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Network Working Group D. Kristol Request for Comments: 2109 Bell Laboratories, Lucent Technologies Category: Standards Track L. Montulli

                                               Netscape Communications
                                                         February 1997
                  HTTP State Management Mechanism

Status of this Memo

 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.


 This document specifies a way to create a stateful session with HTTP
 requests and responses.  It describes two new headers, Cookie and
 Set-Cookie, which carry state information between participating
 origin servers and user agents.  The method described here differs
 from Netscape's Cookie proposal, but it can interoperate with
 HTTP/1.0 user agents that use Netscape's method.  (See the HISTORICAL


 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.
 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.

Kristol & Montulli Standards Track [Page 1] RFC 2109 HTTP State Management Mechanism February 1997

 Hosts names can be specified either as an IP address or a FQHN
 string.  Sometimes we compare one host name with another.  Host A's
 name domain-matches host B's if
  • both host names are IP addresses and their host name strings match

exactly; or

  • both host names are FQDN strings and their host name strings match

exactly; or

  • A is a FQDN string and has the form NB, where N is a non-empty name

string, B has the form .B', and B' is a FQDN string. (So,

   domain-matches but not
 Note that domain-match is not a commutative operation:
 domain-matches, but not the reverse.
 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.


 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 technique 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.
 There are, of course, many different potential contexts and thus many
 different potential types of session.  The designers' paradigm for
 sessions created by the exchange of cookies has these key attributes:
    1.  Each session has a beginning and an end.
    2.  Each session is relatively short-lived.
    3.  Either the user agent or the origin server may terminate a
    4.  The session is implicit in the exchange of state information.

Kristol & Montulli Standards Track [Page 2] RFC 2109 HTTP State Management Mechanism February 1997


 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 goal is to have a minimal
 impact on HTTP and user agents.  Only origin servers that need to
 maintain sessions would suffer any significant impact, and that
 impact can largely be confined to Common Gateway Interface (CGI)
 programs, unless the server provides more sophisticated state
 management support.  (See Implementation Considerations, below.)

4.1 Syntax: General

 The two state management headers, Set-Cookie and Cookie, have common
 syntactic properties involving attribute-value pairs.  The following
 grammar uses the notation, and tokens DIGIT (decimal digits) and
 token (informally, a sequence of non-special, non-white space
 characters) from the HTTP/1.1 specification [RFC 2068] to describe
 their syntax.
 av-pairs        =       av-pair *(";" av-pair)
 av-pair         =       attr ["=" value]        ; optional value
 attr            =       token
 value           =       word
 word            =       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.

4.2 Origin Server Role

4.2.1 General

 The origin server initiates a session, if it so desires.  (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).  To initiate a session, the
 origin server returns an extra response header to the client, Set-
 Cookie.  (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

Kristol & Montulli Standards Track [Page 3] RFC 2109 HTTP State Management Mechanism February 1997

 session.  It may send back to the client a Set-Cookie response header
 with the same or different information, or it may send no Set-Cookie
 header at all.  The origin server effectively ends a session by
 sending the client a Set-Cookie header with Max-Age=0.
 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 could fold multiple such
 headers into a single header.

4.2.2 Set-Cookie Syntax

 The syntax for the Set-Cookie response header is
 set-cookie      =       "Set-Cookie:" cookies
 cookies         =       1#cookie
 cookie          =       NAME "=" VALUE *(";" cookie-av)
 NAME            =       attr
 VALUE           =       value
 cookie-av       =       "Comment" "=" value
                 |       "Domain" "=" value
                 |       "Max-Age" "=" value
                 |       "Path" "=" value
                 |       "Secure"
                 |       "Version" "=" 1*DIGIT
 Informally, the Set-Cookie response header comprises the token Set-
 Cookie:, 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 behavior is undefined.
    Required.  The name of the state information ("cookie") is NAME,
    and its value is VALUE.  NAMEs that begin with $ are reserved for
    other uses and must not be used by applications.

Kristol & Montulli Standards Track [Page 4] RFC 2109 HTTP State Management Mechanism February 1997

    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-Cookie
    Optional.  Because cookies can contain private information about a
    user, the Cookie attribute allows an origin server to document its
    intended use of a cookie.  The user can inspect the information to
    decide whether to initiate or continue a session with this cookie.
    Optional.  The Domain attribute specifies the domain for which the
    cookie is valid.  An explicitly specified domain must always start
    with a dot.
    Optional.  The Max-Age attribute defines the lifetime of the
    cookie, in seconds.  The delta-seconds value is a decimal non-
    negative integer.  After delta-seconds seconds elapse, the client
    should discard the cookie.  A value of zero means the cookie
    should be discarded immediately.
    Optional.  The Path attribute specifies the subset of URLs to
    which this cookie applies.
    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.
    The user agent (possibly under the user's control) 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.
    Required.  The Version attribute, a decimal integer, identifies to
    which version of the state management specification the cookie
    conforms.  For this specification, Version=1 applies.

