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This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the “Internet Official Protocol Standards” (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.
Copyright © The Internet Society (2000). All Rights Reserved.
This document specifies a way to create a stateful session with Hypertext Transfer Protocol (HTTP) requests and responses. It describes three new headers, Cookie, Cookie2, and Set-Cookie2, which carry state information between participating origin servers and user agents. The method described here differs from Netscape's Cookie proposal [Netscape] (, “Persistent Client State -- HTTP Cookies,” .), but it can interoperate with HTTP/1.0 user agents that use Netscape's method. (See the HISTORICAL section.)
This document reflects implementation experience with RFC 2109 and obsoletes it.
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2. STATE AND SESSIONS
3.1. Syntax: General
3.2. Origin Server Role
3.2.2. Set-Cookie2 Syntax
3.2.3. Controlling Caching
3.3. User Agent Role
3.3.1. Interpreting Set-Cookie2
3.3.2. Rejecting Cookies
3.3.3. Cookie Management
3.3.4. Sending Cookies to the Origin Server
3.3.5. Identifying What Version is Understood: Cookie2
3.3.6. Sending Cookies in Unverifiable Transactions
3.4. How an Origin Server Interprets the Cookie Header
3.5. Caching Proxy Role
4.1. Example 1
4.2. Example 2
5. IMPLEMENTATION CONSIDERATIONS
5.1. Set-Cookie2 Content
5.2. Stateless Pages
5.3. Implementation Limits
5.3.1. Denial of Service Attacks
6.1. User Agent Control
6.2. Origin Server Role
6.3. Clear Text
7. SECURITY CONSIDERATIONS
7.1. Protocol Design
7.2. Cookie Spoofing
7.3. Unexpected Cookie Sharing
7.4. Cookies For Account Information
8. OTHER, SIMILAR, PROPOSALS
9.1. Compatibility with Existing Implementations
9.2. Caching and HTTP/1.0
§ Authors' Addresses
§ Intellectual Property and Copyright Statements
The terms user agent, client, server, proxy, origin server, and http_URL have the same meaning as in the HTTP/1.1 specification [RFC2616] (Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” June 1999.). The terms abs_path and absoluteURI have the same meaning as in the URI Syntax specification [RFC2396] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifiers (URI): Generic Syntax,” August 1998.).
Host name (HN) means either the host domain name (HDN) 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.
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 is a HN.
The term effective host name is related to host name. If a host name contains no dots, the effective host name is that name with the string .local appended to it. Otherwise the effective host name is the same as the host name. Note that all effective host names contain at least one dot.
The term request-port refers to the port portion of the absoluteURI (http_URL) of the HTTP request line. If the absoluteURI has no explicit port, the request-port is the HTTP default, 80. The request-port of a cookie is the request-port of the request in which a Set-Cookie2 response header was returned to the user agent.
Host names can be specified either as an IP address or a HDN string. Sometimes we compare one host name with another. (Such comparisons SHALL be case-insensitive.) Host A's name domain-matches host B's if
Note that domain-match is not a commutative operation: a.b.c.com domain-matches .c.com, but not the reverse.
The reach R of a host name H is defined as follows:
For two strings that represent paths, P1 and P2, P1 path-matches P2 if P2 is a prefix of P1 (including the case where P1 and P2 string- compare equal). Thus, the string /tec/waldo path-matches /tec.
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.
The key words "MAY", "MUST", "MUST NOT", "OPTIONAL", "RECOMMENDED", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT" in this document are to be interpreted as described in RFC 2119 [RFC2119] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).
This document describes a way to create stateful sessions with HTTP requests and responses. Currently, HTTP servers respond to each client request without relating that request to previous or subsequent requests; the state management mechanism allows clients and servers that wish to exchange state information to place HTTP requests and responses within a larger context, which we term a "session". This context might be used to create, for example, a "shopping cart", in which user selections can be aggregated before purchase, or a magazine browsing system, in which a user's previous reading affects which offerings are presented.
Neither clients nor servers are required to support cookies. A server MAY refuse to provide content to a client that does not return the cookies it sends.
We describe 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 goal is to have a minimal impact on HTTP and user agents.
