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This document defines the HTTP Cookie and Set-Cookie headers.
NOTE: Much of the text herein is completely wrong. If you have suggestions for improving the draft, please send email to email@example.com. Suggestions with test cases are especially appreciated.
4. Protocol Description
4.3. Controlling Caching
5. User Agent Conformance
5.1. Parsing the Set-Cookie Header
5.1.1. The Max-Age Attribute
5.1.2. The Expires Attribute
5.1.3. The Domain Attribute
5.1.4. The Path Attribute
5.1.5. The Secure Attribute
5.1.6. The HttpOnly Attribute
5.2. Storage Model
5.3. The Cookie Header
6. Implementation Considerations
6.1. Set-Cookie Content
6.2. Implementation Limits
7. Security Considerations
7.1. Clear Text
7.2. Cookie Spoofing
8. Other, Similar, Proposals
Appendix A. Acknowledgements
Appendix B. Tabled Items
§ Author's Address
This document defines the HTTP Cookie and Set-Cookie header.
The terms user agent, client, server, proxy, and origin server have the same meaning as in the HTTP/1.0 specification.
Fully-qualified host name (FQHN) means either the fully-qualified domain name (FQDN) of a host (i.e., a completely specified domain name ending in a top-level domain such as .com or .uk), or the numeric Internet Protocol (IP) address of a host. The fully qualified domain name is preferred; use of numeric IP addresses is strongly discouraged. [TODO: What does "strongly discouraged" mean?]
The terms request-host and request-URI refer to the values the client 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. Note that request-host must be a FQHN. Hosts names can be specified either as an IP address or a FQHN string.
Because it was used in Netscape's original implementation of state management, we will use the term cookie to refer to the state information that passes between an origin server and user agent, and that gets stored by the user agent.
We outline here a way for an origin server to send state information to the user agent, and for the user agent to return the state information to the origin server.
The origin server initiates a session, if it so desires, by including 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).
A user agent returns a Cookie request header (see below) to the origin server if it chooses to continue a session. The origin server may ignore it or use it to determine the current state of the session. It may send the client 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 headers with any response. User agents should send Cookie request headers, subject to other rules detailed below, with every request.
An origin server may include multiple Set-Cookie headers in a response. Note that an intervening gateway MUST NOT fold multiple Set-Cookie headers into a single header.
[TODO: Overview the Set-Cookie and Cookie headers.]
[TODO: Put some examples here.
The cookie protocol consists of two HTTP headers: the Set-Cookie header and the Cookie header. The server sends the Set-Cookie header is to the user agent in an HTTP response, causing the user agent to modify the Cookie header it returns to the server.
This section describes the syntax and semantics of the protocol. Detailed conformance requirements for user agents are given in Section [TODO].
Informally, the Set-Cookie response header comprises the token Set-Cookie:, followed by a cookie. Each cookie begins with a name-value-pair, followed by zero or more semi-colon-separated attribute-value pairs.
set-cookie-header = "Set-Cookie:" name-value-pairs name-value-pairs = name-value-pair *(";" name-value-pair) name-value-pair = name ["=" value] ; optional value name = token value = *CHAR
The valid character for the value production vary depending on the attribute name.
[TODO: Investigate what token actually means.]
Attributes names are case-insensitive. White space is permitted between tokens. Servers MUST NOT include two attributes with the same name. Note that although the above syntax description shows value as optional, some attributes require values.
The cookie-value is opaque to the user agent and MAY be anything the origin server chooses to send, possibly in a server-selected printable ASCII encoding. "Opaque" implies that the content is of interest and relevance only to the origin server. The content may, in fact, be readable by anyone who examines the Set-Cookie header.
NOTE: The syntax above allows whitespace between the attribute and the U+3D ("=") character. Servers wishing to interoperate with some legacy user agents might wish to elide this extra white space to maximize compatibility.
When the user agent receives a Set-Cookie header, the user agent stores the cookie in its cookie store. When the user agent makes another HTTP request to the origin server, the user agent returns the cookie in the Cookie header.
The server can override the default handling of cookies by specifying cookie attributes. User agents ignore unrecognized cookie attributes.
[TODO: Consider removing Max-Age from the server conformance section because it's not supported by IE.]
- A sequence of ASCII numerals.
- The value of the Max-Age attribute represents the maximum lifetime of the cookie, measured in seconds from the moment the user agent receives the cookie. If the server does not supply an Expires or a Max-Age attribute, the lifetime of the cookie is limited to the current session (as defined by the user agent).
- An RFC 1123 date [cite]. (Note that user agents use very forgiving date parers; see Section [TODO]).
- The value of the Expires attribute represents the maximum lifetime of the cookie, represented as the point in time at which the cookie expires. If the server does not supply an Expires or a Max-Age attribute, the lifetime of the cookie is limited to the current session (as defined by the user agent).
[TODO: Test Domain.] The Domain attribute specifies the domain for which the cookie is valid. The leading dot isn't required. If there is no Domain attribute, the default is to return the cookie only to the origin server. [TODO: You can only set cookies for related domains.]