Kristol & Montulli Standards Track [Page 5] RFC 2109 HTTP State Management Mechanism February 1997

4.2.3 Controlling Caching

 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:
  • To suppress caching of the Set-Cookie header: Cache-control: no-


 and one of the following:
  • To suppress caching of a private document in shared caches: Cache-

control: private.

  • To allow caching of a document and require that it be validated

before returning it to the client: Cache-control: must-revalidate.

  • To allow caching of a document, but to require that proxy caches

(not user agent caches) validate it before returning it to the

   client: Cache-control: proxy-revalidate.
  • To allow caching of a document and request that it be validated

before returning it to the client (by "pre-expiring" it):

   Cache-control: max-age=0.  Not all caches will revalidate the
   document in every case.
 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 downsteam 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

Kristol & Montulli Standards Track [Page 6] RFC 2109 HTTP State Management Mechanism February 1997

4.3 User Agent Role

4.3.1 Interpreting Set-Cookie

 The user agent keeps separate track of state information that arrives
 via Set-Cookie response headers from each origin server (as
 distinguished by name or IP address and port).  The user agent
 applies these defaults for optional attributes that are missing:
 VersionDefaults to "old cookie" behavior as originally specified by
        Netscape.  See the HISTORICAL section.
 Domain Defaults to the request-host.  (Note that there is no dot at
        the beginning of request-host.)
 Max-AgeThe default behavior is to discard the cookie when the user
        agent exits.
 Path   Defaults to the path of the request URL that generated the
        Set-Cookie response, up to, but not including, the
        right-most /.
 Secure If absent, the user agent may send the cookie over an
        insecure channel.

4.3.2 Rejecting Cookies

 To prevent possible security or privacy violations, a user agent
 rejects a cookie (shall not store its information) if any of the
 following is true:
  • The value for the Path attribute is not a prefix of the request-


  • The value for the Domain attribute contains no embedded dots or

does not start with a dot.

  • The value for the request-host does not domain-match the Domain


  • The request-host is a FQDN (not IP address) and has the form HD,

where D is the value of the Domain attribute, and H is a string

   that contains one or more dots.
  • A Set-Cookie from request-host for

would be rejected, because H is y.x and contains a dot.

Kristol & Montulli Standards Track [Page 7] RFC 2109 HTTP State Management Mechanism February 1997

  • A Set-Cookie from request-host for would

be accepted.

  • A Set-Cookie with or, will always be

rejected, because there is no embedded dot.

  • A Set-Cookie with will be rejected because the

value for Domain does not begin with a dot.

4.3.3 Cookie Management

 If a user agent receives a Set-Cookie response header whose NAME is
 the same as a pre-existing cookie, and whose Domain and Path
 attribute values exactly (string) match those of a pre-existing
 cookie, the new cookie supersedes the old.  However, if the Set-
 Cookie has a value for Max-Age of zero, the (old and new) cookie is
 discarded.  Otherwise cookies accumulate until they expire (resources
 permitting), at which time they are discarded.
 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-Cookie 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.
 User agents should allow the user to control cookie destruction.  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

4.3.4 Sending Cookies to the Origin Server

 When it sends a request to an origin server, the user agent sends a
 Cookie request header to the origin server if it has cookies that are
 applicable to the request, based on
  • the request-host;

Kristol & Montulli Standards Track [Page 8] RFC 2109 HTTP State Management Mechanism February 1997

  • the request-URI;
  • the cookie's age.
 The syntax for the header is:
 cookie          =       "Cookie:" cookie-version
                         1*((";" | ",") cookie-value)
 cookie-value    =       NAME "=" VALUE [";" path] [";" domain]
 cookie-version  =       "$Version" "=" value
 NAME            =       attr
 VALUE           =       value
 path            =       "$Path" "=" value
 domain          =       "$Domain" "=" value
 The value of the cookie-version attribute must be the value from the
 Version attribute, if any, of the corresponding Set-Cookie 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 any,
 of the corresponding Set-Cookie 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 any, of the corresponding Set-Cookie response header.
 Otherwise the attribute should be omitted from the Cookie request
 Note that there is no Comment attribute in the Cookie request header
 corresponding to the one in the Set-Cookie response header.  The user
 agent does not return the comment information to the origin server.
 The following rules apply to choosing applicable cookie-values from
 among all the cookies the user agent has.
 Domain Selection
      The origin server's fully-qualified host name must domain-match
      the Domain attribute of the cookie.
 Path Selection
      The Path attribute of the cookie must match a prefix of the
 Max-Age Selection
      Cookies that have expired should have been discarded and thus
      are not forwarded to an origin server.

