSTS Policy to
   subdomains of the HSTS Host name may optionally be specified.

   HSTS Hosts manage their advertised HSTS Policies by sending Strict-
   Transport-Security HTTP response header fields to UAs with new values
   for policy time duration and application to subdomains.  UAs cache
   the "freshest" HSTS Policy information on behalf of an HSTS Host.
   Specifying a zero time duration signals to the UA to delete the HSTS
   Policy for that HSTS Host.

   Section 6.2 presents examples of Strict-Transport-Security HTTP
   response header fields.



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6.  Syntax

   This section defines the syntax of the Strict-Transport-Security HTTP
   response header field and its directives, and presents some examples.

   Section 7 "Server Processing Model" then details how hosts employ
   this header field to declare their HSTS Policy, and Section 8 "User
   Agent Processing Model" details how user agents process the header
   field and apply the HSTS Policy.

6.1.  Strict-Transport-Security HTTP Response Header Field

   The Strict-Transport-Security HTTP response header field (STS header
   field) indicates to a UA that it MUST enforce the HSTS Policy in
   regards to the host emitting the response message containing this
   header field.

   The ABNF (Augmented Backus-Naur Form) syntax for the STS header field
   is given below.  It is based on the Generic Grammar defined in
   Section 2 of [RFC2616] (which includes a notion of "implied linear
   whitespace", also known as "implied *LWS").

     Strict-Transport-Security = "Strict-Transport-Security" ":"
                                 [ directive ]  *( ";" [ directive ] )


     directive                 = token [ "=" ( token | quoted-string ) ]

   where:

     token          = <token, defined in [RFC2616], Section 2.2>
     quoted-string  = <quoted-string, defined in [RFC2616], Section 2.2>

   The two directives defined in this specification are described below.
   The overall requirements for directives are:

   1.  The order of appearance of directives is not significant.

   2.  All directives MUST appear only once in an STS header field.
       Directives are either optional or required, as stipulated in
       their definitions.

   3.  Directive names are case-insensitive.

   4.  UAs MUST ignore any STS header fields containing directives, or
       other header field value data, that does not conform to the
       syntax defined in this specification.




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   5.  If an STS header field contains directive(s) not recognized by
       the UA, the UA MUST ignore the unrecognized directives and if the
       STS header field otherwise satisfies the above requirements (1
       through 4), the UA MUST process the recognized directives.

   Additional directives extending the semantic functionality of the STS
   header field can be defined in other specifications.

6.1.1.  The max-age Directive

   The REQUIRED "max-age" directive specifies the number of seconds,
   after the reception of the STS header field, during which the UA
   regards the host (from whom the message was received) as a Known HSTS
   Host.  See also Section 8.1.1 "Noting an HSTS Host".  The delta-
   seconds production is specified in [RFC2616].

   The syntax of the max-age directive's value (after quoted-string
   unescaping, if necessary) is defined as:

    max-age-value = delta-seconds

    delta-seconds = <1*DIGIT, defined in [RFC2616], Section 3.3.2>

   Note:  A max-age value of zero (i.e., "max-age=0") signals the UA to
          cease regarding the host as a Known HSTS Host.

6.1.2.  The includeSubDomains Directive

   The OPTIONAL "includeSubDomains" directive is a valueless flag which,
   if present, signals to the UA that the HSTS Policy applies to this
   HSTS Host as well as any subdomains of the host's domain name.

6.2.  Examples

   The below HSTS header field stipulates that the HSTS Policy is to
   remain in effect for one year (there are approximately 31 536 000
   seconds in a year), and the policy applies only to the domain of the
   HSTS Host issuing it:

     Strict-Transport-Security: max-age=31536000

   The below HSTS header field stipulates that the HSTS Policy is to
   remain in effect for approximately six months and the policy applies
   only to the domain of the issuing HSTS Host and all of its
   subdomains:

     Strict-Transport-Security: max-age=15768000 ; includeSubDomains




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   The max-age directive value can optionally be quoted:

     Strict-Transport-Security: max-age="31536000"


7.  Server Processing Model

   This section describes the processing model that HSTS Hosts
   implement.  The model is comprised of two facets: the first being the
   processing rules for HTTP request messages received over a secure
   transport (e.g., TLS [RFC5246], SSL [I-D.ietf-tls-ssl-version3], or
   perhaps others), the second being the processing rules for HTTP
   request messages received over non-secure transports, i.e. over
   TCP/IP.

7.1.  HTTP-over-Secure-Transport Request Type

   When replying to an HTTP request that was conveyed over a secure
   transport, an HSTS Host SHOULD include in its response message an STS
   header field that MUST satisfy the grammar specified above in
   Section 6.1 "Strict-Transport-Security HTTP Response Header Field".
   If an STS header field is included, the HSTS Host MUST include only
   one such header field.

   Note:  Including the STS header field is stipulated as a "SHOULD" in
          order to accommodate various server- and network-side caches
          and load-balancing configurations where it may be difficult to
          uniformly emit STS header fields on behalf of a given HSTS
          Host.

          Establishing a given host as a Known HSTS Host, in the context
          of a given UA, MAY be accomplished over the HTTP protocol by
          correctly returning, per this specification, at least one
          valid STS header field to the UA.  Other mechanisms, such as a
          client-side pre-loaded Known HSTS Host list MAY also be used.
          E.g., see Section 11 "User Agent Implementation Advice".

7.2.  HTTP Request Type

   If an HSTS Host receives a HTTP request message over a non-secure
   transport, it SHOULD send a HTTP response message containing a status
   code indicating a permanent redirect, such as status code 301
   (Section 10.3.2 of [RFC2616]), and a Location header field value
   containing either the HTTP request's original Effective Request URI
   (see Section 12 "Constructing an Effective Request URI") altered as
   necessary to have a URI scheme of "https", or a URI generated
   according to local policy (which SHOULD employ a URI scheme of
   "https").



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   Note:  The above behavior is a "SHOULD" rather than a "MUST" due to:

      *  Risks in server-side non-secure-to-secure redirects
         [owaspTLSGuide].

      *  Site deployment characteristics.  For example, a site that
         incorporates third-party components may not behave correctly
         when doing server-side non-secure-to-secure redirects in the
         case of being accessed over non-secure transport, but does
         behave correctly when accessed uniformly over secure transport.
         The latter is the case given a HSTS-capable UA that has already
         noted the site as a Known HSTS Host (by whatever means, e.g.,
         prior interaction or UA configuration).

   An HSTS Host MUST NOT include the STS header field in HTTP responses
   conveyed over non-secure transport.


8.  User Agent Processing Model

   This section describes the HTTP Strict Transport Security processing
   model for UAs.  There are several facets to the model, enumerated by
   the following subsections.

   This processing model assumes that the UA implements IDNA2008
   [RFC5890], or possibly IDNA2003 [RFC3490], as noted in Section 13
   "Internationalized Domain Names for Applications (IDNA): Dependency
   and Migration".  It also assumes that all domain names manipulated in
   this specification's context are already IDNA-canonicalized as
   outlined in Section 9 "Domain Name IDNA-Canonicalization" prior to
   the processing specified in this section.

   The above assumptions mean that this processing model also
   specifically assumes that appropriate IDNA and Unicode validations
   and character list testing have occurred on the domain names, in
   conjunction with their IDNA-canonicalization, prior to the processing
   specified in this section.  See the IDNA-specific security
   considerations in Section 14.8 "Internationalized Domain Names" for
   rationale and further details.

8.1.  Strict-Transport-Security Response Header Field Processing

   If an HTTP response, received over a secure transport, includes an
   STS header field, conforming to the grammar specified in Section 6.1
   "Strict-Transport-Security HTTP Response Header Field", and there are
   no underlying secure transport errors or warnings (see Section 8.4),
   the UA MUST either:




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   o  Note the host as a Known HSTS Host if it is not already so noted
      (see Section 8.1.1 "Noting an HSTS Host"),

   or,

   o  Update the UA's cached information for the Known HSTS Host if
      either or both of the max-age and includeSubDomains header field
      value tokens are conveying information different than that already
      maintained by the UA.

