[Docs] [txt|pdf|xml|html] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 RFC 6265

httpstate                                                       A. Barth
Internet-Draft                                             U.C. Berkeley
Obsoletes: 2109 (if approved)                              March 7, 2010
Intended status: Standards Track
Expires: September 8, 2010


                    HTTP State Management Mechanism
                     draft-ietf-httpstate-cookie-05

Abstract

   This document defines the HTTP Cookie and Set-Cookie headers.  These
   headers can be used by HTTP servers to store state on HTTP user
   agents, letting the servers maintain a stateful session over the
   mostly stateless HTTP protocol.  The cookie protocol has many
   historical infelicities that degrade its security and privacy.

      NOTE: If you have suggestions for improving the draft, please send
      email to http-state@ietf.org.  Suggestions with test cases are
      especially appreciated.


Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   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."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on September 8, 2010.

Copyright Notice




Barth                   Expires September 8, 2010               [Page 1]


Internet-Draft       HTTP State Management Mechanism          March 2010


   Copyright (c) 2010 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
   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 BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.



























Barth                   Expires September 8, 2010               [Page 2]


Internet-Draft       HTTP State Management Mechanism          March 2010


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  General Nonsense . . . . . . . . . . . . . . . . . . . . . . .  5
     2.1.  Conformance Criteria . . . . . . . . . . . . . . . . . . .  5
     2.2.  Syntax Notation  . . . . . . . . . . . . . . . . . . . . .  5
     2.3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.1.  Examples . . . . . . . . . . . . . . . . . . . . . . . . .  6
   4.  A Well-Behaved Profile . . . . . . . . . . . . . . . . . . . .  9
     4.1.  Set-Cookie . . . . . . . . . . . . . . . . . . . . . . . .  9
       4.1.1.  Syntax . . . . . . . . . . . . . . . . . . . . . . . .  9
       4.1.2.  Semantics (Non-Normative)  . . . . . . . . . . . . . . 10
     4.2.  Cookie . . . . . . . . . . . . . . . . . . . . . . . . . . 12
       4.2.1.  Syntax . . . . . . . . . . . . . . . . . . . . . . . . 12
       4.2.2.  Semantics  . . . . . . . . . . . . . . . . . . . . . . 12
   5.  The Cookie Protocol  . . . . . . . . . . . . . . . . . . . . . 14
     5.1.  Algorithms . . . . . . . . . . . . . . . . . . . . . . . . 14
       5.1.1.  Dates  . . . . . . . . . . . . . . . . . . . . . . . . 14
       5.1.2.  Domains  . . . . . . . . . . . . . . . . . . . . . . . 16
       5.1.3.  Paths  . . . . . . . . . . . . . . . . . . . . . . . . 16
     5.2.  The Set-Cookie Header  . . . . . . . . . . . . . . . . . . 17
       5.2.1.  The Max-Age Attribute  . . . . . . . . . . . . . . . . 19
       5.2.2.  The Expires Attribute  . . . . . . . . . . . . . . . . 19
       5.2.3.  The Domain Attribute . . . . . . . . . . . . . . . . . 20
       5.2.4.  The Path Attribute . . . . . . . . . . . . . . . . . . 20
       5.2.5.  The Secure Attribute . . . . . . . . . . . . . . . . . 21
       5.2.6.  The HttpOnly Attribute . . . . . . . . . . . . . . . . 21
     5.3.  Storage Model  . . . . . . . . . . . . . . . . . . . . . . 21
     5.4.  The Cookie Header  . . . . . . . . . . . . . . . . . . . . 24
   6.  Implementation Considerations  . . . . . . . . . . . . . . . . 27
     6.1.  Limits . . . . . . . . . . . . . . . . . . . . . . . . . . 27
     6.2.  Application Programmer Interfaces  . . . . . . . . . . . . 27
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 28
     7.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . 28
     7.2.  Ambient Authority  . . . . . . . . . . . . . . . . . . . . 28
     7.3.  Clear Text . . . . . . . . . . . . . . . . . . . . . . . . 29
     7.4.  Session Identifiers  . . . . . . . . . . . . . . . . . . . 29
     7.5.  Weak Confidentiality . . . . . . . . . . . . . . . . . . . 30
     7.6.  Weak Integrity . . . . . . . . . . . . . . . . . . . . . . 30
     7.7.  Reliance on DNS  . . . . . . . . . . . . . . . . . . . . . 31
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 32
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 32
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 32
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 33
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 34





Barth                   Expires September 8, 2010               [Page 3]


Internet-Draft       HTTP State Management Mechanism          March 2010


1.  Introduction

   This document defines the HTTP Cookie and Set-Cookie header.  Using
   the Set-Cookie header, an HTTP server can store name/value pairs and
   associated metadata (called cookies) at the user agent.  When the
   user agent makes subsequent requests to the server, the user agent
   uses the metadata to determine whether to return the name/value pairs
   in the Cookie header.

   Although simple on its surface, the cookie protocol has a number of
   complexities.  For example, the server indicates a scope for each
   cookie when sending them to the user agent.  The scope indicates the
   maximum amount of time the user agent should retain the cookie, to
   which servers the user agent should return the cookie, and for which
   protocols the cookie is applicable.

   For historical reasons, the cookie protocol contains a number of
   security and privacy infelicities.  For example, a server can
   indicate that a given cookie is intended for "secure" connections,
   but the Secure attribute provides only confidentiality (not
   integrity) from active network attackers.  Similarly, cookies for a
   given host are shared across all the ports on that host, even though
   the usual "same-origin policy" used by web browsers isolates content
   retrieved from different ports.



























Barth                   Expires September 8, 2010               [Page 4]


Internet-Draft       HTTP State Management Mechanism          March 2010


2.  General Nonsense

2.1.  Conformance Criteria

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

   Requirements phrased in the imperative as part of algorithms (such as
   "strip any leading space characters" or "return false and abort these
   steps") are to be interpreted with the meaning of the key word
   ("MUST", "SHOULD", "MAY", etc) used in introducing the algorithm.

2.2.  Syntax Notation

   This specification uses the Augmented Backus-Naur Form (ABNF)
   notation of [RFC5234].

   The following core rules are included by reference, as defined in
   [RFC5234], Appendix B.1: ALPHA (letters), CR (carriage return), CRLF
   (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote),
   HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
   sequence of data), SP (space), HTAB (horizontal tab), VCHAR (any
   visible [USASCII] character), and WSP (whitespace).

