Network Working Group                                         P. Hoffman
Internet-Draft                                            VPN Consortium
Intended status: Informational                               A. Melnikov
Expires: January 14, September 8, 2009                                    Isode Ltd.
                                                           July 13, 2008
                                                           March 7, 2009

                     Security Requirements for HTTP

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   Recent IESG practice dictates that IETF protocols must specify
   mandatory-to-implement security mechanisms, so that all conformant
   implementations share a common baseline.  This document examines all
   widely deployed HTTP security technologies, and analyzes the trade-
   offs of each.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Existing HTTP Security Mechanisms  . . . . . . . . . . . . . .  3
     2.1.  Forms And Cookies  . . . . . . . . . . . . . . . . . . . .  3
     2.2.  HTTP Access Authentication . . . . . . . . . . . . . . . .  5
       2.2.1.  Basic Authentication . . . . . . . . . . . . . . . . .  5
       2.2.2.  Digest Authentication  . . . . . . . . . . . . . . . .  5
       2.2.3.  Authentication Using Certificates in TLS . . . . . . .  6
       2.2.4.  Other Access Authentication Schemes  . . . . . . . . .  6
     2.3.  Centrally-Issued Tickets . . . . . . . . . . . . . . . . .  7
     2.4.  Web Services . . . . . . . . . . . . . . . . . . . . . . .  7
     2.5.  Transport Layer Security . . . . . . . . . . . . . . . . .  8
   3.  Revisions To HTTP  . . . . . . . . . . . . . . . . . . . . . .  8
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   5.  Normative References . . . . . . . . . . . . . . . . . . . . .  8
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . .  9
   Appendix B.  Document History  . . . . . . . . . . . . . . . . . . 10
     B.1.  Changes between draft-sayre-http-security-variance-00
           and draft-ietf-httpbis-security-properties-00  . . . . . . 10
     B.2.  Changes between -00 and -01  . . . . . . . . . . . . . . . 10
     B.3.  Changes between -01 and -02  . . . . . . . . . . . . . . . 11
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
   Intellectual Property and Copyright Statements . . . . . . . . . . 12

1.  Introduction

   Recent IESG practice dictates that IETF protocols are required to
   specify mandatory to implement security mechanisms.  "The IETF
   Standards Process" [RFC2026] does not require that protocols specify
   mandatory security mechanisms.  "Strong Security Requirements for
   IETF Standard Protocols" [RFC3365] requires that all IETF protocols
   provide a mechanism for implementers to provide strong security.  RFC
   3365 does not define the term "strong security".

   "Security Mechanisms for the Internet" [RFC3631] is not an IETF
   procedural RFC, but it is perhaps most relevant.  Section 2.2 states:

     We have evolved in the IETF the notion of "mandatory to implement"
     mechanisms.  This philosophy evolves from our primary desire to
     ensure interoperability between different implementations of a
     protocol.  If a protocol offers many options for how to perform a
     particular task, but fails to provide for at least one that all
     must implement, it may be possible that multiple, non-interoperable
     implementations may result.  This is the consequence of the
     selection of non-overlapping mechanisms being deployed in the
     different implementations.

   This document examines the effects of applying security constraints
   to Web applications, documents the properties that result from each
   method, and will make Best Current Practice recommendations for HTTP
   security in a later document version.  At the moment, it is mostly a
   laundry list of security technologies and tradeoffs.

2.  Existing HTTP Security Mechanisms

   For HTTP, the IETF generally defines "security mechanisms" as some
   combination of access authentication and/or a secure transport.

   [[ There is a suggestion that this section be split into "browser-
   like" and "automation-like" subsections. ]]

   [[ NTLM (shudder) was brought up in the WG a few times in the
   discussion of the -00 draft.  Should we add a section on it? ]]

2.1.  Forms And Cookies

   Almost all HTTP authentication that involves a human using a web
   browser is accomplished through HTML forms, with session identifiers
   stored in cookies.  For cookies, most implementations rely on the
   "Netscape specification", which is described loosely in section 10 of
   "HTTP State Management Mechanism" [RFC2109].  The protocol in RFC
   2109 is relatively widely implemented, but most clients don't
   advertise support for it.  RFC 2109 was later updated [RFC2965], but
   the newer version is not widely implemented.

