Movatterモバイル変換

Network Working Group                                            K. MooreRequest for Comments: 2964                        University of TennesseeBCP: 44                                                          N. FreedCategory: Best Current Practice                                  Innosoft                                                             October 2000Use of HTTP State ManagementStatus of this Memo   This document specifies an Internet Best Current Practices for the   Internet Community, and requests discussion and suggestions for   improvements.  Distribution of this memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (2000).  All Rights Reserved.IESG Note   The IESG notes that this mechanism makes use of the .local top-level   domain (TLD) internally when handling host names that don't contain   any dots, and that this mechanism might not work in the expected way   should an actual .local TLD ever be registered.Abstract   The mechanisms described in "HTTP State Management Mechanism" (RFC-2965), and its predecessor (RFC-2109), can be used for many different   purposes.  However, some current and potential uses of the protocol   are controversial because they have significant user privacy and   security implications.  This memo identifies specific uses of   Hypertext Transfer Protocol (HTTP) State Management protocol which   are either (a) not recommended by the IETF, or (b) believed to be   harmful, and discouraged.  This memo also details additional privacy   considerations which are not covered by the HTTP State Management   protocol specification.1.  Introduction   The HTTP State Management mechanism is both useful and controversial.   It is useful because numerous applications of HTTP benefit from the   ability to save state between HTTP transactions, without encoding   such state in URLs.  It is controversial because the mechanism has   been used to accomplish things for which it was not designed and is   not well-suited.  Some of these uses have attracted a great deal of   public criticism because they threaten to violate the privacy of webMoore & Freed            Best Current Practice                  [Page 1]

RFC 2964              Use of HTTP State Management          October 2000   users, specifically by leaking potentially sensitive information to   third parties such as the Web sites a user has visited.  There are   also other uses of HTTP State Management which are inappropriate even   though they do not threaten user privacy.   This memo therefore identifies uses of the HTTP State Management   protocol specified inRFC-2965 which are not recommended by the IETF,   or which are believed to be harmful and are therefore discouraged.   This document occasionally uses terms that appear in capital letters.   When the terms "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"   appear capitalized, they are being used to indicate particular   requirements of this specification.  A discussion of the meanings of   the terms "MUST", "SHOULD", and "MAY" appears in [RFC-1123]; the   terms "MUST NOT" and "SHOULD NOT" are logical extensions of this   usage.2.  Uses of HTTP State Management   The purpose of HTTP State Management is to allow an HTTP-based   service to create stateful "sessions" which persist across multiple   HTTP transactions.  A single session may involve transactions with   multiple server hosts.  Multiple client hosts may also be involved in   a single session when the session data for a particular user is   shared between client hosts (e.g., via a networked file system).  In   other words, the "session" retains state between a "user" and a   "service", not between particular hosts.   It's important to realize that similar capabilities may also be   achieved using the "bare" HTTP protocol, and/or dynamically-generated   HTML, without the State Management extensions.  For example, state   information can be transmitted from the service to the user by   embedding a session identifier in one or more URLs which appear in   HTTP redirects, or dynamically generated HTML; and the state   information may be returned from the user to the service when such   URLs appear in a GET or POST request.  HTML forms can also be used to   pass state information from the service to the user and back, without   the user being aware of this happening.   However, the HTTP State Management facility does provide an increase   in functionality over ordinary HTTP and HTML.  In practice, this   additional functionality includes:   (1)   The ability to exchange URLs between users, of resources         accessed during stateful sessions, without leaking the state         information associated with those sessions.  (e.g. "Here's the         URL for the FooCorp web catalog entry for those sandals that         you wanted.")Moore & Freed            Best Current Practice                  [Page 2]