2.3.  Terminology

   The terms user agent, client, server, proxy, and origin server have
   the same meaning as in the HTTP/1.1 specification ([RFC2616]).

   The terms request-host and request-URI refer to the values the user
   agent would send to the server as, respectively, the host (but not
   port) and abs_path portions of the absoluteURI (http_URL) of the HTTP
   Request-Line.

















Barth                   Expires September 8, 2010               [Page 5]


Internet-Draft       HTTP State Management Mechanism          March 2010


3.  Overview

   We outline here a way for an origin server to send state information
   to a user agent, and for the user agent to return the state
   information to the origin server.

   To initiate a session, the origin server includes a Set-Cookie header
   in an HTTP response.  (Note that "session" here does not refer to a
   persistent network connection but to a logical session created from
   HTTP requests and responses.  The presence or absence of a persistent
   connection should have no effect on the use of cookie-derived
   sessions).

   The user agent returns a Cookie request header to the origin server
   if it chooses to continue a session.  The Cookie header contains a
   number of cookies the user agent received in previous Set-Cookie
   headers.  The origin server MAY ignore the Cookie header or use the
   header to determine the current state of the session.  The origin
   server MAY send the user agent a Set-Cookie response header with the
   same or different information, or it MAY send no Set-Cookie header at
   all.

   Servers MAY return a Set-Cookie response header with any response.
   User agents SHOULD send a Cookie request header, subject to other
   rules detailed below, with every request.

   An origin server MAY include multiple Set-Cookie header fields in a
   single response.  Note that an intervening gateway MUST NOT fold
   multiple Set-Cookie header fields into a single header field.

3.1.  Examples

   Using the cookie protocol, a server can send the user agent a short
   string in an HTTP response that the user agent will return in future
   HTTP requests.  For example, the server can send the user agent a
   "session identifier" named SID with the value 31d4d96e407aad42.  The
   user agent then returns the session identifier in subsequent
   requests.


   == Server -> User Agent ==
   Set-Cookie: SID=31d4d96e407aad42

   == User Agent -> Server ==
   Cookie: SID=31d4d96e407aad42


   The server can alter the default scope of the cookie using the Path



Barth                   Expires September 8, 2010               [Page 6]


Internet-Draft       HTTP State Management Mechanism          March 2010


   and Domain attributes.  For example, the server can instruct the user
   agent to return the cookie to every path and every subdomain of
   example.com.


   == Server -> User Agent ==
   Set-Cookie: SID=31d4d96e407aad42; Path=/; Domain=.example.com

   == User Agent -> Server ==
   Cookie: SID=31d4d96e407aad42


   The server can store multiple cookies in the user agent.  For
   example, the server can store a session identifier as well as the
   user's preferred language by returning two Set-Cookie response
   headers.  Notice that the server uses the Secure and HttpOnly
   attributes to provide additional security protections for the more-
   sensitive session identifier.


   == Server -> User Agent ==
   Set-Cookie: SID=31d4d96e407aad42; Path=/; Secure, HttpOnly
   Set-Cookie: lang=en-US; Path=/; Domain=.example.com

   == User Agent -> Server ==
   Cookie: SID=31d4d96e407aad42; lang=en-US


   If the server wishes the user agent to persist the cookie over
   multiple sessions, the server can specify a expiration date in the
   Expires attribute.  Note that the user agent might might delete the
   cookie before the expiration date if the user agent's cookie store
   exceeds its quota or if the user manually deletes the server's
   cookie.


   == Server -> User Agent ==
   Set-Cookie: lang=en-US; Expires=Wed, 09 Jun 2021 10:18:14 GMT

   == User Agent -> Server ==
   Cookie: lang=en-US


   Finally, to remove a cookie, the server returns a Set-Cookie header
   with an expiration date in the past.  The server will be successful
   in removing the cookie only if the Path and the Domain attribute in
   the Set-Cookie header match the values used when the cookie was
   created.



Barth                   Expires September 8, 2010               [Page 7]


Internet-Draft       HTTP State Management Mechanism          March 2010


   == Server -> User Agent ==
   Set-Cookie: lang=; Expires=Sun, 06 Nov 1994 08:49:37 GMT

   == User Agent -> Server ==
   (No Cookie header)














































Barth                   Expires September 8, 2010               [Page 8]


Internet-Draft       HTTP State Management Mechanism          March 2010


4.  A Well-Behaved Profile

   This section describes the syntax and semantics of a well-behaved
   profile of the protocol.  Servers SHOULD use the profile described in
   this section, both to maximize interoperability with existing user
   agents and because a future version of the cookie protocol could
   remove support for some of the most esoteric aspects of the protocol.
   User agents, however, MUST implement the full protocol to ensure
   interoperability with servers making use of the full protocol.

4.1.  Set-Cookie

   The Set-Cookie header is used to send cookies from the server to the
   user agent.

4.1.1.  Syntax

   Informally, the Set-Cookie response header comprises the token Set-
   Cookie:, followed by a cookie.  Each cookie begins with a name-value-
   pair, followed by zero or more attribute-value pairs.  Servers SHOULD
   NOT send Set-Cookie headers that fail to conform to the following
   grammar:


   set-cookie-header = "Set-Cookie:" OWS set-cookie-string OWS
   set-cookie-string = cookie-pair *( ";" cookie-av )
   cookie-pair       = cookie-name "=" cookie-value
   cookie-name       = token
   cookie-value      = token
   token             = <token, as defined in RFC 2616>

   cookie-av         = expires-av / domain-av / path-av /
                       secure-av / httponly-av
   expires-av        = "Expires" "=" sane-cookie-date
   sane-cookie-date  = <rfc1123-date, as defined in RFC 2616>
   domain-av         = "Domain" "=" domain-value
   domain-value      = token
   path-av           = "Path" "=" path-value
   path-value        = <abs_path, as defined in RFC 2616>
   secure-av         = "Secure"
   httponly-av       = "HttpOnly"


   Servers SHOULD NOT include two attributes with the same name.

   The cookie-value is opaque to the user agent and MAY be anything the
   origin server chooses to send.  "Opaque" implies that the content is
   of interest and relevance only to the origin server.  The content is,



Barth                   Expires September 8, 2010               [Page 9]


Internet-Draft       HTTP State Management Mechanism          March 2010


   in fact, readable by anyone who examines the Set-Cookie header.