   Forms and cookies have many properties that make them an excellent
   solution for some implementers.  However, many of those properties
   introduce serious security trade-offs.

   HTML forms provide a large degree of control over presentation, which
   is an imperative for many websites.  However, this increases user
   reliance on the appearance of the interface.  Many users do not
   understand the construction of URIs [RFC3986], or their presentation
   in common clients [PhishingHOWTO].  As a result, forms are extremely
   vulnerable to spoofing.

   HTML forms provide acceptable internationalization if used carefully,
   at the cost of being transmitted as normal HTTP content in all cases
   (credentials are not differentiated in the protocol).

   Many Web browsers have an auto-complete feature that stores a user's
   information and pre-populates fields in forms.  This is considered to
   be a convenience mechanism, and convenience mechanisms often have
   negative security properties.  The security concerns with auto-
   completion are particularly poignant for web browsers that reside on
   computers with multiple users.  HTML forms provide a facility for
   sites to indicate that a field, such as a password, should never be
   pre-populated.  However, it is clear that some form creators do not
   use this facility when they should.

   The cookies that result from a successful form submission make it
   unnecessary to validate credentials with each HTTP request; this
   makes cookies an excellent property for scalability.  Cookies are
   susceptible to a large variety of XSS (cross-site scripting) attacks,
   and measures to prevent such attacks will never be as stringent as
   necessary for authentication credentials because cookies are used for
   many purposes.  Cookies are also susceptible to a wide variety of
   attacks from malicious intermediaries and observers.  The possible
   attacks depend on the contents of the cookie data.  There is no
   standard format for most of the data.

   HTML forms and cookies provide flexible ways of ending a session from
   the client.

   HTML forms require an HTML rendering engine for which many protocols
   have no use.

2.2.  HTTP Access Authentication

   HTTP 1.1 provides a simple authentication framework, "HTTP
   Authentication: Basic and Digest Access Authentication" [RFC2617],
   which defines two optional mechanisms.  Both of these mechanisms are
   extremely rarely used in comparison to forms and cookies, but some
   degree of support for one or both is available in many
   implementations.  Neither scheme provides presentation control,
   logout capabilities, or interoperable internationalization.

2.2.1.  Basic Authentication

   Basic Authentication (normally called just "Basic") transmits
   usernames and passwords in the clear.  It is very easy to implement,
   but not at all secure unless used over a secure transport.

   Basic has very poor scalability properties because credentials must
   be revalidated with every request, and because secure transports
   negate many of HTTP's caching mechanisms.  Some implementations use
   cookies in combination with Basic credentials, but there is no
   standard method of doing so.

   Since Basic credentials are clear text, they are reusable by any
   party.  This makes them compatible with any authentication database,
   at the cost of making the user vulnerable to mismanaged or malicious
   servers, even over a secure channel.

   Basic is not interoperable when used with credentials that contain
   characters outside of the ISO 8859-1 repertoire.

2.2.2.  Digest Authentication

   In Digest Authentication, the client transmits the results of hashing
   user credentials with properties of the request and values from the
   server challenge.  Digest is susceptible to man-in-the-middle attacks
   when not used over a secure transport.

   Digest has some properties that are preferable to Basic and Cookies.
   Credentials are not immediately reusable by parties that observe or
   receive them, and session data can be transmitted along side
   credentials with each request, allowing servers to validate
   credentials only when absolutely necessary.  Authentication data
   session keys are distinct from other protocol traffic.

   Digest includes many modes of operation, but only the simplest modes
   enjoy any degree of interoperability.  For example, most
   implementations do not implement the mode that provides full message
   integrity.  Perhaps one reason is that implementation experience has
   shown that in some cases, especially those involving large requests
   or responses such as streams, the message integrity mode is
   impractical because it requires servers to analyze the full request
   before determining whether the client knows the shared secret or
   whether message-body integrity has been violated and hence whether
   the request can be processed.

   Digest is extremely susceptible to offline dictionary attacks, making
   it practical for attackers to perform a namespace walk consisting of
   a few million passwords for most users.

   Many of the most widely-deployed HTTP/1.1 clients are not compliant
   when GET requests include a query string [Apache_Digest].