RFC 2964              Use of HTTP State Management          October 2000   (2)   The ability to maintain session state without "cache-busting".         That is, separating the session state from the URL allows a web         cache to maintain only a single copy of the named resource.  If         the state is maintained in session-specific URLs, the cache         would likely have to maintain several identical copies of the         resource.   (3)   The ability to implement sessions with minimal server         configuration and minimal protocol overhead, as compared to         other techniques of maintaining session state.   (4)   The ability to associate the user with session state whenever a         user accesses the service, regardless of whether the user         enters through a particular "home page" or "portal".   (5)   The ability to save session information in stable storage, so         that a "session" can be maintained across client invocations,         system reboots, and client or system crashes.2.1.  Recommended Uses   Use of HTTP State Management is appropriate whenever it is desirable   to maintain state between a user and a service across multiple HTTP   transactions, provided that:   (1)   the user is aware that session state is being maintained and         consents to it,   (2)   the user has the ability to delete the state associated with         such a session at any time,   (3)   the information obtained through the ability to track the         user's usage of the service is not disclosed to other parties         without the user's explicit consent, and   (4)   session information itself cannot contain sensitive information         and cannot be used to obtain sensitive information that is not         otherwise available to an eavesdropper.   This last point is important because cookies are usually sent in the   clear and hence are readily available to eavesdroppers.   An example of such a recommended use would be a "shopping cart",   where the existence of the shopping cart is explicitly made known to   the user, the user can explicitly "empty" his or her shopping cart   (either by requesting that it be emptied or by purchasing thoseMoore & Freed            Best Current Practice                  [Page 3]

RFC 2964              Use of HTTP State Management          October 2000   items) and thus cause the shared state to be discarded, and the   service asserts that it will not disclose the user's shopping or   browsing habits to third parties without the user's consent.   Note that the HTTP State Management protocol effectively allows a   service provider to refuse to provide a service, or provide a reduced   level of service, if the user or a user's client fails to honor a   request to maintain session state.  Absent legal prohibition to the   contrary, the server MAY refuse to provide the service, or provide a   reduced level of service, under these conditions.  As a purely   practical consideration, services designed to utilize HTTP State   Management may be unable to function properly if the client does not   provide it.  Such servers SHOULD gracefully handle such conditions   and explain to the user why the full level of service is not   available.2.2.  Problematic Uses   The following uses of HTTP State Management are deemed inappropriate   and contrary to this specification:2.2.1.  Leakage of Information to Third Parties   HTTP State Management MUST NOT be used to leak information about the   user or the user's browsing habits to other parties besides the user   or service, without the user's explicit consent.  Such usage is   prohibited even if the user's name or other externally-assigned   identifier are not exposed to other parties, because the state   management mechanism itself provides an identifier which can be used   to compile information about the user.   Because such practices encourage users to defeat HTTP State   Management mechanisms, they tend to reduce the effectiveness of HTTP   State Management, and are therefore considered detrimental to the   operation of the web.2.2.2.  Use as an Authentication Mechanism   It is generally inappropriate to use the HTTP State Management   protocol as an authentication mechanism.  HTTP State Management is   not designed with such use in mind, and safeguards for protection of   authentication credentials are lacking in both the protocol   specification and in widely deployed HTTP clients and servers.  Most   HTTP sessions are not encrypted and "cookies" may therefore be   exposed to passive eavesdroppers.  Furthermore, HTTP clients and   servers typically store "cookies" in cleartext with little or no   protection against exposure.  HTTP State Management therefore SHOULDMoore & Freed            Best Current Practice                  [Page 4]

RFC 2964              Use of HTTP State Management          October 2000   NOT be used as an authentication mechanism to protect information   from being exposed to unauthorized parties, even if the HTTP sessions   are encrypted.   The prohibition against using HTTP State Management for   authentication includes both its use to protect information which is   provided by the service, and its use to protect potentially sensitive   information about the user which is entrusted to the service's care.   For example, it would be inappropriate to expose a user's name,   address, telephone number, or billing information to a client that   merely presented a cookie which had been previously associated with   the user.   Similarly, HTTP State Management SHOULD NOT be used to authenticate   user requests if unauthorized requests might have undesirable side-   effects for the user, unless the user is aware of the potential for   such side-effects and explicitly consents to such use.  For example,   a service which allowed a user to order merchandise with a single   "click", based entirely on the user's stored "cookies", could   inconvenience the user by requiring her to dispute charges to her   credit card, and/or return the unwanted merchandise, in the event   that the cookies were exposed to third parties.   Some uses of HTTP State Management to identify users may be   relatively harmless, for example, if the only information which can   be thus exposed belongs to the service, and the service will suffer   little harm from the exposure of such information.3.  User Interface Considerations for HTTP State Management   HTTP State Management has been very controversial because of its   potential to expose information about a user's browsing habits to   third parties, without the knowledge or consent of the user.  While   such exposure is possible, this is less a flaw in the protocol itself   than a failure of HTTP client implementations (and of some providers   of HTTP-based services) to protect users' interests.   As implied above, there are other ways to maintain session state than   using HTTP State Management, and therefore other ways in which users'   browsing habits can be tracked.  Indeed, it is difficult to imagine   how the HTTP protocol or an HTTP client could actually prevent a   service from disclosing a user's "click trail" to other parties if   the service chose to do so.  Protection of such information from   inappropriate exposure must therefore be the responsibility of the   service.  HTTP client implementations inherently cannot provide such   protection, though they can implement countermeasures which make it   more difficult for HTTP State Management to be used as the mechanism   by which such information is exposed.Moore & Freed            Best Current Practice                  [Page 5]