      The max-age value is essentially a "time to live" value relative
      to the reception time of the STS header field.

      If the max-age header field value token has a value of zero, the
      UA MUST remove its cached HSTS Policy information if the HSTS Host
      is known, or, MUST NOT note this HSTS Host if it is not yet known.

      If a UA receives more than one STS header field in a HTTP response
      message over secure transport, then the UA MUST process only the
      first such header field.

   Otherwise:

   o  If an HTTP response is received over insecure transport, the UA
      MUST ignore any present STS header field(s).

   o  The UA MUST ignore any STS header fields not conforming to the
      grammar specified in Section 6.1 "Strict-Transport-Security HTTP
      Response Header Field".

8.1.1.  Noting an HSTS Host

   If the substring matching the host production from the Request-URI
   (of the message to which the host responded) syntactically matches
   the IP-literal or IPv4address productions from Section 3.2.2 of
   [RFC3986], then the UA MUST NOT note this host as a Known HSTS Host.

   Otherwise, if the substring does not congruently match a Known HSTS
   Host's domain name, per the matching procedure specified in
   Section 8.2 "Known HSTS Host Domain Name Matching", then the UA MUST
   note this host as a Known HSTS Host, caching the HSTS Host's domain
   name and noting along with it the expiry time of this information, as
   effectively stipulated per the given max-age value, as well as
   whether the includeSubDomains flag is asserted or not.  See also
   Section 10.1 "HSTS Policy expiration time considerations".






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   Note:  The UA MUST NOT modify the expiry time nor the
          includeSubDomains flag of any superdomain matched Known HSTS
          Host.

8.2.  Known HSTS Host Domain Name Matching

   A given domain name may match a Known HSTS Host's domain name in one
   or both of two fashions: a congruent match, or a superdomain match.
   Or, there may be no match.

   The below steps determine whether there are any matches, and if so,
   of which fashion:

      Compare the given domain name with the domain name of each of the
      UA's Known HSTS Hosts.  For each Known HSTS Host's domain name,
      the comparison is done with the given domain name label-by-label
      (comparing only labels) using an ASCII case-insensitive comparison
      beginning with the rightmost label, and continuing right-to-left.
      See also Section 2.3.2.4. of [RFC5890].

      *  Superdomain Match

         If a label-for-label match between an entire Known HSTS Host's
         domain name and a right-hand portion of the given domain name
         is found, then this Known HSTS Host's domain name is a
         superdomain match for the given domain name.  There could be
         multiple superdomain matches for a given domain name.

         For example:

            Given Domain Name:      qaz.bar.foo.example.com

            Superdomain matched
            Known HSTS Host DN:         bar.foo.example.com

            Superdomain matched
            Known HSTS Host DN:             foo.example.com


      *  Congruent Match

         If a label-for-label match between a Known HSTS Host's domain
         name and the given domain name is found -- i.e., there are no
         further labels to compare -- then the given domain name
         congruently matches this Known HSTS Host.

         For example:




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            Given Domain Name:              foo.example.com

            Congruently matched
            Known HSTS Host DN:             foo.example.com


      *  Otherwise, if no matches are found, the given domain name does
         not represent a Known HSTS Host.

8.3.  URI Loading and Port Mapping

   Whenever the UA prepares to "load", also known as "dereference", any
   "http" URI [RFC3986], the UA MUST first determine whether a domain
   name is given in the URI and whether it matches a Known HSTS Host,
   using these steps:

   1.  Extract from the URI any substring described by the host
       component of the authority component of the URI.

   2.  If the substring is null, then there is no match with any Known
       HSTS Host.

   3.  Else, if the substring is non-null and syntactically matches the
       IP-literal or IPv4address productions from Section 3.2.2 of
       [RFC3986], then there is no match with any Known HSTS Host.

   4.  Otherwise, the substring is a given domain name, which MUST be
       matched against the UA's Known HSTS Hosts using the procedure in
       Section 8.2 "Known HSTS Host Domain Name Matching".

   If a superdomain match with an asserted includeSubDomains flag is
   found, or if a congruent match is found -- without any found
   superdomain matches having asserted includeSubDomains flags -- then
   before proceeding with the load:

      The UA MUST replace the URI scheme with "https" [RFC2818], and,

      if the URI contains an explicit port component of "80", then the
      UA MUST convert the port component to be "443", or,

      if the URI contains an explicit port component that is not equal
      to "80", the port component value MUST be preserved, otherwise,

      if the URI does not contain an explicit port component, the UA
      MUST NOT add one.






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   Note:  The the above steps ensure that the HSTS Policy applies to
          HTTP over any TCP port of an HSTS Host.

8.4.  Errors in Secure Transport Establishment

   When connecting to a Known HSTS Host, the UA MUST terminate the
   connection (see also Section 11 "User Agent Implementation Advice")
   if there are any errors (e.g., certificate errors), whether "warning"
   or "fatal" or any other error level, with the underlying secure
   transport.  This includes any issues with certificate revocation
   checking whether via the Certificate Revocation List (CRL) [RFC5280],
   or via the Online Certificate Status Protocol (OCSP) [RFC2560].

8.5.  HTTP-Equiv <Meta> Element Attribute

   UAs MUST NOT heed http-equiv="Strict-Transport-Security" attribute
   settings on <meta> elements [W3C.REC-html401-19991224] in received
   content.

8.6.  Missing Strict-Transport-Security Response Header Field

   If a UA receives HTTP responses from a Known HSTS Host over a secure
   channel, but they are missing the STS header field, the UA MUST
   continue to treat the host as a Known HSTS Host until the max-age
   value for the knowledge of that Known HSTS Host is reached.  Note
   that the max-age value could be effectively infinite for a given
   Known HSTS Host.  For example, this would be the case if the Known
   HSTS Host is part of a pre-configured list that is implemented such
   that the list entries never "age out".


9.  Domain Name IDNA-Canonicalization

   An IDNA-canonicalized domain name is the output string generated by
   the following steps.  The input is a putative domain name string
   ostensibly composed of any combination of "A-labels", "U-labels", and
   "NR-LDH labels" (see Section 2 of [RFC5890]) concatenated using some
   separator character (typically ".").

   1.  Convert the input putative domain name string to a order-
       preserving sequence of individual label strings.

   2.  When implementing IDNA2008, convert, validate, and test each
       A-label and U-label found among the sequence of individual label
       strings, using the procedures defined in Sections 5.3 through 5.5
       of [RFC5891].

       Otherwise, when implementing IDNA2003, convert each label using



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       the "ToASCII" conversion in Section 4 of [RFC3490] (see also the
       definition of "equivalence of labels" in Section 2 of the latter
       specification).

   3.  If no errors occurred during the foregoing step, concatenate all
       the labels in the sequence, in order, into a string, separating
       each label from the next with a %x2E (".") character.  The
       resulting string, known as a IDNA-canonicalized domain name, is
       appropriate for use in the context of Section 8 "User Agent
       Processing Model".

       Otherwise, errors occurred.  The input putative domain name
       string was not successfully IDNA-canonicalized.  Invokers of this
       procedure should attempt appropriate error recovery.

   See also Section 13 "Internationalized Domain Names for Applications
   (IDNA): Dependency and Migration" and Section 14.8 "Internationalized
   Domain Names" of this specification for further details and
   considerations.


10.  Server Implementation and Deployment Advice

   This section is non-normative.

10.1.  HSTS Policy expiration time considerations

   Server implementations and deploying web sites need to consider
   whether they are setting an expiry time that is a constant value into
   the future, or whether they are setting an expiry time that is a
   fixed point in time.

   The constant value into the future approach can be accomplished by
   constantly sending the same max-age value to UAs.

   For example, a max-age value of 778000 is 90 days:

     Strict-Transport-Security: max-age=778000

   Note that each receipt of this header by a UA will require the UA to
   update its notion of when it must delete its knowledge of this Known
   HSTS Host.