   Digest either requires that authentication databases be expressly
   designed to accommodate it, or requires access to cleartext
   passwords.  As a result, many authentication databases that chose to
   do the former are incompatible, including the most common method of
   storing passwords for use with Forms and Cookies.

   Many Digest capabilities included to prevent replay attacks expose
   the server to Denial of Service attacks.

   Digest is not interoperable when used with credentials that contain
   characters outside of the ISO 8859-1 repertoire.

2.2.3.  Authentication Using Certificates in TLS

   Running HTTP over TLS provides authentication of the HTTP server to
   the client.  HTTP over TLS can also provides authentication of the
   client to the server using certificates.  Although forms are a much
   more common way to authenticate users to HTTP servers, TLS client
   certificates are widely used in some environments.  The public key
   infrastructure (PKI) used to validate certificates in TLS can be
   rooted in public trust anchors or can be based on local trust

2.2.4.  Other Access Authentication Schemes

   There are many niche schemes that make use of the HTTP Authentication
   framework, but very few are well documented.  Some are bound to
   transport layer connections.  Negotiate (GSS-API) Authentication

   Microsoft has designed an HTTP authentication mechanism that utilizes
   SPNEGO [RFC4178] GSSAPI [RFC4559].  In Microsoft's implementation,
   SPNEGO allows selection between Kerberos and NTLM (Microsoft NT Lan
   Manager protocols).

   In Kerberos, clients and servers rely on a trusted third-party
   authentication service which maintains its own authentication
   database.  Kerberos is typically used with shared secret key
   cryptography, but extensions for use of other authentication
   mechnanisms such as PKIX certificates and two-factor tokens are also
   common.  Kerberos was designed to work under the assumption that
   packets traveling along the network can be read, modified, and
   inserted at will.

   Unlike Digest, Negotiate authentication can take multiple round trips
   (client sending authentication data in Authorization, server sending
   authentication data in WWW-Authenticate) to complete.

   Kerberos authentication is generally more secure than Digest.
   However the requirement for having a separate network authentication
   service might be a barrier to deployment.

2.3.  Centrally-Issued Tickets

   Many large Internet services rely on authentication schemes that
   center on clients consulting a single service for a time-limited
   ticket that is validated with undocumented heuristics.  Centralized
   ticket issuing has the advantage that users may employ one set of
   credentials for many services, and clients don't send credentials to
   many servers.  This approach is often no more than a sophisticated
   application of forms and cookies.

   All of the schemes in wide use are proprietary and non-standard, and
   usually are undocumented.  There are many standardization efforts in
   progress, as usual.

2.4.  Web Services

   Many security properties mentioned in this document have been recast
   in XML-based protocols, using HTTP as a substitute for TCP.  Like the
   amalgam of HTTP technologies mentioned above, the XML-based protocols
   are defined by an ever-changing combination of standard and vendor-
   produced specifications, some of which may be obsoleted at any time
   [WS-Pagecount] without any documented change control procedures.
   These protocols usually don't have much in common with the
   Architecture of the World Wide Web. It's not clear why the term "Web"
   is used to group them, but they are obviously out of scope for HTTP-
   based application protocols.

   [[ This section could really use a good definition of "Web Services"
   to differentiate it from REST. ]]

2.5.  Transport Layer Security

   In addition to using TLS for client and/or server authentication, it
   is also very commonly used to protect the confidentiality and
   integrity of the HTTP session.  For instance, both HTTP Basic
   authentication and Cookies are often protected against snooping by

   It should be noted that, in that case, TLS does not protect against a
   breach of the credential store at the server or against a keylogger
   or phishing interface at the client.  TLS does not change the fact
   that Basic Authentication passwords are reusable and does not address
   that weakness.

3.  Revisions To HTTP

   Is is possible that HTTP will be revised in the future.  "HTTP/1.1"
   [RFC2616] and "Use and Interpretation of HTTP Version Numbers"
   [RFC2145] define conformance requirements in relation to version
   numbers.  In HTTP 1.1, all authentication mechanisms are optional,
   and no single transport substrate is specified.  Any HTTP revision
   that adds a mandatory security mechanism or transport substrate will
   have to increment the HTTP version number appropriately.  All widely
   used schemes are non-standard and/or proprietary.