The two state management headers, Set-Cookie2 and Cookie, have common syntactic properties involving attribute-value pairs. The following grammar uses the notation, and tokens DIGIT (decimal digits), token (informally, a sequence of non-special, non-white space characters), and http_URL from the HTTP/1.1 specification [RFC2616] (Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” June 1999.) to describe their syntax.
av-pairs = av-pair *(";" av-pair) av-pair = attr ["=" value] ; optional value attr = token value = token | quoted-string
Attributes (names) (attr) are case-insensitive. White space is permitted between tokens. Note that while the above syntax description shows value as optional, most attrs require them.
NOTE: The syntax above allows whitespace between the attribute and the = sign.
The origin server initiates a session, if it so desires. To do so, it returns an extra response header to the client, Set-Cookie2. (The details follow later.)
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 back to the client a Set-Cookie2 response header with the same or different information, or it MAY send no Set-Cookie2 header at all. The origin server effectively ends a session by sending the client a Set-Cookie2 header with Max-Age=0.
Servers MAY return Set-Cookie2 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-Cookie2 headers in a response. Note that an intervening gateway could fold multiple such headers into a single header.
The syntax for the Set-Cookie2 response header is
set-cookie = "Set-Cookie2:" cookies cookies = 1#cookie cookie = NAME "=" VALUE *(";" set-cookie-av) NAME = attr VALUE = value set-cookie-av = "Comment" "=" value | "CommentURL" "=" <"> http_URL <"> | "Discard" | "Domain" "=" value | "Max-Age" "=" value | "Path" "=" value | "Port" [ "=" <"> portlist <"> ] | "Secure" | "Version" "=" 1*DIGIT portlist = 1#portnum portnum = 1*DIGIT
Informally, the Set-Cookie2 response header comprises the token Set-Cookie2:, followed by a comma-separated list of one or more cookies. Each cookie begins with a NAME=VALUE pair, followed by zero or more semi-colon-separated attribute-value pairs. The syntax for attribute-value pairs was shown earlier. The specific attributes and the semantics of their values follows. The NAME=VALUE attribute-value pair MUST come first in each cookie. The others, if present, can occur in any order. If an attribute appears more than once in a cookie, the client SHALL use only the value associated with the first appearance of the attribute; a client MUST ignore values after the first.
The NAME of a cookie MAY be the same as one of the attributes in this specification. However, because the cookie's NAME must come first in a Set-Cookie2 response header, the NAME and its VALUE cannot be confused with an attribute-value pair.
- REQUIRED. The name of the state information ("cookie") is NAME, and its value is VALUE. NAMEs that begin with $ are reserved and MUST NOT be used by applications.
The 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 that examines the Set-Cookie2 header.
- OPTIONAL. The Discard attribute instructs the user agent to discard the cookie unconditionally when the user agent terminates.
- OPTIONAL. The value of the Domain attribute specifies the domain for which the cookie is valid. If an explicitly specified value does not start with a dot, the user agent supplies a leading dot.
- OPTIONAL. The value of the Max-Age attribute is delta-seconds, the lifetime of the cookie in seconds, a decimal non-negative integer. To handle cached cookies correctly, a client SHOULD calculate the age of the cookie according to the age calculation rules in the HTTP/1.1 specification [RFC2616] (Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” June 1999.). When the age is greater than delta-seconds seconds, the client SHOULD discard the cookie. A value of zero means the cookie SHOULD be discarded immediately.
- OPTIONAL. The value of the Path attribute specifies the subset of URLs on the origin server to which this cookie applies.
- OPTIONAL. The Port attribute restricts the port to which a cookie may be returned in a Cookie request header. Note that the syntax REQUIREs quotes around the OPTIONAL portlist even if there is only one portnum in portlist.
- OPTIONAL. The Secure attribute (with no value) directs the user agent to use only (unspecified) secure means to contact the origin server whenever it sends back this cookie, to protect the confidentially and authenticity of the information in the cookie.
The user agent (possibly with user interaction) MAY determine what level of security it considers appropriate for "secure" cookies. The Secure attribute should be considered security advice from the server to the user agent, indicating that it is in the session's interest to protect the cookie contents. When it sends a "secure" cookie back to a server, the user agent SHOULD use no less than the same level of security as was used when it received the cookie from the server.
- REQUIRED. The value of the Version attribute, a decimal integer, identifies the version of the state management specification to which the cookie conforms. For this specification, Version=1 applies.