   To maximize compatibility with user agents, servers that wish to
   store non-ASCII data in a cookie-value SHOULD encode that data using
   a printable ASCII encoding, such as base64.

   NOTE: The syntax above allows whitespace between the attribute and
   the U+003D ("=") character.  Servers wishing to interoperate with
   some legacy user agents might wish to elide this whitespace.

4.1.2.  Semantics (Non-Normative)

   This section describes a simplified semantics of the Set-Cookie
   header.  These semantics are detailed enough to be useful for
   understanding the most common uses of the cookie protocol.  The full
   semantics are described in Section 5.

   When the user agent receives a Set-Cookie header, the user agent
   stores the cookie in its cookie store.  When the user agent
   subsequently makes an HTTP request, the user agent consults its
   cookie store and includes the applicable, non-expired cookies in the
   Cookie header.

   If the cookie store already contains a cookie with the same cookie-
   name, domain-value, and path-value, the existing cookie is evicted
   from the cookie store and replaced with the new value.  Notice that
   servers can delete cookies by including an Expires attribute with a
   value in the past.

   Unless the cookie's attributes indicate otherwise, the cookie is
   returned only to the origin server, and it expires at the end of the
   current session (as defined by the user agent).  User agents ignore
   unrecognized cookie attributes.

4.1.2.1.  Expires

   The Expires attribute indicates the maximum lifetime of the cookie,
   represented as the date and time at which the cookie expires.  The
   user agent is not required to retain the cookie until the specified
   date has passed.  In fact, user agents often evict cookies from the
   cookie store due to memory pressure or privacy concerns.

4.1.2.2.  Domain

   The Domain attribute specifies those hosts for which the cookie will
   be sent.  For example, if the Domain attribute contains the value
   ".example.com", the user agent will include the cookie in the Cookie
   header when making HTTP requests to example.com, www.example.com, and



Barth                   Expires September 8, 2010              [Page 10]


Internet-Draft       HTTP State Management Mechanism          March 2010


   www.corp.example.com.  (Note that a leading U+002E ("."), if present,
   is ignored.)  If the server omits the Domain attribute, the user
   agent will return the cookie only to the origin server.

      WARNING: Some legacy user agents treat an absent Domain attribute
      as if the Domain attribute were present and contained the current
      host name.  For example, if example.com returns a Set-Cookie
      header without a Domain attribute, these user agents will send the
      cookie to www.example.com.

   The user agent will reject cookies (refuse to store them in the
   cookie store) unless the Domain attribute specifies a scope for the
   cookie that would include the origin server.  For example, the user
   agent will accept a Domain attribute of ".example.com" or of
   ".foo.example.com" from foo.example.com, but the user agent will not
   accept a Domain attribute of ".bar.example.com" or of
   ".baz.foo.example.com".

   NOTE: For security reasons, some user agents are configured to reject
   Domain attributes that do not correspond to a "registry controlled"
   domain (or a subdomain of a registry controlled domain).  For
   example, some user agents will reject Domain attributes of ".com".

4.1.2.3.  Path

   The Path attribute limits the scope of the cookie to a set of paths.
   When a cookie has a Path attribute, the user agent will include the
   cookie in an HTTP request only if the path portion of the Request-URI
   matches (or is a subdirectory of) the cookie's Path attribute, where
   the U+002F ("/") character is interpreted as a directory separator.
   If the server omits the Path attribute, the user agent will use the
   directory of the Request-URI's path component as the default value.

   Although seemingly useful for isolating cookies between different
   paths within a given domain, the Path attribute cannot be relied upon
   for security for two reasons: First, user agents do not prevent one
   path from overwriting the cookies for another path.  For example, if
   a response to a request for /foo/bar.html attempts to set a cookie
   with a Path attribute of "/baz" the user agent will store that cookie
   in the cookie store.  Second, the "same-origin" policy implemented by
   many user agents does not isolate different paths within an origin.
   For example, /foo/bar.html can read cookies with a Path attribute of
   "/baz" because they are within the "same origin".

4.1.2.4.  Secure

   The Secure attribute limits the scope of the cookie to "secure"
   channels (where "secure" is defined by the user agent).  When a



Barth                   Expires September 8, 2010              [Page 11]


Internet-Draft       HTTP State Management Mechanism          March 2010


   cookie has the Secure attribute, the user agent will include the
   cookie in an HTTP request only if the request is transmitted over a
   secure channel (typically TLS [RFC5246]).

   Although seemingly useful for protecting cookies from active network
   attackers, the Secure attribute protects only the cookie's
   confidentiality.  An active network attacker can overwrite Secure
   cookies from an insecure channel, disrupting the integrity of the
   cookies.

4.1.2.5.  HttpOnly

   The HttpOnly attribute limits the scope of the cookie to HTTP
   requests.  In particular, the attribute instructs the user agent to
   elide the cookie when providing access to its cookie store via "non-
   HTTP" APIs (as defined by the user agent).

4.2.  Cookie

4.2.1.  Syntax

   The user agent returns stored cookies to the origin server in the
   Cookie header.  If the server conforms to the requirements in this
   section, the requirements in the next section will cause the user
   agent to return a Cookie header that conforms to the following
   grammar:


   cookie-header = "Cookie:" OWS cookie-string OWS
   cookie-string = cookie-pair *( ";" cookie-pair )


4.2.2.  Semantics

   Each cookie-pair represents a cookie stored by the user agent.  The
   cookie-name and the cookie-value are returned verbatim from the
   corresponding parts of the Set-Cookie header.

   Notice that the cookie attributes are not returned.  In particular,
   the server cannot determine from the Cookie header alone when a
   cookie will expire, for which domains the cookie is valid, for which
   paths the cookie is valid, or whether the cookie was set with the
   Secure or HttpOnly attributes.

   The semantics of individual cookies in the Cookie header is not
   defined by this document.  Servers are expected to imbue these
   cookies with server-specific semantics.




Barth                   Expires September 8, 2010              [Page 12]


Internet-Draft       HTTP State Management Mechanism          March 2010


   Although cookies are serialized linearly in the Cookie header,
   servers SHOULD NOT rely upon the serialization order.  In particular,
   if the Cookie header contains two cookies with the same name, servers
   SHOULD NOT rely upon the order in which these cookies appear in the
   header.














































Barth                   Expires September 8, 2010              [Page 13]


Internet-Draft       HTTP State Management Mechanism          March 2010


5.  The Cookie Protocol

   For historical reasons, the full cookie protocol contains a number of
   exotic quirks.  This section is intended to specify the cookie
   protocol in enough detail to enable a user agent that implements the
   protocol precisely as specified to interoperate with existing
   servers.