   The fixed point in time approach can be accomplished by sending max-
   age values that represent the remaining time until the desired expiry
   time.  This would require the HSTS Host to send a newly-calculated
   max-age value on each HTTP response.




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   A consideration here is whether a deployer wishes to have the
   signaled HSTS Policy expiry time match that for the web site's domain
   certificate.

   Additionally, server implementers should consider employing a default
   max-age value of zero in their deployment configuration systems.
   This will require deployers to wilfully set max-age in order to have
   UAs enforce the HSTS Policy for their host, and protects them from
   inadvertently enabling HSTS with some arbitrary non-zero duration.

10.2.  Using HSTS in conjunction with self-signed public-key
       certificates

   If a web site/organization/enterprise is generating its own secure
   transport public-key certificates for web sites, and that
   organization's root certification authority (CA) certificate is not
   typically embedded by default in browser and/or operating system CA
   certificate stores, and if HSTS Policy is enabled on a host
   identifying itself using a certificate signed by the organization's
   CA (i.e., a "self-signed certificate"), and this certificate does not
   match a usable TLS certificate association (as defined by Section 4
   of the TLSA protocol specification [I-D.ietf-dane-protocol]), then
   secure connections to that site will fail, per the HSTS design.  This
   is to protect against various active attacks, as discussed above.

   However, if said organization wishes to employ its own CA, and self-
   signed certificates, in concert with HSTS, it can do so by deploying
   its root CA certificate to its users' browsers or operating system CA
   root certificate stores.  It can also, in addition or instead,
   distribute to its users' browsers the end-entity certificate(s) for
   specific hosts.  There are various ways in which this can be
   accomplished (details are out of scope for this specification).  Once
   its root CA certificate is installed in the browsers, it may employ
   HSTS Policy on its site(s).

   Alternately, that organization can deploy the TLSA protocol; all
   browsers that also use TLSA will then be able to trust the
   certificates identified by usable TLS certificate associations as
   denoted via TLSA.

   Note:  Interactively distributing root CA certificates to users,
          e.g., via email, and having the users install them, is
          arguably training the users to be susceptible to a possible
          form of phishing attack.  See Section 14.6 "Bogus Root CA
          Certificate Phish plus DNS Cache Poisoning Attack".  Thus care
          should be taken in the manner in which such certificates are
          distributed and installed on users' systems and browsers.




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10.3.  Implications of includeSubDomains

   The includeSubDomains directive has some practical implications.  For
   example, if an HSTS Host offers HTTP-based services on various ports
   or at various subdomains of its host domain name, then they will all
   have to be available over secure transport in order to work properly.

   For example, certification authorities often offer their CRL
   distribution and OCSP services [RFC2560] over plain HTTP, and
   sometimes at a subdomain of a publicly-available web application
   which may be secured by TLS/SSL.  For example,
   <https://ca.example.com/> is a publicly-available web application for
   "Example CA", a certification authority.  Customers use this web
   application to register their public keys and obtain certificates.
   Example CA generates certificates for customers containing
   <http://crl-and-ocsp.ca.example.com/> as the value for the "CRL
   Distribution Points" and "Authority Information Access:OCSP"
   certificate fields.

   If ca.example.com were to issue an HSTS Policy with the
   includeSubDomains directive, then HTTP-based user agents implementing
   HSTS that have interacted with the ca.example.com web application
   would fail to retrieve CRLs, and fail to check OCSP for certificates,
   because these services are offered over plain HTTP.

   In this case, Example CA can either:

   o  not use the includeSubDomains directive, or,

   o  ensure HTTP-based services offered at subdomains of ca.example.com
      are also uniformly offered over TLS/SSL, or,

   o  offer plain HTTP-based services at a different domain name, e.g.,
      crl-and-ocsp.ca.example.NET.


11.  User Agent Implementation Advice

   This section is non-normative.

   In order to provide users and web sites more effective protection, as
   well as controls for managing their UA's caching of HSTS Policy, UA
   implementers should consider including features such as:

11.1.  No User Recourse

   Failing secure connection establishment on any warnings or errors
   (per Section 8.4 "Errors in Secure Transport Establishment") should



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   be done with "no user recourse".  This means that the user should not
   be presented with a dialog giving her the option to proceed.  Rather,
   it should be treated similarly to a server error where there is
   nothing further the user can do with respect to interacting with the
   target web application, other than wait and re-try.

   Essentially, "any warnings or errors" means anything that would cause
   the UA implementation to announce to the user that something is not
   entirely correct with the connection establishment.

   Not doing this, i.e., allowing user recourse such as "clicking-
   through warning/error dialogs", is a recipe for a Man-in-the-Middle
   attack.  If a web application advertises HSTS, then it is opting into
   this scheme, whereby all certificate errors or warnings cause a
   connection termination, with no chance to "fool" the user into making
   the wrong decision and compromising themselves.

11.2.  User-declared HSTS Policy

   A User-declared HSTS Policy is the ability for users to explicitly
   declare a given Domain Name as representing an HSTS Host, thus
   seeding it as a Known HSTS Host before any actual interaction with
   it.  This would help protect against the Section 14.4 "Bootstrap MITM
   Vulnerability".

   Note:  Such a feature is difficult to get right on a per-site basis.
          See the discussion of "rewrite rules" in Section 5.5 of
          [ForceHTTPS].  For example, arbitrary web sites may not
          materialize all their URIs using the "https" scheme, and thus
          could "break" if a UA were to attempt to access the site
          exclusively using such URIs.  Also note that this feature
          would complement, but is independent of, the following
          described facility.

11.3.  HSTS Pre-Loaded List

   An HSTS Pre-Loaded List is a facility whereby web site administrators
   can have UAs pre-configured with HSTS Policy for their site(s) by the
   UA vendor(s) -- a so-called "pre-loaded list" -- in a manner similar
   to how root CA certificates are embedded in browsers "at the
   factory".  This would help protect against the Section 14.4
   "Bootstrap MITM Vulnerability".

   Note:  Such a facility would complement a "User-declared HSTS Policy"
          feature.






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11.4.  Disallow Mixed Security Context Loads

   "Mixed security context" loads are when an web application resource,
   fetched by the UA over a secure transport, subsequently fetches and
   loads one or more other resources without using secure transport.
   This is also generally referred to as "mixed content" loads (see
   Section 5.3 "Mixed Content" in [W3C.REC-wsc-ui-20100812]), but should
   not be confused with the same "mixed content" term that is also used
   in the context of markup languages such as XML and HTML.

   Note:  In order to provide behavioral uniformity across UA
          implementations, the notion of mixed security context will
          require further standardization work, e.g., to define the
          term(s) more clearly and to define specific behaviors with
          respect to it.

11.5.  HSTS Policy Deletion

   HSTS Policy Deletion is the ability to delete a UA's cached HSTS
   Policy on a per HSTS Host basis.

   Note:  Adding such a feature should be done very carefully in both
          the user interface and security senses.  Deleting a cache
          entry for a Known HSTS Host should be a very deliberate and
          well-considered act -- it shouldn't be something users get
          used to just "clicking through" in order to get work done.
          Also, implementations need to guard against allowing an
          attacker to inject code, e.g.  ECMAscript, into the UA that
          silently and programmatically removes entries from the UA's
          cache of Known HSTS Hosts.


12.  Constructing an Effective Request URI

   This section specifies how an HSTS Host must construct the Effective
   Request URI for a received HTTP request.

   HTTP requests often do not carry an absoluteURI for the target
   resource; instead, the URI needs to be inferred from the Request-URI,
   Host header field, and connection context ([RFC2616], Sections 3.2.1
   and 5.1.2).  The result of this process is called the "effective
   request URI (ERU)".  The "target resource" is the resource identified
   by the effective request URI.

12.1.  ERU Fundamental Definitions

   The first line of an HTTP request message, Request-Line, is specified
   by the following ABNF from [RFC2616], Section 5.1:



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     Request-Line   = Method SP Request-URI SP HTTP-Version CRLF

   The Request-URI, within the Request-Line, is specified by the
   following ABNF from [RFC2616], Section 5.1.2:

     Request-URI    = "*" | absoluteURI | abs_path | authority

   The Host request header field is specified by the following ABNF from
   [RFC2616], Section 14.23:

     Host = "Host" ":" host [ ":" port ]

12.2.  Determining the Effective Request URI

   If the Request-URI is an absoluteURI, then the effective request URI
   is the Request-URI.