An origin server must be cognizant of the effect of possible caching of both the returned resource and the Set-Cookie2 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-Cookie2 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-Cookie2 header SHOULD NOT be cached. A Set-Cookie2 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:
and one of the following:
Cache-Control: must-revalidate, max-age=0
Cache-Control: proxy-revalidate, max-age=0
HTTP/1.1 servers MUST send Expires: old-date (where old-date is a date long in the past) on responses containing Set-Cookie2 response headers unless they know for certain (by out of band means) that there are no HTTP/1.0 proxies in the response chain. 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.
The user agent keeps separate track of state information that arrives via Set-Cookie2 response headers from each origin server (as distinguished by name or IP address and port). The user agent MUST ignore attribute-value pairs whose attribute it does not recognize. The user agent applies these defaults for optional attributes that are missing:
- The default behavior is dictated by the presence or absence of a Max-Age attribute.
- Defaults to the effective request-host. (Note that because there is no dot at the beginning of effective request-host, the default Domain can only domain-match itself.)
- The default behavior is to discard the cookie when the user agent exits.
- Defaults to the path of the request URL that generated the Set-Cookie2 response, up to and including the right-most /.
- The default behavior is that a cookie MAY be returned to any request-port.
- If absent, the user agent MAY send the cookie over an insecure channel.
To prevent possible security or privacy violations, a user agent rejects a cookie according to rules below. The goal of the rules is to try to limit the set of servers for which a cookie is valid, based on the values of the Path, Domain, and Port attributes and the request-URI, request-host and request-port.
A user agent rejects (SHALL NOT store its information) if the Version attribute is missing. Moreover, a user agent rejects (SHALL NOT store its information) if any of the following is true of the attributes explicitly present in the Set-Cookie2 response header:
If a user agent receives a Set-Cookie2 response header whose NAME is the same as that of a cookie it has previously stored, the new cookie supersedes the old when: the old and new Domain attribute values compare equal, using a case-insensitive string-compare; and, the old and new Path attribute values string-compare equal (case-sensitive). However, if the Set-Cookie2 has a value for Max-Age of zero, the (old and new) cookie is discarded. Otherwise a cookie persists (resources permitting) until whichever happens first, then gets discarded: its Max-Age lifetime is exceeded; or, if the Discard attribute is set, the user agent terminates the session.
Because user agents have finite space in which to store cookies, they MAY also discard older cookies to make space for newer ones, using, for example, a least-recently-used algorithm, along with constraints on the maximum number of cookies that each origin server may set.
If a Set-Cookie2 response header includes a Comment attribute, the user agent SHOULD store that information in a human-readable form with the cookie and SHOULD display the comment text as part of a cookie inspection user interface.
If a Set-Cookie2 response header includes a CommentURL attribute, the user agent SHOULD store that information in a human-readable form with the cookie, or, preferably, SHOULD allow the user to follow the http_URL link as part of a cookie inspection user interface.
The cookie inspection user interface may include a facility whereby a user can decide, at the time the user agent receives the Set-Cookie2 response header, whether or not to accept the cookie. A potentially confusing situation could arise if the following sequence occurs:
The user agent SHOULD NOT send any cookies in this context. The user agent MAY discard any cookie it receives in this context that the user has not, through some user agent mechanism, deemed acceptable.
User agents SHOULD allow the user to control cookie destruction, but they MUST NOT extend the cookie's lifetime beyond that controlled by the Discard and Max-Age attributes. An infrequently-used cookie may function as a "preferences file" for network applications, and a user may wish to keep it even if it is the least-recently-used cookie. One possible implementation would be an interface that allows the permanent storage of a cookie through a checkbox (or, conversely, its immediate destruction).
Privacy considerations dictate that the user have considerable control over cookie management. The PRIVACY section contains more information.
When it sends a request to an origin server, the user agent includes a Cookie request header if it has stored cookies that are applicable to the request, based on
The syntax for the header is:
cookie = "Cookie:" cookie-version 1*((";" | ",") cookie-value) cookie-value = NAME "=" VALUE [";" path] [";" domain] [";" port] cookie-version = "$Version" "=" value NAME = attr VALUE = value path = "$Path" "=" value domain = "$Domain" "=" value port = "$Port" [ "=" <"> value <"> ]
The value of the cookie-version attribute MUST be the value from the Version attribute of the corresponding Set-Cookie2 response header. Otherwise the value for cookie-version is 0. The value for the path attribute MUST be the value from the Path attribute, if one was present, of the corresponding Set-Cookie2 response header. Otherwise the attribute SHOULD be omitted from the Cookie request header. The value for the domain attribute MUST be the value from the Domain attribute, if one was present, of the corresponding Set-Cookie2 response header. Otherwise the attribute SHOULD be omitted from the Cookie request header.