   Conformance requirements phrased as algorithms or specific steps may
   be implemented in any manner, so long as the end result is
   equivalent.  (In particular, the algorithms defined in this
   specification are intended to be easy to follow, and not intended to
   be performant.)

5.1.  Algorithms

   This section defines a number of algorithms used by the cookie
   protocol.

5.1.1.  Dates

   The user agent MUST use the following algorithm to *parse a cookie-
   date*:

   1.  Using the grammar below, divide the cookie-date into date-tokens.


   cookie-date     = *delimiter date-token-list *delimiter
   date-token-list = date-token *( 1*delimiter date-token )
   delimiter       = %x09 / %x20 / %x21 / %x22 / %x23 / %x24 /
                     %x25 / %x26 / %x27 / %x28 / %x29 / %x2A /
                     %x2B / %x2C / %x2D / %x2E / %x2F / %x3B /
                     %x3C / %x3D / %x3E / %x3F / %x40 / %x5B /
                     %x5C / %x5D / %x5E / %x5F / %x60 / %x7B /
                     %x7C / %x7D / %x7E
   date-token      = day-of-month / month / year / time / mystery
   day-of-month    = 2DIGIT / DIGIT
   month           = "jan" [ mystery ] / "feb" [ mystery ] /
                     "mar" [ mystery ] / "apr" [ mystery ] /
                     "may" [ mystery ] / "jun" [ mystery ] /
                     "jul" [ mystery ] / "aug" [ mystery ] /
                     "sep" [ mystery ] / "oct" [ mystery ] /
                     "nov" [ mystery ] / "dec" [ mystery ]
   year            = 5DIGIT / 4DIGIT / 3DIGIT / 2DIGIT / DIGIT
   time            = 2DIGIT ":" 2DIGIT ":" 2DIGIT
   mystery         = <anything except a delimiter>





Barth                   Expires September 8, 2010              [Page 14]


Internet-Draft       HTTP State Management Mechanism          March 2010


   2.  Process each date-token sequentially in the order the date-tokens
       appear in the cookie-date:

       1.  If the found-day-of-month flag is not set and the date-token
           matches the day-of-month production, set the found-day-of-
           month flag and set the day-of-month-value to the number
           denoted by the date-token.  Skip the remaining sub-steps and
           continue to the next date-token.

       2.  If the found-month flag is not set and the date-token matches
           the month production, set the found-month flag and set the
           month-value to the month denoted by the date-token.  Skip the
           remaining sub-steps and continue to the next date-token.

       3.  If the found-year flag is not set and the date-token matches
           the year production, set the found-year flag and set the
           year-value to the number denoted by the date-token.  Skip the
           remaining sub-steps and continue to the next date-token.

       4.  If the found-time flag is not set and the token matches the
           time production, set the found-time flag and set the hour-
           value, minute-value, and second-value to the numbers denoted
           by the digits in the date-token, respectively.  Skip the
           remaining sub-steps and continue to the next date-token.

   3.  Abort these steps and *fail to parse* if

       *  at least one of the found-day-of-month, found-month, found-
          year, or found-time flags is not set,

       *  the day-of-month-value is less than 1 or greater than 31,

       *  the year-value is less than 1601 or greater than 30827,

       *  the hour-value is greater than 23,

       *  the minute-value is greater than 59, or

       *  the second-value is greater than 59.

   4.  If the year-value is greater than 68 and less than 100, increment
       the year-value by 1900.

   5.  If the year-value is greater than or equal to 0 and less than 69,
       increment the year-value by 2000.

   6.  Let the parsed-cookie-date be the date whose day-of-month, month,
       year, hour, minute, and second (in GMT) are the day-of-month-



Barth                   Expires September 8, 2010              [Page 15]


Internet-Draft       HTTP State Management Mechanism          March 2010


       value, the month-value, the year-value, the hour-value, the
       minute-value, and the second-value, respectively.

   7.  Return the parsed-cookie-date as the result of this algorithm.

5.1.2.  Domains

   A *canonicalized* host-name is the host-name converted to lower case.

   A request-host *domain-matches* a cookie-domain if at least one of
   the following conditions hold:

   o  The cookie-domain and the canonicalized request-host are
      identical.

   o  All of the following conditions hold:

      *  The cookie-domain is a suffix of the canonicalized request-
         host.

      *  The last character of the canonicalized request-host that is
         not included in the cookie-domain is a U+002E (".") character.

      *  The request-host is a host name (i.e., not an IP address).

5.1.3.  Paths

   The user agent MUST use the following algorithm to compute the
   *default-path* of a cookie:

   1.  Let uri-path be the path portion of the Request-URI.

   2.  If the first character of the uri-path is not a U+002F ("/")
       character, output U+002F ("/") and skip the remaining steps.

   3.  If the uri-path contains only a single U+002F ("/") character,
       output U+002F ("/") and skip the remaining steps.

   4.  Output the characters of the uri-path from the first character up
       to, but not including, the right-most U+002F ("/").

   A request-path *path-matches* a cookie-path if at least one of the
   following conditions hold:

   o  The cookie-path and the request-path are identical.

   o  The cookie-path is a prefix of the request-path and the last
      character of the cookie-path is U+002F ("/").



Barth                   Expires September 8, 2010              [Page 16]


Internet-Draft       HTTP State Management Mechanism          March 2010


   o  The cookie-path is a prefix of the request-path and the first
      character of the request-path that is not included in the cookie-
      path is a U+002F ("/") character.

5.2.  The Set-Cookie Header

   When a user agent receives a Set-Cookie header in an HTTP response,
   the user agent *receives a set-cookie-string* consisting of the value
   of the header.

   A user agent MUST use the following algorithm to parse set-cookie-
   strings:

   1.  If the set-cookie-string is empty or consists entirely of WSP
       characters, the user agent MAY ignore the set-cookie-string
       entirely.

   2.  If the set-cookie-string contains a U+003B (";") character:

          The name-value-pair string consists of the characters up to,
          but not including, the first U+003B (";"), and the unparsed-
          attributes consist of the remainder of the set-cookie-string
          (including the U+003B (";") in question).