Kristol & Montulli Standards Track [Page 9] RFC 2109 HTTP State Management Mechanism February 1997

 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.

4.3.5 Sending Cookies in Unverifiable Transactions

 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 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
 When it makes an unverifiable transaction, a user agent must enable a
 session only if a cookie with a domain attribute D was sent or
 received in its origin transaction, such that the host name in the
 Request-URI of the unverifiable transaction domain-matches D.
 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".
 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

Kristol & Montulli Standards Track [Page 10] RFC 2109 HTTP State Management Mechanism February 1997

 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.

4.4 How an Origin Server Interprets the Cookie Header

 A user agent returns much of the information in the Set-Cookie header
 to the origin server when the Path attribute matches that of a new
 request.  When it receives a Cookie header, the origin server should
 treat cookies with NAMEs whose prefix is $ specially, as an attribute
 for the adjacent cookie.  The value for such a NAME is to be
 interpreted as applying to the lexically (left-to-right) most recent
 cookie whose name does not have the $ prefix.  If there is no
 previous cookie, the value applies to the cookie mechanism as a
 whole.  For example, consider the cookie
 Cookie: $Version="1"; Customer="WILE_E_COYOTE";
 $Version applies to the cookie mechanism as a whole (and gives the
 version number for the cookie mechanism).  $Path is an attribute
 whose value (/acme) defines the Path attribute that was used when the
 Customer cookie was defined in a Set-Cookie response header.

4.5 Caching Proxy Role

 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:
  • Honor requests from the cache, if possible, based on cache validity


  • Pass along a Cookie request header in any request that the proxy

must make of another server.

  • Return the response to the client. Include any Set-Cookie response


Kristol & Montulli Standards Track [Page 11] RFC 2109 HTTP State Management Mechanism February 1997

  • Cache the received response subject to the control of the usual

headers, such as Expires, Cache-control: no-cache, and Cache-

   control: private,
  • Cache the Set-Cookie subject to the control of the usual header,

Cache-control: no-cache="set-cookie". (The Set-Cookie header

   should usually not be cached.)
 Proxies must not introduce Set-Cookie (Cookie) headers of their own
 in proxy responses (requests).


5.1 Example 1

 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-Cookie: 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-Cookie: Part_Number="Rocket_Launcher_0001"; Version="1";
       Shopping basket contains an item.

Kristol & Montulli Standards Track [Page 12] RFC 2109 HTTP State Management Mechanism February 1997

   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-Cookie: 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 URLs
 all have /acme as a prefix, and that matches the Path attribute, each
 request contains all the cookies received so far.

5.2 Example 2

 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

Kristol & Montulli Standards Track [Page 13] RFC 2109 HTTP State Management Mechanism February 1997

 Set-Cookie: Part_Number="Rocket_Launcher_0001"; Version="1";
 Set-Cookie: Part_Number="Riding_Rocket_0023"; Version="1";
 A subsequent request by the user agent to the (same) server for URLs
 of the form /acme/ammo/...  would include the following request
 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 speculate on likely or desirable details for an origin server
 that implements state management.

6.1 Set-Cookie Content

 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
 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

Kristol & Montulli Standards Track [Page 14] RFC 2109 HTTP State Management Mechanism February 1997

 a database creates some problems that this state management
 specification was meant to avoid, namely:
   1.  keeping real state on the server side;
   2.  how and when to garbage-collect the database entry, in case the
       user agent terminates the session by, for example, exiting.

6.2 Stateless Pages

 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

6.3 Implementation Limits

 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:
  • at least 300 cookies
  • at least 4096 bytes per cookie (as measured by the size of the

characters that comprise the cookie non-terminal in the syntax

      description of the Set-Cookie header)
  • at least 20 cookies per unique host or domain name
 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-Cookie 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.

Kristol & Montulli Standards Track [Page 15] RFC 2109 HTTP State Management Mechanism February 1997

6.3.1 Denial of Service Attacks

 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.


7.1 User Agent Control

 An origin server could create a Set-Cookie 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 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
  • to completely disable the sending and saving of cookies.
  • to determine whether a stateful session is in progress.
  • to control the saving of a cookie on the basis of the cookie's

Domain attribute.

 Such control could be provided by, for example, mechanisms
  • to notify the user when the user agent is about to send a cookie

to the origin server, offering the option not to begin a session.