- A sequence of characters beginning with a "/" character.
- The Path attribute specifies the scope of the cookie within a given FQDN. The user agent will include a cookie in an HTTP request only if the Request-URI's path matches, or is a subdirectory of, the cookie's Path attribute (where the "/" character is interpreted as a directory separator). The default value for the Path attribute is the directory of the Request-URI when the cookie was received.
- Servers MUST NOT include a value.
- The user agent SHOULD protect the confidentiality of cookies with the Secure attribute by not transmitting Secure cookies over an "insecure" channel (where "insecure" is defined by the user agent).
- Servers MUST NOT include a value.
- The user agent SHOULD protect confidentiality of cookies with the HttpOnly attribute by including HttpOnly cookies only when generating cookie strings for use in HTTP requests.
The user agent returns stored cookies to the origin server in the Cookie header. The Cookie header shares a common syntax with the Set-Cookie header, but the semantics of the header differ dramatically.
cookie-header = "Cookie:" name-value-pairs name-value-pairs = name-value-pair *(";" name-value-pair) name-value-pair = name "=" value name = token value = *CHAR
NOTE: If the server supplies a Set-Cookie header that does not conform to the grammar in Section [TODO], the user agent might not supply a Cookie header that conforms to the preceding grammar.
Each name-value-pair represents a cookie stored by the user agent. The cookie name is returned in as the name and the cookie value is returned as the value.
The meaning of the cookies in the Cookie header is not defined by this document. Servers are expected to imbue these cookies with server-specific semantics.
[TODO: Should we go into this much detail here? This seems redundant with the HTTP specs.]
An origin server must be cognizant of the effect of possible caching of both the returned resource and the Set-Cookie header. Caching "public" documents is desirable. For example, if the origin server wants to use a public document such as a "front door" page as a sentinel to indicate the beginning of a session for which a Set-Cookie response header must be generated, the page should be stored in caches "pre-expired" so that the origin server will see further requests. "Private documents", for example those that contain information strictly private to a session, should not be cached in shared caches.
If the cookie is intended for use by a single user, the Set-Cookie header should not be cached. A Set-Cookie header that is intended to be shared by multiple users may be cached.
The origin server should send the following additional HTTP/1.1 response headers, depending on circumstances: [TODO: Is this good advice?]
and one of the following:
HTTP/1.1 servers must send Expires: old-date (where old-date is a date long in the past) on responses containing Set-Cookie response headers unless they know for certain (by out of band means) that there are no downstream HTTP/1.0 proxies. HTTP/1.1 servers may send other Cache-Control directives that permit caching by HTTP/1.1 proxies in addition to the Expires: old-date directive; the Cache-Control directive will override the Expires: old-date for HTTP/1.1 proxies.
Not all origin servers conform to the behavior specified in the previous section. To ensure interoperability, user agents MUST process cookies in a manner that is "black-box" indistinguishable from the requirements in this section.
Let an LWS character be either a U+20 (SPACE) or a U+09 (TAB) character.
A user agent MUST use the following algorithm to parse the Set-Cookie header:
the name-value-pair string is characters up to, but not including, the first U+3B (";"), and the unparsed-cookie-attributes are the remainder of the header (including the U+3B (";") in question).
the name-value-pair string is all the character contained in the header, and the unparsed-cookie-attributes is the empty string.
the (possibly empty) name string is the characters up to, but not including, the first U+3D ("=") character, and the (possibly empty) value string is the characters after the first U+3D ("=") character.
the name string is empty, and the value string is the entire name-value-pair string.
The user agent MUST use the following algorithm to parse the unparsed-attributes:
Consume the characters of the unparsed-attributes up to, but not including, the first U+3B (";") character.
The characters consumed in this step comprise the attribute-value-pair string.
Consume the remainder of the unparsed-attributes.
the (possibly empty) name string is the characters up to, but not including, the first U+3D ("=") character, and the (possibly empty) value string is the characters after the first U+3D ("=") character .
the name string is the entire attribute-value-pair string, and the value string is empty. (Note that this step differs from the analogous step when parsing the name-value-pair string.)
Attribute | Instruction ------------+--------------------- Max-Age | See Section [TODO] Expires | See Section [TODO] Domain | See Section [TODO] Path | See Section [TODO] Secure | See Section [TODO] HttpOnly | See Section [TODO]
[TODO: Can parsing a cookie ever fail? Doesn't look like it! Well, unless you count "Set-Cookie: " as a fail...]
When the user agent finishes parsing the Set-Cookie header, the user agent *receives a cookie* from the origin server with name cookie-name, value cookie-value, and attributes cookie-attribute-list.
When the user agent receives a cookie attribute with a name string that case-insensitively matches the string "Max-Age", the user agent MUST process the value string as follows.
If the first character of the value string is not a DIGIT or a "-" character, the user agent MUST ignore the attribute.
If the remainder of value string contains a non-DIGIT character, the user agent MUST ignore the attribute.
Let delta-seconds be the contents of the value string converted to an integer.