       Otherwise:

          The name-value-pair string consists of all the characters
          contained in the set-cookie-string, and the unparsed-
          attributes is the empty string.

   3.  If the name-value-pair string contains a U+003D ("=") character:

          The (possibly empty) name string consists of the characters up
          to, but not including, the first U+003D ("=") character, and
          the (possibly empty) value string consists of the characters
          after the first U+003D ("=") character.

       Otherwise:

          The name string is empty, and the value string consists of the
          entire name-value-pair string.

   4.  Remove any leading or trailing WSP characters from the name
       string and the value string.

   5.  The cookie-name is the name string, and the cookie-value is the
       value string.




Barth                   Expires September 8, 2010              [Page 17]


Internet-Draft       HTTP State Management Mechanism          March 2010


   The user agent MUST use the following algorithm to parse the
   unparsed-attributes:

   1.  If the unparsed-attributes string is empty, skip the rest of
       these steps.

   2.  Consume the first character of the unparsed-attributes (which
       will be a U+003B (";") character).

   3.  If the remaining unparsed-attributes contains a U+003B (";")
       character:

          Consume the characters of the unparsed-attributes up to, but
          not including, the first U+003B (";") character.

       Otherwise:

          Consume the remainder of the unparsed-attributes.

       Let the cookie-av string be the characters consumed in this step.

   4.  If the cookie-av string contains a U+003D ("=") character:

          The (possibly empty) attribute-name string consists of the
          characters up to, but not including, the first U+003D ("=")
          character, and the (possibly empty) attribute-value string
          consists of the characters after the first U+003D ("=")
          character.

       Otherwise:

          The attribute-name string consists of the entire cookie-av
          string, and the attribute-value string is empty.  (Note that
          this step differs from the analogous step when parsing the
          name-value-pair string.)

   5.  Remove any leading or trailing WSP characters from the attribute-
       name string and the attribute-value string.

   6.  Process the attribute-name and attribute-value according to the
       requirements in the following subsections.

   7.  Return to Step 1.

   When the user agent finishes parsing the set-cookie-string, the user
   agent *receives a cookie* from the Request-URI with name cookie-name,
   value cookie-value, and attributes cookie-attribute-list.




Barth                   Expires September 8, 2010              [Page 18]


Internet-Draft       HTTP State Management Mechanism          March 2010


5.2.1.  The Max-Age Attribute

   If the attribute-name case-insensitively matches the string "Max-
   Age", the user agent MUST process the cookie-av as follows.

   If the first character of the attribute-value is not a DIGIT or a "-"
   character, ignore the cookie-av.

   If the remainder of attribute-value contains a non-DIGIT character,
   ignore the cookie-av.

   Let delta-seconds be the attribute-value converted to an integer.

   If delta-seconds is less than or equal to zero (0), let expiry-time
   be the current date and time.  Otherwise, let the expiry-time be the
   current date and time plus delta-seconds seconds.

   Append an attribute to the cookie-attribute-list with an attribute-
   name of Max-Age and an attribute-value of expiry-time.

5.2.2.  The Expires Attribute

   If the attribute-name case-insensitively matches the string
   "Expires", the user agent MUST process the cookie-av as follows.

   Let the parsed-cookie-date be the result of parsing the attribute-
   value as cookie-date.

   If the attribute-value failed to parse as a cookie date, ignore the
   cookie-av.

   If the user agent received the set-cookie-string from an HTTP
   response that contains a Date header field and the contents of the
   last Date header field successfully parse as a cookie-date:

      Let server-date be the date obtained by parsing the contents of
      the last Date header field as a cookie-date.

      Let delta-seconds be the number of seconds between the server-date
      and the parsed-cookie-date (i.e., parsed-cookie-date - server-
      date).

      Let the expiry-time be the current date and time plus delta-
      seconds seconds.

   Otherwise:





Barth                   Expires September 8, 2010              [Page 19]


Internet-Draft       HTTP State Management Mechanism          March 2010


      Let the expiry-time be the parsed-cookie-date.

   If the expiry-time is later than the last date the user agent can
   represent, the user agent MAY replace the expiry-time with the last
   representable date.

   If the expiry-time is earlier than the first date the user agent can
   represent, the user agent MAY replace the expiry-time with the first
   representable date.

   Append an attribute to the cookie-attribute-list with an attribute-
   name of Expires and an attribute-value of expiry-time.

5.2.3.  The Domain Attribute

   If the attribute-name case-insensitively matches the string "Domain",
   the user agent MUST process the cookie-av as follows.

   If the attribute-value is empty, the behavior is undefined.  However,
   user agent SHOULD ignore the cookie-av entirely.

   If the first character of the attribute-value string is U+002E ("."):

      Let cookie-domain be the attribute-value without the leading
      U+002E (".") character.

   Otherwise:

      Let cookie-domain be the entire attribute-value.

   Convert the cookie-domain to lower case.

   Append an attribute to the cookie-attribute-list with an attribute-
   name of Domain and an attribute-value of cookie-domain.

5.2.4.  The Path Attribute

   If the attribute-name case-insensitively matches the string "Path",
   the user agent MUST process the cookie-av as follows.

   If the attribute-value is empty or if the first character of the
   attribute-value is not U+002F ("/"):

      Let cookie-path be the default-path.

   Otherwise:





Barth                   Expires September 8, 2010              [Page 20]


Internet-Draft       HTTP State Management Mechanism          March 2010


      Let cookie-path be the attribute-value.

   Append an attribute to the cookie-attribute-list with an attribute-
   name of Path and an attribute-value of cookie-path.

5.2.5.  The Secure Attribute

   If the attribute-name case-insensitively matches the string "Secure",
   the user agent MUST append an attribute to the cookie-attribute-list
   with an attribute-name of Secure and an empty attribute-value.

5.2.6.  The HttpOnly Attribute

   If the attribute-name case-insensitively matches the string
   "HttpOnly", the user agent MUST append an attribute to the cookie-
   attribute-list with an attribute-name of HttpOnly and an empty
   attribute-value.

5.3.  Storage Model

   When the user agent receives a cookie, the user agent SHOULD record
   the cookie in its cookie store as follows.

   A user agent MAY ignore a received cookie in its entirety if the user
   agent is configured to block receiving cookies.  For example, the
   user agent might wish to block receiving cookies from "third-party"
   responses.