  • to display a visual indication that a stateful session is in


  • to let the user decide which cookies, if any, should be saved

when the user concludes a window or user agent session.

  • to let the user examine the contents of a cookie at any time.
 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

Kristol & Montulli Standards Track [Page 16] RFC 2109 HTTP State Management Mechanism February 1997

 an origin server.  (The user agent would then behave like one that is
 unaware of how to handle Set-Cookie 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 messed

7.2 Protocol Design

 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, 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 and to share cookies, but not and
 Similarly, a server can only set a Path for cookies that are related
 to the request-URI.


8.1 Clear Text

 The information in the Set-Cookie and Cookie headers is unprotected.
 Two consequences are:
 1.  Any sensitive information that is conveyed in them is exposed
     to intruders.
 2.  A malicious intermediary could alter the headers as they travel
     in either direction, with unpredictable results.
 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.

Kristol & Montulli Standards Track [Page 17] RFC 2109 HTTP State Management Mechanism February 1997

8.2 Cookie Spoofing

 Proper application design can avoid spoofing attacks from related
 domains.  Consider:
   1.  User agent makes request to, gets back
       cookie session_id="1234" and sets the default domain
   2.  User agent makes request to, gets back
       cookie session-id="1111", with Domain="".
   3.  User agent makes request to again, and
       Cookie: $Version="1";
                       session_id="1111"; $Domain=""
       The server at should detect that the second
       cookie was not one it originated by noticing that the Domain
       attribute is not for itself and ignore it.

8.3 Unexpected Cookie Sharing

 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 in a URI for a CGI on host  User
 agent implementors are strongly encouraged to prevent this sort of
 exchange whenever possible.


 Three other proposals have been made to accomplish similar goals.
 This specification is an amalgam of Kristol's State-Info proposal and
 Netscape's Cookie proposal.
 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.

Kristol & Montulli Standards Track [Page 18] RFC 2109 HTTP State Management Mechanism February 1997

 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.


10.1 Compatibility With Netscape's Implementation

 HTTP/1.0 clients and servers may use Set-Cookie and Cookie headers
 that reflect Netscape's original cookie proposal.  These notes cover
 inter-operation between "old" and "new" cookies.

10.1.1 Extended Cookie Header

 This proposal adds attribute-value pairs to the Cookie request header
 in a compatible way.  An "old" client that receives a "new" cookie
 will ignore attributes it does not understand; it returns what it
 does understand to the origin server.  A "new" client always sends
 cookies in the new form.
 An "old" server that receives a "new" cookie will see what it thinks
 are many cookies with names that begin with a $, and it will ignore
 them.  (The "old" server expects these cookies to be separated by
 semi-colon, not comma.)  A "new" server can detect cookies that have
 passed through an "old" client, because they lack a $Version

10.1.2 Expires and Max-Age

 Netscape's original proposal defined an Expires header that took a
 date value in a fixed-length variant format in place of Max-Age:
 Note that the Expires date format contains embedded spaces, and that
 "old" cookies did not have quotes around values.  Clients that
 implement to this specification should be aware of "old" cookies and

Kristol & Montulli Standards Track [Page 19] RFC 2109 HTTP State Management Mechanism February 1997

10.1.3 Punctuation

 In Netscape's original proposal, the values in attribute-value pairs
 did not accept "-quoted strings.  Origin servers should be cautious
 about sending values that require quotes unless they know the
 receiving user agent understands them (i.e., "new" cookies).  A
 ("new") user agent should only use quotes around values in Cookie
 headers when the cookie's version(s) is (are) all compliant with this
 specification or later.
 In Netscape's original proposal, no whitespace was permitted around
 the = that separates attribute-value pairs.  Therefore such
 whitespace should be used with caution in new implementations.

10.2 Caching and HTTP/1.0

 Some caches, such as those conforming to HTTP/1.0, will inevitably
 cache the Set-Cookie header, 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-Cookie headers will usually either 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-Cookie header is 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
 following people, in addition to the authors: Roy Fielding, Marc
 Hedlund, Ted Hardie, Koen Holtman, Shel Kaphan, Rohit Khare.

Kristol & Montulli Standards Track [Page 20] RFC 2109 HTTP State Management Mechanism February 1997


 David M. Kristol
 Bell Laboratories, Lucent Technologies
 600 Mountain Ave.  Room 2A-227
 Murray Hill, NJ  07974
 Phone: (908) 582-2250
 Fax: (908) 582-5809
 Lou Montulli
 Netscape Communications Corp.
 501 E. Middlefield Rd.
 Mountain View, CA  94043
 Phone: (415) 528-2600

Kristol & Montulli Standards Track [Page 21]

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