If delta-seconds is less than or equal to 0, then append an attribute named Expires (note the name conversion) to the cookie-attribute-list with a value equal to the current date and time.
If delta-seconds is strictly greater than 0, then append an attributes named Expires (note the name conversion) to the cookie-attribute-list with a value equal to the current date and time plus delta-seconds seconds.
[TODO: Basically, cookie dates are a mess for historical reasons.]
cookie-date = rfc1123-like-date / mystery-date rfc1123-like-date = weekday "," SP rfc1123-like-dmy SP time SP "GMT" weekday = "Monday" / "Mon" / "Tuesday" / "Tue" / ... rfc1123-like-dmy = day dmy-div month dmy-div year dmy-div = SP / "-" day = 2DIGIT / *1SP DIGIT month = "Jan" / "Feb" / ... year = 2DIGIT / 4DIGIT time = 2DIGIT ":" 2DIGIT ":" 2DIGIT mystery-date = *CHAR ; see below
[TODO: More information about mystery-date.]
The user agent MUST use the following algorithm to compute the default-path of a cookie:
A request-path path-matches a cookie-path if the cookie-path is a prefix of the request-path and at least one of the following conditions hold:
When the user agent receives a cookie attribute with a name string that case-insensitively matches the string "Path", the user agent MUST process the value string as follows:
[TODO: Test \ ? ; # $ % etc]
When the user agent receives a cookie attribute with a name string that case-insensitively matches the string "Secure", the user agent MUST append an attribute named Secure to the cookie-attribute-list with an empty value regardless of the value string.
When the user agent receives a cookie attribute with a name string that case-insensitively matches the string "HttpOnly", the user agent MUST append an attribute named Secure to the cookie-attribute-list with an empty value regardless of the value string.
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 received cookies in their entirety if the user agent is configured to block receiving cookie for a particular response. 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:
When the user agent receives a cookie, the user agent MUST follow the following algorithm:
- If the cookie-attributes contains at least one Expires attribute, store the value of the last such attribute in the expiry field. Store the value true in the persistent field. [TODO: Test that this really works when mixing Max-Age and Expires.]
- If the cookie-attributes contains at least one Domain attribute, store the value of the last such attribute in the domain field. Store the value false in the host-only field. [TODO: Reject cookies for unrelated domains.] [TODO: If the URL's host is an IP address, let Domain to be an IP address if it matches the URL's host exactly, but set the host-only flag. ]
- If the cookie-attributes contains at least one Path attribute, store the value of the last such attribute in the path field.
- If the cookie-attributes contains at least one Secure attribute, store the value true in the secure-only field.
- If the cookie-attributes contains at least one HttpOnly attribute, store the value true in the http-only field.
The user agent MUST evict a cookie from the cookie store if 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). [TODO: Explain where 50 comes from.]
The user agent MAY evict cookies from the cookie store if the cookie store exceeds some maximum storage bound (such as 3000 cookies). [TODO: Explain where 3000 comes from.]
When the user agent evicts cookies from the cookie store, the user agent MUST evict cookies in the following priority order:
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 user agent exits, the user agent MUST remove from the cookie store all cookies with the persistent field set to false.
When the user agent generates an HTTP request for a particular URI, the user agent SHOULD attach exactly one HTTP header named Cookie if the cookie-string (defined below) for that URI is non-empty.
A user agent MAY elide the Cookie header in its entirety if the user agent is configured to block sending cookie for a particular request. 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 from a URI:
NOTE: The notion of an "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.
An origin server's content should probably be divided into disjoint application areas, some of which require the use of state information. The application areas can be distinguished by their request URLs. The Set-Cookie header can incorporate information about the application areas by setting the Path attribute for each one.
The session information can obviously be clear or encoded text that describes state. However, if it grows too large, it can become unwieldy. Therefore, an implementor might choose for the session information to be a key to a server-side resource. [TODO: Describe briefly how to generate a decent session key.]
[TODO: We could recommend that servers encrypt and mac their cookie data.]
[TODO: Mention issues that arise from having multiple concurrent sessions.]
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:
The information in a Set-Cookie response header must be retained in its entirety. If for some reason there is inadequate space to store the cookie, the cookie must be discarded, not truncated.
Applications should use as few and as small cookies as possible, and they should cope gracefully with the loss of a cookie. [TODO: Could mention latency issues that arise from having tons of cookies.]
The information in the Set-Cookie and Cookie headers is transmitted in the clear. Three consequences are:
These facts imply that information of a personal and/or financial nature should be sent over a secure channel. For less sensitive information, or when the content of the header is a database key, an origin server should be vigilant to prevent a bad Cookie value from causing failures.
[TODO: Mention integrity issue where a sibling domain can inject cookies.]
[TODO: Mention integrity issue where a HTTP can inject cookies into HTTPS.]
[TODO: Describe relation to the Netscape Cookie Spec, RFC 2109, RFC 2629, and cookie-v2.]
This document borrows heavily from RFC 2109. [TODO: Figure out the proper way to credit the authors of RFC 2109.]
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