   The user agent stores the following fields about each cookie: name,
   value, expiry-time, domain, path, creation-time, last-access-time,
   persistent-flag, host-only-flag, secure-only-flag, and http-only-
   flag.

   When the user agent receives a cookie from a Request-URI with name
   cookie-name, value cookie-value, and attributes cookie-attribute-
   list, the user agent MUST process the cookie as follows:

   1.   Create a new cookie with name cookie-name, value cookie-value.
        Set the creation-time and the last-access-time to the current
        date and time.

   2.   If the cookie-attribute-list contains an attribute with an
        attribute-name of "Max-Age":

           Set the cookie's persistent-flag to true.

           Set the cookie's expiry-time to attribute-value of the last
           attribute in the cookie-attribute-list with an attribute-name



Barth                   Expires September 8, 2010              [Page 21]


Internet-Draft       HTTP State Management Mechanism          March 2010


           of "Max-Age".

        Otherwise, if the cookie-attribute-list contains an attribute
        with an attribute-name of "Expires" (and does not contain an
        attribute with an attribute-name of "Max-Age"):

           Set the cookie's persistent-flag to true.

           Set the cookie's expiry-time to attribute-value of the last
           attribute in the cookie-attribute-list with an attribute-name
           of "Expires".

        Otherwise:

           Set the cookie's persistent-flag to false.

           Set the cookie's expiry-time to the latest representable
           date.

   3.   If the cookie-attribute-list contains an attribute with an
        attribute-name of "Domain":

           Let the domain-attribute be the attribute-value of the last
           attribute in the cookie-attribute-list with an attribute-name
           of "Domain".

        Otherwise:

           Let the domain-attribute be the empty string.

   4.   If the user agent is configured to use a "public suffix" list
        and the domain-attribute is a public suffix:

           If the domain-attribute is identical to the canonicalized
           Request-URI's host:

              Let the domain-attribute be the empty string.

           Otherwise:

              Ignore the cookie entirely and abort these steps

           NOTE: A "public suffix" is a domain that is controlled by a
           public registry, such as "com", "co.uk", and "pvt.k12.wy.us".
           This step is essential for preventing attacker.com from
           disrupting the integrity of example.com by setting a cookie
           with a Domain attribute of "com".  Unfortunately, the set of
           public suffixes (also known as "registry controlled domains")



Barth                   Expires September 8, 2010              [Page 22]


Internet-Draft       HTTP State Management Mechanism          March 2010


           changes over time.  If feasible, user agents SHOULD use an
           up-to-date public suffix list, such as the one maintained by
           the Mozilla project at http://publicsuffix.org/.

   5.   If the domain-attribute is non-empty:

           If the Request-URI's host does not domain-match the domain-
           attribute, ignore the cookie entirely and abort these steps.

           Set the cookie's host-only-flag to false.

           Set the cookie's domain to the domain-attribute.

        Otherwise:

           Set the cookie's host-only-flag to true.

           Set the cookie's domain to the host of the Request-URI.

   6.   If the cookie-attribute-list contains an attribute with an
        attribute-name of "Path", set the cookie's path to attribute-
        value of the last attribute in the cookie-attribute-list with an
        attribute-name of "Path".  Otherwise, set cookie's path to the
        default-path of the Request-URI.

   7.   If the cookie-attribute-list contains an attribute with an
        attribute-name of "Secure", set the cookie's secure-only-flag to
        true.  Otherwise, set cookie's secure-only-flag to false.

   8.   If the cookie-attribute-list contains an attribute with an
        attribute-name of "HttpOnly", set the cookie's http-only-flag to
        true.  Otherwise, set cookie's http-only-flag to false.

   9.   If the cookie's name and value are both empty, abort these steps
        and ignore the cookie entirely.

   10.  If the cookie's expiry-time is not in the future, abort these
        steps and ignore the cookie entirely.

   11.  If the cookie was received from a non-HTTP context and the
        cookie's http-only-flag is set, abort these steps and ignore the
        cookie entirely.

   12.  If the cookie store contains a cookie with the same name,
        domain, and path as the newly created cookie:

        1.  Let old-cookie be the existing cookie with the same name,
            domain, and path as the newly created cookie.  (Notice that



Barth                   Expires September 8, 2010              [Page 23]


Internet-Draft       HTTP State Management Mechanism          March 2010


            this algorithm maintains the invariant that there is at most
            one such cookie.)

        2.  If the newly created cookie was received from an non-HTTP
            context and the old-cookie's host-only-flag is set, abort
            these steps and ignore the newly created cookie entirely.

        3.  Update the creation-time of the newly created cookie to
            match the creation-time of the old-cookie.

        4.  Remove the old-cookie from the cookie store.

   13.  Insert the newly created cookie into the cookie store.

   The user agent MUST evict a cookie from the cookie store if, at any
   time, a cookie exists in the cookie store with an expiry date in the
   past.

   The user agent MAY evict a cookie from the cookie store if the number
   of cookies sharing a domain field exceeds some predetermined upper
   bound (such as 50 cookies).

   The user agent MAY evict a cookie from the cookie store if the cookie
   store exceeds some predetermined upper bound (such as 3000 cookies).

   When the user agent evicts a cookie from the cookie store, the user
   agent MUST evict cookies in the following priority order:

   1.  Cookies with an expiry date in the past.

   2.  Cookies that share a domain field with more than a predetermined
       number of other cookies.

   3.  All cookies.

   If two cookies have the same removal priority, the user agent MUST
   evict the cookie with the least recent last-access date first.

   When "the current session is over" (as defined by the user agent),
   the user agent MUST remove from the cookie store all cookies with the
   persistent-flag set to false.

5.4.  The Cookie Header

   When the user agent generates an HTTP request, the user agent SHOULD
   attach exactly one HTTP header named Cookie if the cookie-string
   (defined below) for the Request-URI is non-empty.




Barth                   Expires September 8, 2010              [Page 24]


Internet-Draft       HTTP State Management Mechanism          March 2010


   A user agent MAY elide the Cookie header in its entirety if the user
   agent is configured to block sending cookies.  For example, the user
   agent might wish to block sending cookies during "third-party"
   requests.

   The user agent MUST use the following algorithm to compute the
   cookie-string from a cookie store and a Request-URI:

   1.  Let cookie-list be the set of cookies from the cookie store that
       meet all of the following requirements:

       *  Let request-host be the Request-URI's host.  Either:

             The cookie's host-only-flag is true and the canonicalized
             request-host is identical to the cookie's domain.