The port attribute of the Cookie request header MUST mirror the Port attribute, if one was present, in the corresponding Set-Cookie2 response header. That is, the port attribute MUST be present if the Port attribute was present in the Set-Cookie2 header, and it MUST have the same value, if any. Otherwise, if the Port attribute was absent from the Set-Cookie2 header, the attribute likewise MUST be omitted from the Cookie request header.
Note that there is neither a Comment nor a CommentURL attribute in the Cookie request header corresponding to the ones in the Set-Cookie2 response header. The user agent does not return the comment information to the origin server.
The user agent applies the following rules to choose applicable cookie-values to send in Cookie request headers from among all the cookies it has received.
- Domain Selection
- The origin server's effective host name MUST domain-match the Domain attribute of the cookie.
- Port Selection
- There are three possible behaviors, depending on the Port attribute in the Set-Cookie2 response header:
- By default (no Port attribute), the cookie MAY be sent to any port.
- If the attribute is present but has no value (e.g., Port), the cookie MUST only be sent to the request-port it was received from.
- If the attribute has a port-list, the cookie MUST only be returned if the new request-port is one of those listed in port-list.
- Path Selection
- The request-URI MUST path-match the Path attribute of the cookie.
- Max-Age Selection
- Cookies that have expired should have been discarded and thus are not forwarded to an origin server.
If multiple cookies satisfy the criteria above, they are ordered in the Cookie header such that those with more specific Path attributes precede those with less specific. Ordering with respect to other attributes (e.g., Domain) is unspecified.
Note: For backward compatibility, the separator in the Cookie header is semi-colon (;) everywhere. A server SHOULD also accept comma (,) as the separator between cookie-values for future compatibility.
The Cookie2 request header facilitates interoperation between clients and servers that understand different versions of the cookie specification. When the client sends one or more cookies to an origin server, if at least one of those cookies contains a $Version attribute whose value is different from the version that the client understands, then the client MUST also send a Cookie2 request header, the syntax for which is
cookie2 = "Cookie2:" cookie-version
Here the value for cookie-version is the highest version of cookie specification (currently 1) that the client understands. The client needs to send at most one such request header per request.
Users MUST have control over sessions in order to ensure privacy. (See PRIVACY section below.) To simplify implementation and to prevent an additional layer of complexity where adequate safeguards exist, however, this document distinguishes between transactions that are verifiable and those that are unverifiable. A transaction is verifiable if the user, or a user-designated agent, has the option to review the request-URI prior to its use in the transaction. A transaction is unverifiable if the user does not have that option. Unverifiable transactions typically arise when a user agent automatically requests inlined or embedded entities or when it resolves redirection (3xx) responses from an origin server. Typically the origin transaction, the transaction that the user initiates, is verifiable, and that transaction may directly or indirectly induce the user agent to make unverifiable transactions.
An unverifiable transaction is to a third-party host if its request-host U does not domain-match the reach R of the request-host O in the origin transaction.
When it makes an unverifiable transaction, a user agent MUST disable all cookie processing (i.e., MUST NOT send cookies, and MUST NOT accept any received cookies) if the transaction is to a third-party host.
This restriction prevents a malicious service author from using unverifiable transactions to induce a user agent to start or continue a session with a server in a different domain. The starting or continuation of such sessions could be contrary to the privacy expectations of the user, and could also be a security problem.
User agents MAY offer configurable options that allow the user agent, or any autonomous programs that the user agent executes, to ignore the above rule, so long as these override options default to "off".
(N.B. Mechanisms may be proposed that will automate overriding the third-party restrictions under controlled conditions.)
Many current user agents already provide a review option that would render many links verifiable. For instance, some user agents display the URL that would be referenced for a particular link when the mouse pointer is placed over that link. The user can therefore determine whether to visit that site before causing the browser to do so. (Though not implemented on current user agents, a similar technique could be used for a button used to submit a form -- the user agent could display the action to be taken if the user were to select that button.) However, even this would not make all links verifiable; for example, links to automatically loaded images would not normally be subject to "mouse pointer" verification.