   If the Request-URI uses the abs_path form or the asterisk form, and
   the Host header field is present, then the effective request URI is
   constructed by concatenating:

   o  the scheme name: "http" if the request was received over an
      insecure TCP connection, or "https" when received over a TLS/
      SSL-secured TCP connection, and,

   o  the octet sequence "://", and,

   o  the host, and the port (if present), from the Host header field,
      and

   o  the Request-URI obtained from the Request-Line, unless the
      Request-URI is just the asterisk "*".

   If the Request-URI uses the abs_path form or the asterisk form, and
   the Host header field is not present, then the effective request URI
   is undefined.

   Otherwise, when Request-URI uses the authority form, the effective
   request URI is undefined.

   Effective request URIs are compared using the rules described in
   [RFC2616] Section 3.2.3, except that empty path components MUST NOT
   be treated as equivalent to an absolute path of "/".








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12.2.1.  Effective Request URI Examples

   Example 1: the effective request URI for the message

     GET /pub/WWW/TheProject.html HTTP/1.1
     Host: www.example.org:8080

   (received over an insecure TCP connection) is "http", plus "://",
   plus the authority component "www.example.org:8080", plus the
   request-target "/pub/WWW/TheProject.html".  Thus it is:
   "http://www.example.org:8080/pub/WWW/TheProject.html".

   Example 2: the effective request URI for the message

     OPTIONS * HTTP/1.1
     Host: www.example.org

   (received over an SSL/TLS secured TCP connection) is "https", plus
   "://", plus the authority component "www.example.org".  Thus it is:
   "https://www.example.org".


13.  Internationalized Domain Names for Applications (IDNA): Dependency
     and Migration

   Textual domain names on the modern Internet may contain one or more
   "internationalized" domain name labels.  Such domain names are
   referred to as "internationalized domain names" (IDNs).  The
   specification suites defining IDNs and the protocols for their use
   are named "Internationalized Domain Names for Applications (IDNA)".
   At this time, there are two such specification suites: IDNA2008
   [RFC5890] and its predecessor IDNA2003 [RFC3490].

   IDNA2008 obsoletes IDNA2003, but there are differences between the
   two specifications, and thus there can be differences in processing
   (e.g., converting) domain name labels that have been registered under
   one from those registered under the other.  There will be a
   transition period of some time during which IDNA2003-based domain
   name labels will exist in the wild.  In order to facilitate their
   IDNA transition, user agents SHOULD implement IDNA2008 [RFC5890] and
   MAY implement [RFC5895] (see also Section 7 of [RFC5894]) or [UTS46].
   If a user agent does not implement IDNA2008, the user agent MUST
   implement IDNA2003.


14.  Security Considerations

   This specification concerns the expression, conveyance, and



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   enforcement of the HSTS Policy.  The overall HSTS Policy threat
   model, including addressed and unaddressed threats, is given in
   Section 2.3 "Threat Model".

   Additionally, the below sections discuss operational ramifications of
   the HSTS Policy, provide feature rationale, discuss potential HSTS
   Policy misuse, and highlight some known vulnerabilities in the HSTS
   Policy regime.

14.1.  Ramifications of HSTS Policy Establishment only over Error-free
       Secure Transport

   The User Agent Processing Model defined in Section 8, stipulates that
   a host is initially noted as a Known HSTS Host, or that updates are
   made to a Known HSTS Host's cached information, only if the UA
   receives the STS header field over a secure transport connection
   having no underlying secure transport errors or warnings.

   The rationale behind this is that if there is a man-in-the-middle
   (MITM) -- whether a legitimately deployed proxy or an illegitimate
   entity -- it could cause various mischief (see also Appendix A
   "Design Decision Notes", item 3, as well as Section 14.4 "Bootstrap
   MITM Vulnerability"), for example:

   o  Unauthorized notation of the host as a Known HSTS Host,
      potentially leading to a denial of service situation if the host
      does not uniformly offer its services over secure transport (see
      also Section 14.3 "Denial of Service").

   o  Resetting the time-to-live for the host's designation as a Known
      HSTS Host by manipulating the max-age header field parameter value
      that is returned to the UA.  If max-age is returned as zero, this
      will cause the host to cease being regarded as an Known HSTS Host
      by the UA, leading to either insecure connections to the host or
      possibly denial-of-service if the host delivers its services only
      over secure transport.

   However, this means that if a UA is "behind" a proxy -- within a
   corporate intranet, for example -- and interacts with an unknown HSTS
   Host beyond the proxy, the user will possibly be presented with the
   legacy secure connection error dialogs.  Even if the risk is accepted
   and the user clicks-through, the host will not be noted as an HSTS
   Host.  Thus, as long as the UA is behind such a proxy the user will
   be vulnerable, and possibly be presented with the legacy secure
   connection error dialogs for as yet unknown HSTS Hosts.

   Once the UA successfully connects to an unknown HSTS Host over error-
   free secure transport, the host will be noted as a Known HSTS Host.



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   This will result in the failure of subsequent connection attempts
   from behind interfering proxies.

   The above discussion relates to the recommendation in Section 11
   "User Agent Implementation Advice" that the secure connection be
   terminated with "no user recourse" whenever there are warnings and
   errors and the host is a Known HSTS Host.  Such a posture protects
   users from "clicking through" security warnings and putting
   themselves at risk.

14.2.  The Need for includeSubDomains

   Without the includeSubDomains directive, a web application would not
   be able to adequately protect so-called "domain cookies" (even if
   these cookies have their "Secure" flag set and thus are conveyed only
   on secure channels).  These are cookies the web application expects
   UAs to return to any and all subdomains of the web application.

   For example, suppose example.com represents the top-level DNS name
   for a web application.  Further suppose that this cookie is set for
   the entire example.com domain, i.e. it is a "domain cookie", and it
   has its Secure flag set.  Suppose example.com is a Known HSTS Host
   for this UA, but the includeSubDomains flag is not set.

   Now, if an attacker causes the UA to request a subdomain name that is
   unlikely to already exist in the web application, such as
   "https://uxdhbpahpdsf.example.com/", but that the attacker has
   managed to register in the DNS and point at an HTTP server under the
   attacker's control, then:

   1.  The UA is unlikely to already have an HSTS Policy established for
       "uxdhbpahpdsf.example.com", and,

   2.  The HTTP request sent to uxdhbpahpdsf.example.com will include
       the Secure-flagged domain cookie.

   3.  If "uxdhbpahpdsf.example.com" returns a certificate during TLS
       establishment, and the user clicks through any warning that might
       be presented (it is possible, but not certain, that one may
       obtain a requisite certificate for such a domain name such that a
       warning may or may not appear), then the attacker can obtain the
       Secure-flagged domain cookie that's ostensibly being protected.

   Without the "includeSubDomains" directive, HSTS is unable to protect
   such Secure-flagged domain cookies.






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14.3.  Denial of Service

   HSTS could be used to mount certain forms of Denial-of- Service (DoS)
   attacks against web sites.  A DoS attack is an attack in which one or
   more network entities target a victim entity and attempt to prevent
   the victim from doing useful work.  This section discusses such
   scenarios in terms of HSTS, though this list is not exhaustive.  See
   also [RFC4732] for a discussion of overall Internet DoS
   considerations.

   o  Web applications available over HTTP

      There is an opportunity for perpetrating DoS attacks with web
      applications that are -- or critical portions of them are --
      available only over HTTP without secure transport, if attackers
      can cause UAs to set HSTS Policy for such web applications'
      host(s).

      This is because once the HSTS Policy is set for a web
      application's host in a UA, the UA will only use secure transport
      to communicate with the host.  If the host is not using secure
      transport, or is not for critical portions of its web application,
      then the web application will be rendered unusable for the UA's
      user.

