Network Working Group D. Kristol
Request for Comments: 2965 Bell Laboratories, Lucent Technologies
Obsoletes: 2109 L. Montulli
Category: Standards Track Epinions.com, Inc.
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.
Copyright (C) The Internet Society (2000). All Rights Reserved.
The IESG notes that this mechanism makes use of the .local top-level
domain (TLD) internally when handling host names that don't contain
any dots, and that this mechanism might not work in the expected way
should an actual .local TLD ever be registered.
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], 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
The terms user agent, client, server, proxy, origin server, and
http_URL have the same meaning as in the HTTP/1.1 specification
[RFC2616]. The terms abs_path and absoluteURI have the same meaning
as in the URI Syntax specification [RFC2396].
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
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
* their host name strings string-compare equal; or
* A is a HDN string and has the form NB, where N is a non-empty
name string, B has the form .B', and B' is a HDN string. (So,
x.y.com domain-matches .Y.com but not Y.com.)
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:
- H is the host domain name of a host; and,
- H has the form A.B; and
- A has no embedded (that is, interior) dots; and
- B has at least one embedded dot, or B is the string "local".
then the reach of H is .B.
* Otherwise, the reach of H is H.
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].
2. STATE AND SESSIONS
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.
3.1 Syntax: General
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] to describe
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.
3.2 Origin Server Role
3.2.1 General 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.
3.2.2 Set-Cookie2 Syntax The syntax for the Set-Cookie2 response
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 <">
| "Domain" "=" value
| "Max-Age" "=" value
| "Path" "=" value
| "Port" [ "=" <"> portlist <"> ]
| "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
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
information about a user, the value of the Comment attribute
allows an origin server to document how it intends to use the
cookie. The user can inspect the information to decide whether to
initiate or continue a session with this cookie. Characters in
value MUST be in UTF-8 encoding. [RFC2279]
information about a user, the CommentURL attribute allows an
origin server to document how it intends to use the cookie. The
user can inspect the information identified by the URL to decide
whether to initiate or continue a session with this cookie.
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]. 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
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
3.2.3 Controlling Caching 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:
* To suppress caching of the Set-Cookie2 header:
and one of the following:
* To suppress caching of a private document in shared caches:
* To allow caching of a document and require that it be validated
before returning it to the client:
Cache-Control: must-revalidate, max-age=0
* 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, max-age=0
* To allow caching of a document and request that it be validated
before returning it to the client (by "pre-expiring" it):
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-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
3.3 User Agent Role
3.3.1 Interpreting Set-Cookie2 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:
Discard The default behavior is dictated by the presence or absence
of a Max-Age attribute.
Domain 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.)
Max-Age The default behavior is to discard the cookie when the user
Path Defaults to the path of the request URL that generated the
Set-Cookie2 response, up to and including the right-most /.
Port The default behavior is that a cookie MAY be returned to any
Secure If absent, the user agent MAY send the cookie over an
3.3.2 Rejecting Cookies 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:
* The value for the Path attribute is not a prefix of the
* The value for the Domain attribute contains no embedded dots,
and the value is not .local.
* The effective host name that derives from the request-host does
not domain-match the Domain attribute.
* The request-host is a HDN (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.
* The Port attribute has a "port-list", and the request-port was
not in the list.
* A Set-Cookie2 from request-host y.x.foo.com for Domain=.foo.com
would be rejected, because H is y.x and contains a dot.
* A Set-Cookie2 from request-host x.foo.com for Domain=.foo.com
would be accepted.
* A Set-Cookie2 with Domain=.com or Domain=.com., will always be
rejected, because there is no embedded dot.
* A Set-Cookie2 with Domain=ajax.com will be accepted, and the
value for Domain will be taken to be .ajax.com, because a dot
gets prepended to the value.
* A Set-Cookie2 with Port="80,8000" will be accepted if the
request was made to port 80 or 8000 and will be rejected
* A Set-Cookie2 from request-host example for Domain=.local will
be accepted, because the effective host name for the request-
host is example.local, and example.local domain-matches .local.
3.3.3 Cookie Management 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 receives a cookie that contains a CommentURL
* the user agent's cookie inspection interface is configured so
that it presents a dialog to the user before the user agent
accepts the cookie;
* the dialog allows the user to follow the CommentURL link when
the user agent receives the cookie; and,
* when the user follows the CommentURL link, the origin server
(or another server, via other links in the returned content)
returns another cookie.