4.  Security Considerations

   This entire document is about security considerations.

5.  Normative References

              Apache Software Foundation, "Apache HTTP Server -
              mod_auth_digest", <

              Gutmann, P., "Phishing Tips and Techniques",
              February 2008,

   [RFC2026]  Bradner, S., "The Internet Standards Process -- Revision
              3", BCP 9, RFC 2026, October 1996.

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

   [RFC2145]  Mogul, J., Fielding, R., Gettys, J., and H. Nielsen, "Use
              and Interpretation of HTTP Version Numbers", RFC 2145,
              May 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.

   [RFC2617]  Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
              Leach, P., Luotonen, A., and L. Stewart, "HTTP
              Authentication: Basic and Digest Access Authentication",
              RFC 2617, June 1999.

   [RFC2965]  Kristol, D. and L. Montulli, "HTTP State Management
              Mechanism", RFC 2965, October 2000.

   [RFC3365]  Schiller, J., "Strong Security Requirements for Internet
              Engineering Task Force Standard Protocols", BCP 61,
              RFC 3365, August 2002.

   [RFC3631]  Bellovin, S., Schiller, J., and C. Kaufman, "Security
              Mechanisms for the Internet", RFC 3631, December 2003.

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

   [RFC4178]  Zhu, L., Leach, P., Jaganathan, K., and W. Ingersoll, "The
              Simple and Protected Generic Security Service Application
              Program Interface (GSS-API) Negotiation Mechanism",
              RFC 4178, October 2005.

   [RFC4559]  Jaganathan, K., Zhu, L., and J. Brezak, "SPNEGO-based
              Kerberos and NTLM HTTP Authentication in Microsoft
              Windows", RFC 4559, June 2006.

              Bray, T., "WS-Pagecount", September 2004, <http://

Appendix A.  Acknowledgements

   Much of the material in this document was written by Rob Sayre, who
   first promoted the topic.  Many others on the HTTPbis Working Group
   have contributed to this document in the discussion.

Appendix B.  Document History

   [This entire section is to be removed when published as an RFC.]

B.1.  Changes between draft-sayre-http-security-variance-00 and

   Changed the authors to Paul Hoffman and Alexey Melnikov, with
   permission of Rob Sayre.

   Made lots of minor editorial changes.

   Removed what was section 2 (Requirements Notation), the reference to
   RFC 2119, and any use of 2119ish all-caps words.

   In 3.2.1 and 3.2.2, changed "Latin-1 range" to "ISO 8859-1
   repertoire" to match the definition of "TEXT" in RFC 2616.

   Added minor text to the Security Considerations section.

   Added URLs to the two non-RFC references.

B.2.  Changes between -00 and -01

   Fixed some editorial nits reported by Iain Calder.

   Added the suggestions about splitting for browsers and automation,
   and about adding NTLM, to be beginning of 2.

   In 2.1, added "that involves a human using a web browser" in the
   first sentence.

   In 2.1, changed "session key" to "session identifier".

   In 2.2.2, changed

   Digest includes many modes of operation, but only the simplest modes
   enjoy any degree of interoperability.  For example, most
   implementations do not implement the mode that provides full message
   integrity.  Additionally, implementation experience has shown that
   the message integrity mode is impractical because it requires servers
   to analyze the full request before determining whether the client
   knows the shared secret.

   Digest includes many modes of operation, but only the simplest
   modes enjoy any degree of interoperability.  For example, most
   implementations do not implement the mode that provides full message
   integrity.  Perhaps one reason is that implementation experience has
   shown that in some cases, especially those involving large requests
   or responses such as streams, the message integrity mode is
   impractical because it requires servers to analyze the full request
   before determining whether the client knows the shared secret or
   whether message-body integrity has been violated and hence whether
   the request can be processed.

   In 2.4, asked for a definition of "Web Services".

   In A, added the WG.

B.3.  Changes between -01 and -02

   In section 2.1, added more to the paragraph on auto-completion of
   HTML forms.

   Added the section on TLS for authentication.

   Filled in section 2.5.

Authors' Addresses

   Paul Hoffman
   VPN Consortium


   Alexey Melnikov
   Isode Ltd.


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