Many user agents also provide the option for a user to view the HTML source of a document, or to save the source to an external file where it can be viewed by another application. While such an option does provide a crude review mechanism, some users might not consider it acceptable for this purpose.
A user agent returns much of the information in the Set-Cookie2 header to the origin server when the request-URI path-matches the Path attribute of the cookie. When it receives a Cookie header, the origin server SHOULD treat cookies with NAMEs whose prefix is $ specially, as an attribute for the cookie.
One reason for separating state information from both a URL and document content is to facilitate the scaling that caching permits. To support cookies, a caching proxy MUST obey these rules already in the HTTP specification:
Proxies MUST NOT introduce Set-Cookie2 (Cookie) headers of their own in proxy responses (requests).
Most detail of request and response headers has been omitted. Assume the user agent has no stored cookies.
1. User Agent -> Server
POST /acme/login HTTP/1.1 [form data]
User identifies self via a form.
2. Server -> User Agent
HTTP/1.1 200 OK Set-Cookie2: Customer="WILE_E_COYOTE"; Version="1"; Path="/acme"
Cookie reflects user's identity.
3. User Agent -> Server
POST /acme/pickitem HTTP/1.1 Cookie: $Version="1"; Customer="WILE_E_COYOTE"; $Path="/acme" [form data]
User selects an item for "shopping basket".
4. Server -> User Agent
HTTP/1.1 200 OK Set-Cookie2: Part_Number="Rocket_Launcher_0001"; Version="1"; Path="/acme"
Shopping basket contains an item.
5. User Agent -> Server
POST /acme/shipping HTTP/1.1 Cookie: $Version="1"; Customer="WILE_E_COYOTE"; $Path="/acme"; Part_Number="Rocket_Launcher_0001"; $Path="/acme" [form data]
User selects shipping method from form.
6. Server -> User Agent
HTTP/1.1 200 OK Set-Cookie2: Shipping="FedEx"; Version="1"; Path="/acme"
New cookie reflects shipping method.
7. User Agent -> Server
POST /acme/process HTTP/1.1 Cookie: $Version="1"; Customer="WILE_E_COYOTE"; $Path="/acme"; Part_Number="Rocket_Launcher_0001"; $Path="/acme"; Shipping="FedEx"; $Path="/acme" [form data]
User chooses to process order.
8. Server -> User Agent
HTTP/1.1 200 OK
Transaction is complete.
The user agent makes a series of requests on the origin server, after each of which it receives a new cookie. All the cookies have the same Path attribute and (default) domain. Because the request-URIs all path-match /acme, the Path attribute of each cookie, each request contains all the cookies received so far.
This example illustrates the effect of the Path attribute. All detail of request and response headers has been omitted. Assume the user agent has no stored cookies.
Imagine the user agent has received, in response to earlier requests, the response headers
Set-Cookie2: Part_Number="Rocket_Launcher_0001"; Version="1"; Path="/acme"
Set-Cookie2: Part_Number="Riding_Rocket_0023"; Version="1"; Path="/acme/ammo"
A subsequent request by the user agent to the (same) server for URLs of the form /acme/ammo/... would include the following request header:
Cookie: $Version="1"; Part_Number="Riding_Rocket_0023"; $Path="/acme/ammo"; Part_Number="Rocket_Launcher_0001"; $Path="/acme"
Note that the NAME=VALUE pair for the cookie with the more specific Path attribute, /acme/ammo, comes before the one with the less specific Path attribute, /acme. Further note that the same cookie name appears more than once.
A subsequent request by the user agent to the (same) server for a URL of the form /acme/parts/ would include the following request header:
Cookie: $Version="1"; Part_Number="Rocket_Launcher_0001"; $Path="/acme"
Here, the second cookie's Path attribute /acme/ammo is not a prefix of the request URL, /acme/parts/, so the cookie does not get forwarded to the server.
Here we provide guidance on likely or desirable details for an origin server that implements state management.
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-Cookie2 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. Of course, using a database creates some problems that this state management specification was meant to avoid, namely:
Caching benefits the scalability of WWW. Therefore it is important to reduce the number of documents that have state embedded in them inherently. For example, if a shopping-basket-style application always displays a user's current basket contents on each page, those pages cannot be cached, because each user's basket's contents would be different. On the other hand, if each page contains just a link that allows the user to "Look at My Shopping Basket", the page can be cached.
