HTTP State Management Mechanism

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.¶

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 terms user agent, client, server, proxy, and origin server have the same meaning as in the HTTP/1.1 specification.¶

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.¶

[TODO: Put some examples here.¶

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

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

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.

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.¶

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 "Expires":
   • 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". [TODO: Test that this really works when mixing Max-Age and 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") 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. Remove from the cookie store all cookies that share the same name, domain, path, and host-only-flag as the newly created cookie. [TODO: Validate this list!] [TODO: There's some funny business around http-only here.]
10. If the cookie's name and value are both empty, abort these steps.
11. If the cookie's expiry-time is not in the future, abort these steps.
12. 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.¶

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.¶

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 field 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 field 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.
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 ("; ").

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.¶

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.¶

The cookie protocol is NOT RECOMMENDED for new applications.¶

For applications that do use the cookie protocol, servers SHOULD NOT rely upon cookies for security.¶

For servers that do use cookies for security, servers SHOULD use a redundant form of authentication, such as HTTP authentication or TLS client certificates.¶

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. User agents can mitigate this issue to some degree by providing APIs for suppressing the Cookie header on outgoing requests.¶

Although this security concern goes by a number of names (e.g., cross-site scripting and cross-site request forgery), 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). Disentangling designation and authorization can cause the server and its clients to become confused deputies and undertake undesirable actions.¶

Instead of using cookies for authorization, server operators might wish to consider entangling designation and authorization by treating URLs as object-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.¶

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 their cookies when transmitting them to the user agent (even when sending the cookies over a secure channel). However, encrypting and signing cookies does not prevent an attacker from transplanting a cookie from one user agent to another.¶

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.¶

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 are also available to other schemes, such as ftp and gopher. This lack of isolation is most easily seen when a user agent retrieves a URI with a gopher scheme via HTTP, but the lack of isolation by scheme is also apparent via non-HTTP APIs that permit access to cookies, such as HTML's document.cookie API.¶

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", 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.¶

Similarly, an active network attacker can 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 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.¶

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.¶