      An HSTS Policy can be set for a victim host in various ways:

      *  If the web application has a HTTP response splitting
         vulnerability [CWE-113] (which can be abused in order to
         facilitate "HTTP Header Injection").

      *  If an attacker can spoof a redirect from an insecure victim
         site, e.g., <http://example.com/> to <httpS://example.com/>,
         where the latter is attacker-controlled and has an apparently
         valid certificate, then the attacker can set an HSTS Policy for
         example.com, and also for all subdomains of example.com.

      *  If an attacker can convince users to manually configure HSTS
         Policy for a victim host.  This assumes their UAs offer such a
         capability (see Section 11 "User Agent Implementation Advice").
         Or, if such UA configuration is scriptable, then an attacker
         can cause UAs to execute his script and set HSTS Policies for
         whichever desired domains.

   o  Inadvertent use of includeSubDomains

      The includeSubDomains directive instructs UAs to automatically
      regard all subdomains of the given HSTS Host as Known HSTS Hosts.



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      If any such subdomains do not support properly configured secure
      transport, then they will be rendered unreachable from such UAs.

14.4.  Bootstrap MITM Vulnerability

   The bootstrap MITM (Man-In-The-Middle) vulnerability is a
   vulnerability users and HSTS Hosts encounter in the situation where
   the user manually enters, or follows a link, to an unknown HSTS Host
   using a "http" URI rather than a "https" URI.  Because the UA uses an
   insecure channel in the initial attempt to interact with the
   specified server, such an initial interaction is vulnerable to
   various attacks (see Section 5.3 of [ForceHTTPS]).

   Note:  There are various features/facilities that UA implementations
          may employ in order to mitigate this vulnerability.  Please
          see Section 11 "User Agent Implementation Advice".

14.5.  Network Time Attacks

   Active network attacks can subvert network time protocols (such as
   Network Time Protocol (NTP) [RFC5905]) - making HSTS less effective
   against clients that trust NTP or lack a real time clock.  Network
   time attacks are beyond the scope of this specification.  Note that
   modern operating systems use NTP by default.  See also Section 2.10
   of [RFC4732].

14.6.  Bogus Root CA Certificate Phish plus DNS Cache Poisoning Attack

   An attacker could conceivably obtain a victim HSTS-protected web
   application's users' login credentials via a bogus root CA
   certificate phish plus DNS cache poisoning attack.

   For example, the attacker could first convince users of a victim web
   application (which is protected by HSTS Policy) to install the
   attacker's version of a root CA certificate purporting (falsely) to
   represent the CA of the victim web application.  This might be
   accomplished by sending the users a phishing email message with a
   link to such a certificate, which their browsers may offer to install
   if clicked on.

   Then, if the attacker can perform an attack on the users' DNS
   servers, (e.g., via cache poisoning) and turn on HSTS Policy for
   their fake web application, the affected users' browsers would access
   the attackers' web application rather than the legitimate web
   application.

   This type of attack leverages vectors that are outside of the scope
   of HSTS.  However, the feasibility such threats can be mitigated by



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   appropriate employment of security facilities such as DNS Security
   [RFC4033], and DomainKeys Identified Mail (DKIM) [RFC6376], in
   addition to HSTS, on the part of a web application's overall
   deployment approach,

14.7.  Creative Manipulation of HSTS Policy Store

   Since an HSTS Host may select its own host name and subdomains
   thereof, and this information is cached in the HSTS Policy store of
   conforming UAs, it is possible for those who control an HSTS Host(s)
   to encode information into domain names they control and cause such
   UAs to cache this information as a matter of course in the process of
   noting the HSTS Host.  This information can be retrieved by other
   hosts through clever loaded page construction causing the UA to send
   queries to (variations of) the encoded domain names.  Such queries
   can reveal whether the UA had prior visited the original HSTS Host
   (and subdomains).

   Such a technique could potentially be abused as yet another form of
   "web tracking" [WebTracking].

14.8.  Internationalized Domain Names

   Internet security relies in part on the DNS and the domain names it
   hosts.  Domain names are used by users to identify and connect to
   Internet hosts and other network resources.  For example, Internet
   security is compromised if a user entering an internationalized
   domain name (IDN) is connected to different hosts based on different
   interpretations of the IDN.

   The processing models specified in this specification assume that the
   domain names they manipulate are IDNA-canonicalized, and that the
   canonicalization process correctly performed all appropriate IDNA and
   Unicode validations and character list testing per the requisite
   specifications (e.g., as noted in Section 9 "Domain Name IDNA-
   Canonicalization").  These steps are necessary in order to avoid
   various potentially compromising situations.

   In brief, some examples of issues that could stem from lack of
   careful and consistent Unicode and IDNA validations are things such
   as unexpected processing exceptions, truncation errors, and buffer
   overflows, as well as false-positive and/or false-negative domain
   name matching results.  Any of the foregoing issues could possibly be
   leveraged by attackers in various ways.

   Additionally, IDNA2008 [RFC5890] differs from IDNA2003 [RFC3490] in
   terms of disallowed characters and character mapping conventions.
   This situation can also lead to false-positive and/or false-negative



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   domain name matching results, resulting in, for example, users
   possibly communicating with unintended hosts, or not being able to
   reach intended hosts.

   For details, refer to the Security Considerations sections of
   [RFC5890], [RFC5891], and [RFC3490], as well as the specifications
   they normatively reference.  Additionally, [RFC5894] provides
   detailed background and rationale for IDNA2008 in particular, as well
   as IDNA and its issues in general, and should be consulted in
   conjunction with the former specifications.


15.  IANA Considerations

   Below is the Internet Assigned Numbers Authority (IANA) Permanent
   Message Header Field registration information per [RFC3864].

     Header field name:           Strict-Transport-Security
     Applicable protocol:         HTTP
     Status:                      standard
     Author/Change controller:    IETF
     Specification document(s):   this one


16.  References

16.1.  Normative References

   [I-D.ietf-dane-protocol]
              Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Protocol for Transport Layer
              Security (TLS)", draft-ietf-dane-protocol-19 (work in
              progress), April 2012.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2560]  Myers, M., Ankney, R., Malpani, A., Galperin, S., and C.
              Adams, "X.509 Internet Public Key Infrastructure Online
              Certificate Status Protocol - OCSP", RFC 2560, June 1999.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

   [RFC3490]  Faltstrom, P., Hoffman, P., and A. Costello,



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              "Internationalizing Domain Names in Applications (IDNA)",
              RFC 3490, March 2003.

              This specification is referenced due to its ongoing
              relevance to actual deployments for the foreseeable
              future.

   [RFC3492]  Costello, A., "Punycode: A Bootstring encoding of Unicode
              for Internationalized Domain Names in Applications
              (IDNA)", RFC 3492, March 2003.

              This specification is referenced due to its ongoing
              relevance to actual deployments for the foreseeable
              future.

   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              September 2004.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, May 2008.

   [RFC5890]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, August 2010.

   [RFC5891]  Klensin, J., "Internationalized Domain Names in
              Applications (IDNA): Protocol", RFC 5891, August 2010.

   [RFC5894]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Background, Explanation, and
              Rationale", RFC 5894, August 2010.

   [RFC5895]  Resnick, P. and P. Hoffman, "Mapping Characters for
              Internationalized Domain Names in Applications (IDNA)
              2008", RFC 5895, September 2010.

   [UTS46]    Davis, M. and M. Suignard, "Unicode IDNA Compatibility
              Processing", Unicode Technical Standards # 46, 2010,



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              <http://unicode.org/reports/tr46/>.

   [Unicode]  The Unicode Consortium, "The Unicode Standard",
              <http://www.unicode.org/versions/latest/>.

   [W3C.REC-html401-19991224]
              Hors, A., Raggett, D., and I. Jacobs, "HTML 4.01
              Specification", World Wide Web Consortium
              Recommendation REC-html401-19991224, December 1999,
              <http://www.w3.org/TR/1999/REC-html401-19991224>.

