Network Working Group D. Kristol

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.

1. ABSTRACT

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
section.)

2. TERMINOLOGY

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, 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.

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.

3. 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 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
session.

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

4. OUTLINE

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.

NAME=VALUE
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
header.

Comment=comment
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.

Domain=domain
Optional. The Domain attribute specifies the domain for which the
cookie is valid. An explicitly specified domain must always start
with a dot.

Max-Age=delta-seconds
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.

Path=path
Optional. The Path attribute specifies the subset of URLs to
which this cookie applies.

Secure
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.

Version=version
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-
cache="set-cookie".

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
proxies.

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-
URI.

* 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
attribute.

* 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.

Examples:

* A Set-Cookie from request-host y.x.foo.com for Domain=.foo.com
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 x.foo.com for Domain=.foo.com would
be accepted.

* A Set-Cookie with Domain=.com or Domain=.com., will always be
rejected, because there is no embedded dot.

* A Set-Cookie with Domain=ajax.com 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
information.

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
header.

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
request-URI.

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
transactions.

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";
$Path="/acme"

$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
rules.

* 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
header.

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. EXAMPLES

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";
Path="/acme"

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";
Path="/acme"

and

Set-Cookie: 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.

6. IMPLEMENTATION CONSIDERATIONS

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
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

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
cached.

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. PRIVACY

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
progress.

* 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
up.

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 host1.foo.com and
host2.foo.com to share cookies, but not a.com and b.com.

Similarly, a server can only set a Path for cookies that are related
to the request-URI.

8. SECURITY CONSIDERATIONS

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 victim.cracker.edu, gets back
cookie session_id="1234" and sets the default domain
victim.cracker.edu.

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
passes

Cookie: $Version="1";
session_id="1234";
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.

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 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.

9. OTHER, SIMILAR, PROPOSALS

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. HISTORICAL

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
attribute.

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:

Wdy, DD-Mon-YY HH:MM:SS GMT

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
Expires.

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.

11. ACKNOWLEDGEMENTS

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

12. AUTHORS' ADDRESSES

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
EMail: [email protected]

Lou Montulli
Netscape Communications Corp.
501 E. Middlefield Rd.
Mountain View, CA 94043

Phone: (415) 528-2600
EMail: [email protected]

Kristol & Montulli Standards Track [Page 21]