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
Privacy considerations dictate that the user have considerable
control over cookie management. The PRIVACY section contains more
3.3.4 Sending Cookies to the Origin Server 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 request-host and request-port;
* 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] [";" 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.
The origin server's effective host name MUST domain-match the
Domain attribute of the cookie.
There are three possible behaviors, depending on the Port
attribute in the Set-Cookie2 response header:
1. By default (no Port attribute), the cookie MAY be sent to any
2. 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
3. If the attribute has a port-list, the cookie MUST only be
returned if the new request-port is one of those listed in
The request-URI MUST path-match the Path attribute of the cookie.
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.
3.3.5 Identifying What Version is Understood: Cookie2 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
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.
3.3.6 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, 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
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
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.
3.4 How an Origin Server Interprets the Cookie Header
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.
3.5 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
* 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-Cookie2
* Cache the received response subject to the control of the usual
headers, such as Expires,
* Cache the Set-Cookie2 subject to the control of the usual
(The Set-Cookie2 header should usually not be cached.)
Proxies MUST NOT introduce Set-Cookie2 (Cookie) headers of their own
in proxy responses (requests).
4.1 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
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"
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";
Shopping basket contains an item.
5. User Agent -> Server
POST /acme/shipping HTTP/1.1
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
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.
4.2 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
Set-Cookie2: Part_Number="Rocket_Launcher_0001"; Version="1";
Set-Cookie2: 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
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";
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.
5. IMPLEMENTATION CONSIDERATIONS
Here we provide guidance on likely or desirable details for an origin
server that implements state management.
5.1 Set-Cookie2 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-Cookie2 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
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.
5.2 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
5.3 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 characters
that comprise the cookie non-terminal in the syntax description
of the Set-Cookie2 header, and as received in the Set-Cookie2
* 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-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.
5.3.1 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.
A user should be able to find out how a web site plans to use
information in a cookie and should be able to choose whether or not
those policies are acceptable. Both the user agent and the origin
server must assist informed consent.
6.1 User Agent Control
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
* 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
Such control could be provided, for example, by mechanisms
* to notify the user when the user agent is about to send a
cookie to the origin server, to offer the option not to begin a
* 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 and delete the contents of a cookie at
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
6.2 Origin Server Role
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.
6.3 Clear Text
The information in the Set-Cookie2 and Cookie headers is unprotected.
As a consequence:
1. Any sensitive information that is conveyed in them is exposed
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
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.
7. SECURITY CONSIDERATIONS
7.1 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 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.
7.2 Cookie Spoofing
Proper application design can avoid spoofing attacks from related
1. User agent makes request to victim.cracker.edu, gets back
cookie session_id="1234" and sets the default domain
2. User agent makes request to spoof.cracker.edu, gets back cookie
session-id="1111", with Domain=".cracker.edu".
3. User agent makes request to victim.cracker.edu again, and
Cookie: $Version="1"; session_id="1234",
$Version="1"; session_id="1111"; $Domain=".cracker.edu"
The server at victim.cracker.edu should detect that the second
cookie was not one it originated by noticing that the Domain
attribute is not for itself and ignore it.
7.3 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 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.
7.4 Cookies For Account Information
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.
8. OTHER, SIMILAR, PROPOSALS
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] 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.
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,
9.1 Compatibility with Existing Implementations
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
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
9.2 Caching and HTTP/1.0
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.
11. AUTHORS' ADDRESSES
David M. Kristol
Bell Laboratories, Lucent Technologies
600 Mountain Ave. Room 2A-333
Murray Hill, NJ 07974
Phone: (908) 582-2250
Fax: (908) 582-1239
2037 Landings Dr.
Mountain View, CA 94301
[DMK95] Kristol, D.M., "Proposed HTTP State-Info Mechanism",
available at <http://portal.research.bell-
labs.com/~dmk/state-info.html>, September, 1995.
[Netscape] "Persistent Client State -- HTTP Cookies", available at
[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 Unicode
and ISO-10646", RFC 2279, January 1998.
[RFC2396] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC 2396,
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H. and T.
Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1",
RFC 2616, June 1999.
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