16.2.  Informative References

   [Aircrack-ng]
              d'Otreppe, T., "Aircrack-ng", Accessed: 11-Jul-2010,
              <http://www.aircrack-ng.org/>.

   [BeckTews09]
              Beck, M. and E. Tews, "Practical Attacks Against WEP and
              WPA", Second ACM Conference on Wireless Network
              Security Zurich, Switzerland, 2009, <http://
              wirelesscenter.dk/Crypt/wifi-security-attacks/
              Practical%20Attacks%20Against%20WEP%20and%20WPA.pdf>.

   [CWE-113]  "CWE-113: Improper Neutralization of CRLF Sequences in
              HTTP Headers ('HTTP Response Splitting')", Common Weakness
              Enumeration <http://cwe.mitre.org/>, The Mitre
              Corporation <http://www.mitre.org/>,
              <http://cwe.mitre.org/data/definitions/113.html>.

   [Firesheep]
              Various, "Firesheep", Wikipedia Online, on-going,
              <https://secure.wikimedia.org/wikipedia/en/wiki/
              Firesheep>.

   [ForceHTTPS]
              Jackson, C. and A. Barth, "ForceHTTPS:  Protecting High-
              Security Web Sites from Network Attacks", In Proceedings
              of the 17th International World Wide Web Conference
              (WWW2008) , 2008,
              <https://crypto.stanford.edu/forcehttps/>.

   [GoodDhamijaEtAl05]
              Good, N., Dhamija, R., Grossklags, J., Thaw, D.,
              Aronowitz, S., Mulligan, D., and J. Konstan, "Stopping
              Spyware at the Gate: A User Study of Privacy, Notice and
              Spyware", In Proceedings of Symposium On Usable Privacy
              and Security (SOUPS) Pittsburgh, PA, USA, July 2005, <http



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              ://people.ischool.berkeley.edu/~rachna/papers/
              spyware_study.pdf>.

   [I-D.ietf-tls-ssl-version3]
              Freier, A., Karlton, P., and P. Kocher, "The SSL Protocol
              Version 3.0", draft-ietf-tls-ssl-version3-00 (work in
              progress), November 1996, <http://tools.ietf.org/html/
              draft-ietf-tls-ssl-version3-00>.

              This is the canonical reference for SSLv3.0.

   [JacksonBarth2008]
              Jackson, C. and A. Barth, "Beware of Finer-Grained
              Origins", Web 2.0 Security and Privacy Oakland, CA, USA,
              2008,
              <http://www.adambarth.com/papers/2008/
              jackson-barth-b.pdf>.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, March 2005.

   [RFC4732]  Handley, M., Rescorla, E., and IAB, "Internet Denial-of-
              Service Considerations", RFC 4732, December 2006.

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
              RFC 4949, August 2007.

   [RFC5905]  Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network
              Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, June 2010.

   [RFC6101]  Freier, A., Karlton, P., and P. Kocher, "The Secure
              Sockets Layer (SSL) Protocol Version 3.0", RFC 6101,
              August 2011.

   [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              April 2011.

   [RFC6376]  Crocker, D., Hansen, T., and M. Kucherawy, "DomainKeys
              Identified Mail (DKIM) Signatures", RFC 6376,
              September 2011.

   [RFC6454]  Barth, A., "The Web Origin Concept", RFC 6454,
              December 2011.



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   [SunshineEgelmanEtAl09]
              Sunshine, J., Egelman, S., Almuhimedi, H., Atri, N., and
              L. Cranor, "Crying Wolf: An Empirical Study of SSL Warning
              Effectiveness", In Proceedings of 18th USENIX Security
              Symposium Montreal, Canada, Augus 2009, <http://
              www.usenix.org/events/sec09/tech/full_papers/
              sunshine.pdf>.

   [W3C.REC-wsc-ui-20100812]
              Saldhana, A. and T. Roessler, "Web Security Context: User
              Interface Guidelines", World Wide Web Consortium
              Recommendation REC-wsc-ui-20100812, August 2010,
              <http://www.w3.org/TR/2010/REC-wsc-ui-20100812>.

   [WEBSEC]   "WebSec -- HTTP Application Security Minus Authentication
              and Transport",
              <https://www.ietf.org/mailman/listinfo/websec>.

              Mailing list for IETF WebSec Working Group.  [RFCEditor:
              please remove this reference upon publication as an RFC.]

   [WebTracking]
              Schmucker, N., "Web Tracking", SNET2 Seminar Paper Summer
              Term, 2011, <http://www.snet.tu-berlin.de/fileadmin/fg220/
              courses/SS11/snet-project/web-tracking_schmuecker.pdf>.

   [owaspTLSGuide]
              Coates, M., Wichers, d., Boberski, M., and T. Reguly,
              "Transport Layer Protection Cheat Sheet", Accessed: 11-
              Jul-2010, <http://www.owasp.org/index.php/
              Transport_Layer_Protection_Cheat_Sheet>.


Appendix A.  Design Decision Notes

   This appendix documents various design decisions.

   1.  Cookies aren't appropriate for HSTS Policy expression as they are
       potentially mutable (while stored in the UA), therefore an HTTP
       header field is employed.

   2.  We chose to not attempt to specify how "mixed security context
       loads" (AMA "mixed content loads") are handled due to UA
       implementation considerations as well as classification
       difficulties.

   3.  An HSTS Host may update UA notions of HSTS Policy via new HSTS
       header field parameter values.  We chose to have UAs honor the



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       "freshest" information received from a server because there is
       the chance of a web site sending out an errornous HSTS Policy,
       such as a multi-year max-age value, and/or an incorrect
       includeSubDomains flag.  If the HSTS Host couldn't correct such
       errors over protocol, it would require some form of annunciation
       to users and manual intervention on the users' part, which could
       be a non-trivial problem for both web application providers and
       their users.

   4.  HSTS Hosts are identified only via domain names -- explicit IP
       address identification of all forms is excluded.  This is for
       simplification and also is in recognition of various issues with
       using direct IP address identification in concert with PKI-based
       security.

   5.  The max-age approch of having the HSTS Host provide a simple
       integer number of seconds for a cached HSTS Policy time-to-live
       value, as opposed to an approach of stating an expiration time in
       the future was chosen for various reasons.  Amongst the reasons
       are no need for clock syncronization, no need to define date and
       time value syntaxes (specification simplicity), and
       implementation simplicity.


Appendix B.  Differences between HSTS Policy and Same-Origin Policy

   HSTS Policy has the following primary characteristics:

      HSTS Policy stipulates requirements for the security
      characteristics of UA-to-host connection establishment, on a per-
      host basis.

      Hosts explicitly declare HSTS Policy to UAs.  Conformant UAs are
      obliged to implement hosts' declared HSTS Policies.

      HSTS Policy is conveyed over protocol from the host to the UA.

      The UA maintains a cache of Known HSTS Hosts.

      UAs apply HSTS Policy whenever making a HTTP connection to a Known
      HSTS Host, regardless of host port number.  I.e., it applies to
      all ports on a Known HSTS Host.  Hosts are unable to affect this
      aspect of HSTS Policy.

      Hosts may optionally declare that their HSTS Policy applies to all
      subdomains of their host domain name.

   In contrast, the Same-Origin Policy (SOP) [RFC6454] has the following



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   primary characteristics:

      An origin is the scheme, host, and port of a URI identifying a
      resource.

      A UA may dereference a URI, thus loading a representation of the
      resource the URI identifies.  UAs label resource representations
      with their origins, which are derived from their URIs.

      The SOP refers to a collection of principles, implemented within
      UAs, governing the isolation of and communication between resource
      representations within the UA, as well as resource
      representations' access to network resources.

   In summary, although both HSTS Policy and SOP are enforced by UAs,
   HSTS Policy is optionally declared by hosts and is not origin-based,
   while the SOP applies to all resource representations loaded from all
   hosts by conformant UAs.


