Network Working Group Keith Moore, ed. Internet-Draft University of Tennessee 18 November 1998 Expires: 18 May 1999 Applicability Statement for HTTP State Management draft-iesg-http-cookies-00.txt This document is an Internet-Draft. 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." To view the entire list of current Internet-Drafts, please check the "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow Directories on ftp.is.co.za (Africa), ftp.nordu.net (Northern Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au (Pacific Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast). Abstract The mechanisms described in "HTTP State Management Mechanism" [RFC- XXXX] and its predecessor [RFC-2109], can be used for many different purposes. Even though this protocol has been approved for the Internet standards track, some current and potential uses of the protocol are not within the scope of the standard approved by IESG. This memo identifies specific uses of HTTP State Management protocol which are either (a) nonstandard and thus not recommended by IETF, or (b) nonstandard, 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 widely misused to accomplish things for which it was not designed and is not well-suited. Some of these misuses have received a great deal of public attention because they threaten to Moore Expires 18 May 1999 [Page 1] HTTP Cookies A/S INTERNET-DRAFT 18 November 1998 violate the privacy of web users, specifically by leaking information to third parties about which web pages the user has visited. However, there are other uses of HTTP State Management which are inappropriate beyond those that threaten users privacy. This memo therefore identifies uses of the HTTP State Management protocol which are either outside of the scope of the standard and thus not recommended by IETF, or which are believed to be harmful and are therefore discouraged. Uses in the latter category should be considered violations of the standard, even though the actual protocol implementations may conform to the standard. This memo, along with RFC- XXXX, is considered part of the HTTP State Management protocol specification. The following keywords, when spelled in upper case letters, have specific meanings in the context of the HTTP State Management Specification: - The word "MUST" means that the definition is an absolute require- ment of the specification. - The word "MUST NOT" means that the definition is an absolute prohi- bition of the specification. - The word "SHOULD" means that the definition is generally a require- ment of the specification, but there may exist valid reasons in particular circumstances to ignore the requirement. The full implications must be understood and carefully weighted in light of the intent of both the protocol specification and this document, before choosing to ignore the requirement.. - The word "SHOULD NOT" means that the definition is generally pro- hibited by the specification, but there may exist valid reasons in particular circumstances where the particular behavior is accept- able or even useful, but the full implications should be understood and the case carefully weighed before implementing any behavior described with this label. 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. Moore Expires 18 May 1999 [Page 2] HTTP Cookies A/S INTERNET-DRAFT 18 November 1998 It's important to realize that similar capabilities may 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, a Content-Location response header field, 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. The HTTP State Management facility thus provides only a marginal (but still useful) increase in functionality over ordinary HTTP and HTML. In practice, this additional functionality includes: - 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.'') - 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. - The ability to implement sessions with minimal server configuration and minimal protocol overhead, as compared to other techniques of maintaining session state. - 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''. - 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: - the user is aware that session state is being maintained and con- sents to it, Moore Expires 18 May 1999 [Page 3] HTTP Cookies A/S INTERNET-DRAFT 18 November 1998 - the user has the ability to terminate the session at any time, - 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 - the session information itself does not contain information which is sensitive to the user, and cannot be used to obtain information which is sensitive to the user, that is not otherwise available to an eavesdropper. 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 those 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, such 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. Inappropriate Uses The following uses of HTTP State Management are deemed inappropri- ate and a violation of the standard: 2.2.1. Leakage of Information to Third Parties HTTP State Management MUST NOT be used to is 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. Such usage is not within the intended purpose of the HTTP State Management protocol. Because such practices encourage users to defeat HTTP State Management mechanisms, they tend to reduce the effectiveness Moore Expires 18 May 1999 [Page 4] HTTP Cookies A/S INTERNET-DRAFT 18 November 1998 of HTTP State Management, and are therefore considered detrimental to the operation of the web. 2.2.1. 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 SHOULD NOT be used as an authentication mechanism to protect information from being exposed to unauthorized parties, even if the HTTP sessions are encrypted. This 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 which 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 not a flaw in the protocol itself so much as the failure of HTTP client implementations (and of some providers of HTTP-based services) to protect users' interests. Moore Expires 18 May 1999 [Page 5] HTTP Cookies A/S INTERNET-DRAFT 18 November 1998 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. 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: - Clients MUST NOT respond to HTTP State Management requests unless explicitly enabled by the user. - 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. - 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. - Clients SHOULD provide an effective interface which allows a user to disable future transmission of any state information to a ser- vice, 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. Moore Expires 18 May 1999 [Page 6] HTTP Cookies A/S INTERNET-DRAFT 18 November 1998 - Clients SHOULD provide an effective interface which allows a user to terminate a previous user request to ignore a particular ser- vice's requests for the client to maintain state management infor- mation. 3.2 Limitations of the domain-match algorithm The domain-match algorithm in RFC-XXXX 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 the 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. Editor's Address Keith Moore 104 Ayres Hall Knoxville TN 37996 moore@cs.utk.edu 6. References [RFC-XXXX] David M. Kristol, Lou Montulli. HTTP State Management Mechanism. Internet-Draft , work in progress. (to be replaced by the RFC name when this memo is pub- lished) [RFC-2109] David M. Kristol, Lou Montulli. HTTP State Management Mechanism. RFC 2109, February 1997. Moore Expires 18 May 1999 [Page 7] HTTP Cookies A/S INTERNET-DRAFT 18 November 1998 Moore Expires 18 May 1999 [Page 8]