Practical user agent implementations have limits on the number and size of cookies that they can store. In general, user agents' cookie support should have no fixed limits. They should strive to store as many frequently-used cookies as possible. Furthermore, general-use user agents SHOULD provide each of the following minimum capabilities individually, although not necessarily simultaneously:
User agents created for specific purposes or for limited-capacity devices SHOULD provide at least 20 cookies of 4096 bytes, to ensure that the user can interact with a session-based origin server.
The information in a Set-Cookie2 response header MUST be retained in its entirety. If for some reason there is inadequate space to store the cookie, it 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.
User agents MAY choose to set an upper bound on the number of cookies to be stored from a given host or domain name or on the size of the cookie information. Otherwise a malicious server could attempt to flood a user agent with many cookies, or large cookies, on successive responses, which would force out cookies the user agent had received from other servers. However, the minima specified above SHOULD still be supported.
An origin server could create a Set-Cookie2 header to track the path of a user through the server. Users may object to this behavior as an intrusive accumulation of information, even if their identity is not evident. (Identity might become evident, for example, if a user subsequently fills out a form that contains identifying information.) This state management specification therefore requires that a user agent give the user control over such a possible intrusion, although the interface through which the user is given this control is left unspecified. However, the control mechanisms provided SHALL at least allow the user
Such control could be provided, for example, by mechanisms
A user agent usually begins execution with no remembered state information. It SHOULD be possible to configure a user agent never to send Cookie headers, in which case it can never sustain state with an origin server. (The user agent would then behave like one that is unaware of how to handle Set-Cookie2 response headers.)
When the user agent terminates execution, it SHOULD let the user discard all state information. Alternatively, the user agent MAY ask the user whether state information should be retained; the default should be "no". If the user chooses to retain state information, it would be restored the next time the user agent runs.
NOTE: User agents should probably be cautious about using files to store cookies long-term. If a user runs more than one instance of the user agent, the cookies could be commingled or otherwise corrupted.
An origin server SHOULD promote informed consent by adding CommentURL or Comment information to the cookies it sends. CommentURL is preferred because of the opportunity to provide richer information in a multiplicity of languages.
The information in the Set-Cookie2 and Cookie headers is unprotected. As a consequence:
These facts imply that information of a personal and/or financial nature should only 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.
A user agent in a shared user environment poses a further risk. Using a cookie inspection interface, User B could examine the contents of cookies that were saved when User A used the machine.
The restrictions on the value of the Domain attribute, and the rules concerning unverifiable transactions, are meant to reduce the ways that cookies can "leak" to the "wrong" site. The intent is to restrict cookies to one host, or a closely related set of hosts. Therefore a request-host is limited as to what values it can set for Domain. We consider it acceptable for hosts host1.foo.com and host2.foo.com to share cookies, but not a.com and b.com.
Similarly, a server can set a Path only for cookies that are related to the request-URI.
Proper application design can avoid spoofing attacks from related domains. Consider:
Cookie: $Version="1"; session_id="1234", $Version="1"; session_id="1111"; $Domain=".cracker.edu"
A user agent SHOULD make every attempt to prevent the sharing of session information between hosts that are in different domains. Embedded or inlined objects may cause particularly severe privacy problems if they can be used to share cookies between disparate hosts. For example, a malicious server could embed cookie information for host a.com in a URI for a CGI on host b.com. User agent implementors are strongly encouraged to prevent this sort of exchange whenever possible.
While it is common practice to use them this way, cookies are not designed or intended to be used to hold authentication information, such as account names and passwords. Unless such cookies are exchanged over an encrypted path, the account information they contain is highly vulnerable to perusal and theft.
Apart from RFC 2109, three other proposals have been made to accomplish similar goals. This specification began as an amalgam of Kristol's State-Info proposal [DMK95] (Kristol, D., “Proposed HTTP State-Info Mechanism,” September 1995.) and Netscape's Cookie proposal [Netscape] (, “Persistent Client State -- HTTP Cookies,” .).
Brian Behlendorf proposed a Session-ID header that would be user-agent-initiated and could be used by an origin server to track "clicktrails". It would not carry any origin-server-defined state, however. Phillip Hallam-Baker has proposed another client-defined session ID mechanism for similar purposes.
