httpstate Working GroupA. Barth
Internet-DraftU.C. Berkeley
Obsoletes: 2109 (if approved)April 16, 2010
Intended status: Standards Track
Expires: October 18, 2010

HTTP State Management Mechanism

draft-ietf-httpstate-cookie-06

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.

Status of this Memo

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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.


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).

The OWS (optional whitespace) rule is used where zero or more linear whitespace characters may appear. OWS SHOULD either not be produced or be produced as a single SP character. Multiple OWS characters that occur within field-content SHOULD be replaced with a single SP before interpreting the field value or forwarding the message downstream.

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.


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.

If a server sends multiple responses containing Set-Cookie headers concurrently to the user agent (e.g., when communicating with the user agent over multiple sockets), these responses create a "race condition" that can lead to unpredictable behavior.

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 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.

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

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

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.



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:

  • At least 4096 bytes per cookie (as measured by the sum of the length of the cookie's name, value, and attributes).
  • At least 50 cookies per domain.
  • 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.


7. Privacy Considerations

The cookie protocol is often criticized for letting servers track users. For example, a number of "web analytics" companies use cookies to recognize when a user returns to a web site or visits another web site. Although cookies are not the only mechanism servers can use to track users across HTTP requests, cookies facilitate tracking because they are persistent across user agent sessions and can be shared between host names.

7.1 Third-Party Cookies

Particularly worrisome are so-called "third-party" cookies. In rendering an HTML document, a user agent often requests resources from other servers (such as advertising networks). These third-party servers can use the cookie protocol to track the user even if the user never visits the server directly.

Some user agents restrict how third-party cookies behave. For example, some user agents refuse to send the Cookie header in third-party requests. Other user agents refuse to process the Set-Cookie header in responses to third-party requests. Among user agents, there is a wide variety of third-party cookie policies. This document grants user agents wide latitude to experiment with third-party cookie policies that balance the privacy and compatibility needs of their users. However, this document does not endorse any particular third-party cookie policy.

Third-party cookie blocking policies are often ineffective at achieving their privacy goals if servers attempt to work around their restrictions to track users. In particular, two collaborating servers can often track users without using cookies at all.

7.2 User Controls

User agents SHOULD provide users with a mechanism for managing the cookies stored in the cookie store. For example, a user agent might let users delete all cookies received during a specified time period or all the cookies related to a particular domain. In addition, many user agent include a user interface element that lets users examine the cookies stored in the cookie store.

User agents SHOULD provide users with a mechanism for disabling cookies. When cookies are disabled, the user agent MUST NOT include a Cookie header in outbound HTTP requests and the user agent MUST NOT process Set-Cookie headers in inbound HTTP responses.

Some user agents provide users the option of preventing persistent storage of cookies across sessions. When configured thusly, user agents MUST treat all received cookies as if the persistent-flag were set to false.

Some user agents provide users with the ability to approve individual writes to the cookie store. In many common usage scenarios, these controls generate a large number of prompts. However, some privacy-conscious users find these controls useful nonetheless.


8. Security Considerations

8.1 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.

8.2 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.

8.3 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.

8.4 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 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.

8.5 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.

8.6 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.

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.

8.7 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.


9. References

9.1 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.
[RFC3492]
Costello, A., “Punycode: A Bootstring encoding of Unicode for Internationalized Domain Names in Applications (IDNA)”, RFC 3492, March 2003.
[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.

9.2 Informative References

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

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.


Author's Address

Adam Barth
University of California, Berkeley
EMail: abarth@eecs.berkeley.edu
URI: http://www.adambarth.com/