RFC 2964              Use of HTTP State Management          October 2000   It is arguable that HTTP clients should provide more protection in   general against inappropriate exposure of tracking information,   regardless of whether the exposure were facilitated by use of HTTP   State Management or by some other means.  However, issues related to   other mechanisms are beyond the scope of this memo.3.1.  Capabilities Required of an HTTP Client   A user's willingness to consent to use of HTTP State Management is   likely to vary from one service to another, according to whether the   user trusts the service to use the information appropriately and to   limit its exposure to other parties.  The user therefore SHOULD be   able to control whether his client supports a service's request to   use HTTP State Management, on a per-service basis.  In particular:   (1)   Clients MUST NOT respond to HTTP State Management requests         unless explicitly enabled by the user.   (2)   Clients SHOULD provide an effective interface which allows         users to review, and approve or refuse, any particular requests         from a server to maintain state information, before the client         provides any state information to the server.   (3)   Clients SHOULD provide an effective interface which allows         users to instruct their clients to ignore all requests from a         particular service to maintain state information, on a per-         service basis, immediately in response to any particular         request from a server, before the client provides any state         information to the server.   (4)   Clients SHOULD provide an effective interface which allows a         user to disable future transmission of any state information to         a service, and/or discard any saved state information for that         service, even though the user has previously approved a         service's request to maintain state information.   (5)   Clients SHOULD provide an effective interface which allows a         user to terminate a previous request not to retain state         management information for a given service.3.2.  Limitations of the domain-match algorithm   The domain-match algorithm inRFC-2965 section 2 is intended as a   heuristic to allow a client to "guess" whether or not two domains are   part of the same service.  There are few rules about how domain names   can be used, and the structure of domain names and how they are   delegated varies from one top-level domain to another (i.e. the   client cannot tell which part of the domain was assigned to theMoore & Freed            Best Current Practice                  [Page 6]

RFC 2964              Use of HTTP State Management          October 2000   service).  Therefore NO string comparison algorithm (including the   domain-match algorithm) can be relied on to distinguish a domain that   belongs to a particular service, from a domain that belongs to   another party.   As stated above, each service is ultimately responsible for ensuring   that user information is not inappropriately leaked to third parties.   Leaking information to third parties via State Management by careful   selection of domain names, or by assigning domain names to hosts   maintained by third parties, is at least as inappropriate as leaking   the same information by other means.4.  Security Considerations   This entire memo is about security considerations.5.  Authors' Addresses   Keith Moore   University of Tennessee Computer Science Department   1122 Volunteer Blvd, Suite 203   Knoxville TN, 37996-3450   EMail: moore@cs.utk.edu   Ned Freed   Innosoft International, Inc.   1050 Lakes Drive   West Covina, CA 81790   EMail: ned.freed@innosoft.com6.  References   [RFC 1123] Braden, R., "Requirements for Internet Hosts --              Application and Support", STD 3,RFC 1123, October 1989.   [RFC 2965] Kristol, D. and L. Montulli, "HTTP State Management              Mechanism",RFC 2965, October 2000.   [RFC 2109] Kristol, D. and L. Montulli, "HTTP State Management              Mechanism",RFC 2109, February 1997.Moore & Freed            Best Current Practice                  [Page 7]

RFC 2964              Use of HTTP State Management          October 20007.  Full Copyright Statement   Copyright (C) The Internet Society (2000).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Moore & Freed            Best Current Practice                  [Page 8]