While both session IDs and cookies can provide a way to sustain stateful sessions, their intended purpose is different, and, consequently, the privacy requirements for them are different. A user initiates session IDs to allow servers to track progress through them, or to distinguish multiple users on a shared machine. Cookies are server-initiated, so the cookie mechanism described here gives users control over something that would otherwise take place without the users' awareness. Furthermore, cookies convey rich, server-selected information, whereas session IDs comprise user-selected, simple information.
Existing cookie implementations, based on the Netscape specification, use the Set-Cookie (not Set-Cookie2) header. User agents that receive in the same response both a Set-Cookie and Set-Cookie2 response header for the same cookie MUST discard the Set-Cookie information and use only the Set-Cookie2 information. Furthermore, a user agent MUST assume, if it received a Set-Cookie2 response header, that the sending server complies with this document and will understand Cookie request headers that also follow this specification.
New cookies MUST replace both equivalent old- and new-style cookies. That is, if a user agent that follows both this specification and Netscape's original specification receives a Set-Cookie2 response header, and the NAME and the Domain and Path attributes match (per the Cookie Management section) a Netscape-style cookie, the Netscape-style cookie MUST be discarded, and the user agent MUST retain only the cookie adhering to this specification.
Older user agents that do not understand this specification, but that do understand Netscape's original specification, will not recognize the Set-Cookie2 response header and will receive and send cookies according to the older specification.
A user agent that supports both this specification and Netscape-style cookies SHOULD send a Cookie request header that follows the older Netscape specification if it received the cookie in a Set-Cookie response header and not in a Set-Cookie2 response header. However, it SHOULD send the following request header as well:
The Cookie2 header advises the server that the user agent understands new-style cookies. If the server understands new-style cookies, as well, it SHOULD continue the stateful session by sending a Set-Cookie2 response header, rather than Set-Cookie. A server that does not understand new-style cookies will simply ignore the Cookie2 request header.
Some caches, such as those conforming to HTTP/1.0, will inevitably cache the Set-Cookie2 and Set-Cookie headers, because there was no mechanism to suppress caching of headers prior to HTTP/1.1. This caching can lead to security problems. Documents transmitted by an origin server along with Set-Cookie2 and Set-Cookie headers usually either will be uncachable, or will be "pre-expired". As long as caches obey instructions not to cache documents (following Expires: <a date in the past> or Pragma: no-cache (HTTP/1.0), or Cache-control: no-cache (HTTP/1.1)) uncachable documents present no problem. However, pre-expired documents may be stored in caches. They require validation (a conditional GET) on each new request, but some cache operators loosen the rules for their caches, and sometimes serve expired documents without first validating them. This combination of factors can lead to cookies meant for one user later being sent to another user. The Set-Cookie2 and Set-Cookie headers are stored in the cache, and, although the document is stale (expired), the cache returns the document in response to later requests, including cached headers.
This document really represents the collective efforts of the HTTP Working Group of the IETF and, particularly, the following people, in addition to the authors: Roy Fielding, Yaron Goland, Marc Hedlund, Ted Hardie, Koen Holtman, Shel Kaphan, Rohit Khare, Foteos Macrides, David W. Morris.
|[DMK95]||Kristol, D., “Proposed HTTP State-Info Mechanism,” September 1995.|
available at <http://portal.research.bell-labs.com/~dmk/state-info.html>
|[Netscape]||“Persistent Client State -- HTTP Cookies.”|
available at <http://www.netscape.com/newsref/std/cookie_spec.html>
|[RFC2109]||Kristol, D. and L. Montulli, “HTTP State Management Mechanism,” RFC 2109, February 1997.|
|[RFC2119]||Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” BCP 14, RFC 2119, March 1997.|
|[RFC2279]||Yergeau, F., “UTF-8, a transformation format of ISO 10646,” RFC 2279, January 1998.|
|[RFC2396]||Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifiers (URI): Generic Syntax,” RFC 2396, August 1998.|
|[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.|
|David M. Kristol|
|Bell Laboratories, Lucent Technologies|
|600 Mountain Ave. Room 2A-333|
|Murray Hill, NJ 07974|
|2037 Landings Dr.|
|Mountain View, CA 94301|
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