          Or:

             The cookie's host-only-flag is false and the request-host
             domain-matches cookie's domain.

       *  The Request-URI's path patch-matches cookie's path.

       *  If the cookie's secure-only-flag is true, then the Request-
          URI's scheme must denote a "secure" protocol (as defined by
          the user agent).

             NOTE: The notion of a "secure" protocol is not defined by
             this document.  Typically, user agents consider a protocol
             secure if the protocol makes use of transport-layer
             security, such as TLS.  For example, most user agents
             consider "https" to be a scheme that denotes a secure
             protocol.

       *  If the cookie's http-only-flag is true, then exclude the
          cookie unless the cookie-string is being generated for an
          "HTTP" API (as defined by the user agent).

   2.  Sort the cookie-list in the following order:

       *  Cookies with longer paths are listed before cookies with
          shorter paths.

       *  Among cookies that have equal length path fields, cookies with
          earlier creation-times are listed before cookies with later
          creation-times.





Barth                   Expires September 8, 2010              [Page 25]


Internet-Draft       HTTP State Management Mechanism          March 2010


   3.  Update the last-access-time of each cookie in the cookie-list to
       the current date and time.

   4.  Serialize the cookie-list into a cookie-string by processing each
       cookie in the cookie-list in order:

       1.  If the cookie's name is non-empty, output the cookie's name
           followed by the U+003D ("=") character.

       2.  Output the cookie's value.

       3.  If there is an unprocessed cookie in the cookie-list, output
           the characters U+003B and U+0020 ("; ").

   Note: Despite its name, the cookie-string is actually a sequence of
   octets, not a sequence of characters.  To convert the cookie-string
   into a sequence of characters (e.g., for presentation to the user),
   the user agent SHOULD use the UTF-8 character encoding [RFC3629].

































Barth                   Expires September 8, 2010              [Page 26]


Internet-Draft       HTTP State Management Mechanism          March 2010


6.  Implementation Considerations

6.1.  Limits

   Practical user agent implementations have limits on the number and
   size of cookies that they can store.  General-use user agents SHOULD
   provide each of the following minimum capabilities:

   o  At least 4096 bytes per cookie (as measured by the sum of the
      length of the cookie's name, value, and attributes).

   o  At least 50 cookies per domain.

   o  At least 3000 cookies total.

   Servers SHOULD use as few and as small cookies as possible to avoid
   reaching these implementation limits and to avoid network latency due
   to the Cookie header being included in every request.

   Servers should gracefully degrade if the user agent fails to return
   one or more cookies in the Cookie header because the user agent might
   evict any cookie at any time on orders from the user.

6.2.  Application Programmer Interfaces

   One reason the cookie protocol uses such an esoteric syntax is
   because many platforms (both in servers and user agents) provide
   string-based application programmer interfaces (APIs), requiring
   application-layer programmers to generate and parse the syntax used
   by the cookie protocol.

   Instead of providing string-based APIs to the cookie protocols,
   implementations would be well-served by providing more semantic APIs.
   It is beyond the scope of this document to recommend specific API
   designs, but there are clear benefits to accepting a abstract "Date"
   object instead of a serialized date string.















Barth                   Expires September 8, 2010              [Page 27]


Internet-Draft       HTTP State Management Mechanism          March 2010


7.  Security Considerations

7.1.  Overview

   The cookie protocol has a number of security and privacy pitfalls.

   In particular, cookies encourage developers to rely on ambient
   authority for authentication, often creating vulnerabilities such as
   cross-site request forgery.  When storing session identifiers in
   cookies, developers often create session fixation vulnerabilities.

   Transport-layer encryption, such as that employed in HTTPS, is
   insufficient to prevent a network attacker from obtaining or altering
   a victim's cookies because the cookie protocol itself has various
   vulnerabilities (see "Weak Confidentiality" and "Weak Integrity",
   below).  In addition, by default, the cookie protocol does not
   provide confidentiality or integrity from network attackers, even
   when used in conjunction with HTTPS.

7.2.  Ambient Authority

   A server that uses cookies to authenticate users can suffer security
   vulnerabilities because some user agents let remote parties issue
   HTTP requests from the user agent (e.g., via HTTP redirects and HTML
   forms).  When issuing those requests, user agent attaches cookies
   even if the entity does not know the contents of the cookies,
   possibly letting the remote entity exercise authority at an unwary
   server.

   Although this security concern goes by a number of names (e.g.,
   cross-site request forgery, confused deputy), the issue stems from
   cookies being a form of ambient authority.  Cookies encourage server
   operators to separate designation (in the form of URLs) from
   authorization (in the form of cookies).  Consequently, the user agent
   might supply the authorization for a resource designated by the
   attacker, possibly causing the server or its clients to undertake
   actions designated by the attacker as though they were authorized by
   the user.

   Instead of using cookies for authorization, server operators might
   wish to consider entangling designation and authorization by treating
   URLs as capabilities.  Instead of storing secrets in cookies, this
   approach stores secrets in URLs, requiring the remote entity to
   supply the secret itself.  Although this approach is not a panacea,
   judicious use of these principles can lead to more robust security.






Barth                   Expires September 8, 2010              [Page 28]


Internet-Draft       HTTP State Management Mechanism          March 2010


7.3.  Clear Text

   Unless sent over a secure channel (such as TLS), the information in
   the Set-Cookie and Cookie headers is transmitted in the clear.

   1.  All sensitive information conveyed in these headers is exposed to
       an eavesdropper.

   2.  A malicious intermediary could alter the headers as they travel
       in either direction, with unpredictable results.

   3.  A malicious client could alter the Cookie header before
       transmission, with unpredictable results.

   Servers SHOULD encrypt and sign the contents of cookies when
   transmitting them to the user agent (even when sending the cookies
   over a secure channel).  However, encrypting and signing cookie
   contents does not prevent an attacker from transplanting a cookie
   from one user agent to another or from replaying the cookie at a
   later time.

   In addition to encrypting and signing the contents of every cookie,
   servers that require a higher level of security SHOULD use the cookie
   protocol only over a secure channel.  When using the cookie protocol
   over a secure channel, servers SHOULD set the Secure attribute in
   every cookie.  If a server does not set the Secure attribute, the
   protection provided by the secure channel will be largely moot.