Appendix C.  Acknowledgments

   The authors thank Devdatta Akhawe, Michael Barrett, Tobias Gondrom,
   Paul Hoffman, Murray Kucherawy, Barry Leiba, James Manger, Alexey
   Melnikov, Yoav Nir, Laksh Raghavan, Marsh Ray, Julian Reschke, Tom
   Ritter, Peter Saint-Andre, Sid Stamm, Maciej Stachowiak, Andy
   Steingrubl, Brandon Sterne, Martin Thomson, Daniel Veditz, as well as
   all the websec working group participants and others for their
   various reviews and helpful contributions.

   Thanks to Julian Reschke for his elegant re-writing of the effective
   request URI text, which he did when incorporating the ERU notion into
   the HTTPbis work.  Subsequently, the ERU text in this spec was lifted
   from Julian's work in [I-D.draft-ietf-httpbis-p1-messaging-17] and
   adapted to the [RFC2616] ABNF.


Appendix D.  Change Log

   [RFCEditor: please remove this section upon publication as an RFC.]

   Changes are grouped by spec revision listed in reverse issuance
   order.

D.1.  For draft-ietf-websec-strict-transport-sec






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      Changes from -08 to -09:

      1.  Added IESG Note to Section 3 "Conformance Criteria" per Barry
          Leiba's suggestion on the mailing list.  <https://
          www.ietf.org/mail-archive/web/websec/current/msg01200.html>

      2.  Added additional requirement #5 to requirements for STS header
          field directives in Section 6.1 per Alexey's review.  This
          completes the addressing of issue ticket #45.
          <http://trac.tools.ietf.org/wg/websec/trac/ticket/45>

      3.  Addressed editorial feedback in Murray's AppsDir review of
          -06.

          Most all of these changes were addressing detailed/small
          editorial items, however note the addition of a couple of
          introductory paragraphs in the Security Considerations
          section, as well as a re-written and expanded Section 14.6
          "Bogus Root CA Certificate Phish plus DNS Cache Poisoning
          Attack", as well the new item #5 to Appendix A "Design
          Decision Notes".

          This addresses issue ticket #46.
          <http://trac.tools.ietf.org/wg/websec/trac/ticket/46>

      Changes from -07 to -08:

      1.  Clarified requirement #4 for STS header field directives in
          Section 6.1, and removed "(which "update" this
          specification)".  Also added explicit "max-age=0" to Section
          6.1.1.  Reworked final sentence in 2nd para of Section 13.
          This addresses issue ticket #45.
          <http://trac.tools.ietf.org/wg/websec/trac/ticket/45>

      Changes from -06 to -07:

      1.  Various minor/modest editorial tweaks throughout as I went
          through it pursuing the below issue tickets.  Viewing a visual
          diff against -06 revision recommended.

      2.  fixed some minor editorial issues noted in review by Alexey,
          fixes noted in here: <https://www.ietf.org/mail-archive/web/
          websec/current/msg01163.html>

      3.  Addressed ABNF exposition issues, specifically inclusion of
          quoted-string syntax for directive values.  Fix STS header
          ABNF such that a leading ";" isn't required.  Add example of
          quoted-string-encoded max-age-value.  This addresses (re-



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          opened) issue ticket #33.
          <http://trac.tools.ietf.org/wg/websec/trac/ticket/33>

      4.  Reworked sections 8.1 through 8.3 to ensure matching algorithm
          and resultant HSTS Policy application is more clear, and that
          it is explicitly stipulated to not muck with attributes of
          superdomain matching Known HSTS Hosts.  This addresses issue
          ticket #37.
          <http://trac.tools.ietf.org/wg/websec/trac/ticket/37>

      5.  Added reference to [I-D.ietf-dane-protocol], pared back
          extraneous discussion in section 2.2, and updated discussion
          in 10.2 to accomodate TLSA (nee DANE).  This addresses issue
          ticket #39.
          <http://trac.tools.ietf.org/wg/websec/trac/ticket/39>

      6.  Addressed various editorial items from issue ticket #40.
          <http://trac.tools.ietf.org/wg/websec/trac/ticket/40>

      7.  Loosened up the language regarding redirecting "http" requests
          to "https" in section 7.2 such that future flavors of
          permanent redirects are accommodated.  This addresses issue
          ticket #43.
          <http://trac.tools.ietf.org/wg/websec/trac/ticket/43>

      8.  Reworked the terminology and language in Section 9, in
          particular defining the term "putative domain name string" to
          replace "valid Unicode-encoded string-serialized domain name".
          This addresses issue ticket #44.
          <http://trac.tools.ietf.org/wg/websec/trac/ticket/44>

      Changes from -05 to -06:

      1.  Addressed various editorial comments provided by Tobias G.
          This addresses issue ticket #38.
          <http://trac.tools.ietf.org/wg/websec/trac/ticket/38>

      Changes from -04 to -05:

      1.  Fixed up references to move certain ones back to the normative
          section -- as requested by Alexey M. Added explanation for
          referencing obsoleted [RFC3490] and [RFC3492].  This addresses
          issue ticket #36.
          <http://trac.tools.ietf.org/wg/websec/trac/ticket/36>

      2.  Made minor change to Strict-Transport-Security header field
          ABNF in order to address further feedback as appended to
          ticket #33.  This addresses issue ticket #33.



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          <http://trac.tools.ietf.org/wg/websec/trac/ticket/33>

      Changes from -03 to -04:

      1.   Clarified that max-age=0 will cause UA to forget a known HSTS
           Host, and more generally clarified that the "freshest" info
           from the HSTS Host is cached, and thus HSTS Hosts are able to
           alter the cached max-age in UAs.  This addresses issue ticket
           #13. <http://trac.tools.ietf.org/wg/websec/trac/ticket/13>

      2.   Updated section on "Constructing an Effective Request URI" to
           remove remaining reference to RFC3986 and reference RFC2616
           instead.  Further addresses issue ticket #14.
           <http://trac.tools.ietf.org/wg/websec/trac/ticket/14>

      3.   Addresses further ABNF issues noted in comment:1 of issue
           ticket #27.  <http://trac.tools.ietf.org/wg/websec/trac/
           ticket/27#comment:1>

      4.   Reworked the introduction to clarify the denotation of "HSTS
           Policy" and added the new Appendix B summarizing the primary
           characteristics of HSTS Policy and Same-Origin Policy, and
           identifying their differences.  Added ref to [RFC4732].  This
           addresses issue ticket #28.
           <http://trac.tools.ietf.org/wg/websec/trac/ticket/28>

      5.   Reworked language in Section 2.3.1.3. wrt "mixed content",
           more clearly explain such vulnerability, disambiguate "mixed
           content" in web security context from its usage in markup
           language context.  This addresses issue ticket #29.
           <http://trac.tools.ietf.org/wg/websec/trac/ticket/29>

      6.   Expanded Denial of Service discussion in Security
           Considerations.  Added refs to [RFC4732] and [CWE-113].  This
           addresses issue ticket #30.
           <http://trac.tools.ietf.org/wg/websec/trac/ticket/30>

      7.   Mentioned in prose the case-insensitivity of directive names.
           This addresses issue ticket #31.
           <http://trac.tools.ietf.org/wg/websec/trac/ticket/31>

      8.   Added Section 10.3 "Implications of includeSubDomains".  This
           addresses issue ticket #32.
           <http://trac.tools.ietf.org/wg/websec/trac/ticket/32>

      9.   Further refines text and ABNF definitions of STS header field
           directives.  Retains use of quoted-string in directive
           grammar.  This addresses issue ticket #33.