7.4.  Session Identifiers

   Instead of storing session information directly in a cookie (where it
   might be exposed to or replayed by an attacker), servers commonly
   store a nonce (or "session identifier") in a cookie.  When the server
   receives an HTTP request with a nonce, the server can look up state
   information associated with the cookie using the nonce as a key.

   Using session identifier cookies limits the damage an attacker can
   cause if the attacker learns the contents of a cookie because the
   nonce is useful only for interacting with the server (unlike non-
   nonce cookie content, which might itself be sensitive).  Furthermore,
   using a single nonce prevents an attacker from "splicing" together
   cookie content from two interactions with the server, which could
   cause the server to behave unexpectedly.

   Using session identifiers is not without risk.  For example, the
   server SHOULD take care to avoid "session fixation" vulnerabilities.
   A session fixation attack proceeds in three steps.  First, the
   attacker transplants a session identifier from his or her user agent



Barth                   Expires September 8, 2010              [Page 29]


Internet-Draft       HTTP State Management Mechanism          March 2010


   to the victim's user agent.  Second, the victim uses that session
   identifier to interact with the server, possibly imbuing the session
   identifier with the user's credentials or confidential information.
   Third, the attacker uses the session identifier to interact with
   server directly, possibly obtaining the user's authority or
   confidential information.

7.5.  Weak Confidentiality

   Cookies do not provide isolation by port.  If a cookie is readable by
   a service running on one port, the cookie is also readable by a
   service running on another port of the same server.  If a cookie is
   writable by a service on one port, the cookie is also writable by a
   service running on another port of the same server.  For this reason,
   servers SHOULD NOT both run mutually distrusting services on
   different ports of the same host and use cookies to store security-
   sensitive information.

   Cookies do not provide isolation by scheme.  Although most commonly
   used with the http and https schemes, the cookies for a given host
   might also available to other schemes, such as ftp and gopher.
   Although this lack of isolation by scheme is most apparent in via
   non-HTTP APIs that permit access to cookies (e.g., HTML's
   document.cookie API), the lack of isolation by scheme is actually
   present in the cookie protocol itself (e.g., consider retrieving a
   URI with the gopher scheme via HTTP).

   Cookies do not always provide isolation by path.  Although the
   network-level protocol does not send cookie stored for one path to
   another, some user agents expose cookies via non-HTTP APIs, such as
   HTML's document.cookie API.  Because some of these user agents (e.g.,
   web browsers) do not isolate resources received from different paths,
   a resource retrieved from one path might be able to access cookies
   stored for another path.

7.6.  Weak Integrity

   Cookies do not provide integrity guarantees for sibling domains (and
   their subdomains).  For example, consider foo.example.com and
   bar.example.com.  The foo.example.com server can set a cookie with a
   Domain attribute of ".example.com" (possibly overwriting an existing
   ".example.com" cookie set by bar.example.com), and the user agent
   will include that cookie in HTTP requests to bar.example.com.  In the
   worst case, bar.example.com will be unable to distinguish this cookie
   from a cookie it set itself.  The foo.example.com server might be
   able to leverage this ability to mount an attack against
   bar.example.com.




Barth                   Expires September 8, 2010              [Page 30]


Internet-Draft       HTTP State Management Mechanism          March 2010


   Even though the cookie protocol supports the Path attribute, the Path
   attribute does not provide any integrity protection because the user
   agent will accept an arbitrary Path attribute in a Set-Cookie header.
   For example, an HTTP response to a request for
   http://example.com/foo/bar can set a cookie with a Path attribute of
   "/qux".  Consequently, servers SHOULD NOT both run mutually
   distrusting services on different paths of the same host and use
   cookies store security sensitive information.

   An active network attacker can also inject cookies into the Cookie
   header sent to https://example.com/ by impersonating a response from
   http://example.com/ and injecting a Set-Cookie header.  The HTTPS
   server at example.com will be unable to distinguish these cookies
   from cookies that it set itself in an HTTPS response.  An active
   network attacker might be able to leverage this ability to mount an
   attack against example.com even if example.com uses HTTPS
   exclusively.

   Servers can partially mitigate these attacks by encrypting and
   signing the contents of their cookies.  However, using cryptography
   does not mitigate the issue completely because an attacker can replay
   a cookie he or she received from the authentic example.com server in
   the user's session, with unpredictable results.

   Finally, an attacker might be able to force the user agent to delete
   cookies by storing large number of cookies.  Once the user agent
   reaches its storage limit, the user agent will be forced to evict
   some cookies.  Servers SHOULD NOT rely upon user agents retaining
   cookies.

7.7.  Reliance on DNS

   The cookie protocol relies upon the Domain Name System (DNS) for
   security.  If the DNS is partially or fully compromised, the cookie
   protocol might fail to provide the security properties required by
   applications.















Barth                   Expires September 8, 2010              [Page 31]


Internet-Draft       HTTP State Management Mechanism          March 2010


8.  References

8.1.  Normative References

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

   [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.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

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

8.2.  Informative References

   [RFC2109]  Kristol, D. and L. Montulli, "HTTP State Management
              Mechanism", RFC 2109, February 1997.



























Barth                   Expires September 8, 2010              [Page 32]


Internet-Draft       HTTP State Management Mechanism          March 2010


Appendix A.  Acknowledgements

   This document borrows heavily from RFC 2109 [RFC2109].  We are
   indebted to David M. Kristol and Lou Montulli for their efforts to
   specify the cookie protocol.  David M. Kristol, in particular,
   provided invaluable advice on navigating the IETF process.  We would
   also like to thank Thomas Broyer, Tyler Close, Bil Corry, corvid, Roy
   T. Fielding, Blake Frantz, Eran Hammer-Lahav, Jeff Hodges, Achim
   Hoffmann, Georg Koppen, Dean McNamee, Mark Miller, Yngve N.
   Pettersen, Julian Reschke, Mark Seaborn, Maciej Stachowiak, Daniel
   Stenberg, David Wagner, Dan Winship, and Dan Witte for their valuable
   feedback on this document.







































Barth                   Expires September 8, 2010              [Page 33]


Internet-Draft       HTTP State Management Mechanism          March 2010


Author's Address

   Adam Barth
   University of California, Berkeley

   Email: abarth@eecs.berkeley.edu
   URI:   http://www.adambarth.com/












































Barth                   Expires September 8, 2010              [Page 34]


Html markup produced by rfcmarkup 1.128b, available from https://tools.ietf.org/tools/rfcmarkup/