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           <http://trac.tools.ietf.org/wg/websec/trac/ticket/33>

      10.  Added Section 14.7 "Creative Manipulation of HSTS Policy
           Store", including reference to [WebTracking].  This addresses
           issue ticket #34.
           <http://trac.tools.ietf.org/wg/websec/trac/ticket/34>

      11.  Added Section 14.1 "Ramifications of HSTS Policy
           Establishment only over Error-free Secure Transport" and made
           some accompanying editorial fixes in some other sections.
           This addresses issue ticket #35.
           <http://trac.tools.ietf.org/wg/websec/trac/ticket/35>

      12.  Refined references.  Cleaned out un-used ones, updated to
           latest RFCs for others, consigned many to Informational.
           This addresses issue ticket #36.
           <http://trac.tools.ietf.org/wg/websec/trac/ticket/36>

      13.  Fixed-up some inaccuracies in the "Changes from -02 to -03"
           section.

      Changes from -02 to -03:

      1.  Updated section on "Constructing an Effective Request URI" to
          remove references to RFC3986.  Addresses issue ticket #14.
          <http://trac.tools.ietf.org/wg/websec/trac/ticket/14>

      2.  Reference RFC5890 for IDNA, retaining subordinate refs to
          RFC3490.  Updated IDNA-specific language, e.g. domain name
          canonicalization and IDNA dependencies.  Addresses issue
          ticket #26
          <http://trac.tools.ietf.org/wg/websec/trac/ticket/26>.

      3.  Completely re-wrote the STS header ABNF to be fully based on
          RFC2616, rather than a hybrid of RFC2616 and httpbis.
          Addresses issue ticket #27
          <http://trac.tools.ietf.org/wg/websec/trac/ticket/27>.

      Changes from -01 to -02:

      1.   Updated Section 8.3 "URI Loading and Port Mapping" fairly
           thoroughly in terms of refining the presentation of the
           steps, and to ensure the various aspects of port mapping are
           clear.  Nominally fixes issue ticket #1
           <http://trac.tools.ietf.org/wg/websec/trac/ticket/1>

      2.   Removed dependencies on
           [I-D.draft-ietf-httpbis-p1-messaging-15].  Thus updated STS



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           ABNF in Section 6.1 "Strict-Transport-Security HTTP Response
           Header Field" by lifting some productions entirely from
           [I-D.draft-ietf-httpbis-p1-messaging-15] and leveraging
           [RFC2616].  Addresses issue ticket #2
           <http://trac.tools.ietf.org/wg/websec/trac/ticket/2>.

      3.   Updated Effective Request URI section and definition to use
           language from [I-D.draft-ietf-httpbis-p1-messaging-15] and
           ABNF from [RFC2616].  Fixes issue ticket #3
           <http://trac.tools.ietf.org/wg/websec/trac/ticket/3>.

      4.   Added explicit mention that the HSTS Policy applies to all
           TCP ports of a host advertising the HSTS Policy.  Nominally
           fixes issue ticket #4
           <http://trac.tools.ietf.org/wg/websec/trac/ticket/4>

      5.   Clarified the need for the "includeSubDomains" directive,
           e.g. to protect Secure-flagged domain cookies.  In
           Section 14.2 "The Need for includeSubDomains".  Nominally
           fixes issue ticket #5
           <http://trac.tools.ietf.org/wg/websec/trac/ticket/5>

      6.   Cited Firesheep as real-live threat in Section 2.3.1.1
           "Passive Network Attackers".  Nominally fixes issue ticket #6
           <http://trac.tools.ietf.org/wg/websec/trac/ticket/6>.

      7.   Added text to Section 11 "User Agent Implementation Advice"
           justifying connection termination due to tls warnings/errors.
           Nominally fixes issue ticket #7
           <http://trac.tools.ietf.org/wg/websec/trac/ticket/7>.

      8.   Added new subsection Section 8.6 "Missing Strict-Transport-
           Security Response Header Field".  Nominally fixes issue
           ticket #8
           <http://trac.tools.ietf.org/wg/websec/trac/ticket/8>.

      9.   Added text to Section 8.4 "Errors in Secure Transport
           Establishment" explicitly note revocation check failures as
           errors causing connection termination.  Added references to
           [RFC5280] and [RFC2560].  Nominally fixes issue ticket #9
           <http://trac.tools.ietf.org/wg/websec/trac/ticket/9>.

      10.  Added a sentence, noting that distributing specific end-
           entity certificates to browsers will also work for self-
           signed/private-CA cases, to Section 10 "Server Implementation
           and Deployment Advice" Nominally fixes issue ticket #10
           <http://trac.tools.ietf.org/wg/websec/trac/ticket/10>.




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      11.  Moved "with no user recourse" language from Section 8.4
           "Errors in Secure Transport Establishment" to Section 11
           "User Agent Implementation Advice".  This nominally fixes
           issue ticket #11
           <http://trac.tools.ietf.org/wg/websec/trac/ticket/11>.

      12.  Removed any and all dependencies on
           [I-D.draft-ietf-httpbis-p1-messaging-15], instead depending
           on [RFC2616] only.  Fixes issue ticket #12
           <http://trac.tools.ietf.org/wg/websec/trac/ticket/12>.

      13.  Removed the inline "XXX1" issue because no one had commented
           on it and it seems reasonable to suggest as a SHOULD that web
           apps should redirect incoming insecure connections to secure
           connections.

      14.  Removed the inline "XXX2" issue because it was simply for
           raising consciousness about having some means for
           distributing secure web application metadata.

      15.  Removed "TODO1" because description prose for "max-age" in
           the Note following the ABNF in Section 6 seems to be fine.

      16.  Decided for "TODO2" that "the first STS header field wins".
           TODO2 had read: "Decide UA behavior in face of encountering
           multiple HSTS headers in a message.  Use first header?
           Last?".  Removed TODO2.

      17.  Added Section 1.1 "Organization of this specification" for
           readers' convenience.

      18.  Moved design decision notes to be a proper appendix
           Appendix A.

      Changes from -00 to -01:

      1.  Changed the "URI Loading" section to be "URI Loading and Port
          Mapping".

      2.  [HASMAT] reference changed to [WEBSEC].

      3.  Changed "server" -> "host" where applicable, notably when
          discussing "HSTS Hosts".  Left as "server" when discussing
          e.g. "http server"s.

      4.  Fixed minor editorial nits.





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      Changes from draft-hodges-strict-transport-sec-02 to
      draft-ietf-websec-strict-transport-sec-00:

      1.  Altered spec metadata (e.g. filename, date) in order to submit
          as a WebSec working group Internet-Draft.

D.2.  For draft-hodges-strict-transport-sec

      Changes from -01 to -02:

      1.   updated abstract such that means for expressing HSTS Policy
           other than via HSTS header field is noted.

      2.   Changed spec title to "HTTP Strict Transport Security (HSTS)"
           from "Strict Transport Security".  Updated use of "STS"
           acronym throughout spec to HSTS (except for when specifically
           discussing syntax of Strict-Transport-Security HTTP Response
           Header field), updated "Terminology" appropriately.

      3.   Updated the discussion of "Passive Network Attackers" to be
           more precise and offered references.

      4.   Removed para on nomative/non-normative from "Conformance
           Criteria" pending polishing said section to IETF RFC norms.

      5.   Added examples subsection to "Syntax" section.

      6.   Added OWS to maxAge production in Strict-Transport-Security
           ABNF.

      7.   Cleaned up explanation in the "Note:" in the "HTTP-over-
           Secure-Transport Request Type" section, folded 3d para into
           "Note:", added conformance clauses to the latter.

      8.   Added exaplanatory "Note:" and reference to "HTTP Request
           Type" section.  Added "XXX1" issue.

      9.   Added conformance clause to "URI Loading".

      10.  Moved "Notes for STS Server implementers:" from "UA
           Implementation dvice " to "HSTS Policy expiration time
           considerations:" in "Server Implementation Advice", and also
           noted another option.

      11.  Added cautionary "Note:" to "Ability to delete UA's cached
           HSTS Policy on a per HSTS Server basis".





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      12.  Added some informative references.

      13.  Various minor editorial fixes.

      Changes from -00 to -01:

      1.  Added reference to HASMAT mailing list and request that this
          spec be discussed there.


Authors' Addresses

   Jeff Hodges
   PayPal
   2211 North First Street
   San Jose, California  95131
   US

   Email: Jeff.Hodges@PayPal.com


   Collin Jackson
   Carnegie Mellon University

   Email: collin.jackson@sv.cmu.edu


   Adam Barth
   Google, Inc.

   Email: ietf@adambarth.com
   URI:   http://www.adambarth.com/



















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