HTTP Working Group David M. Kristol
INTERNET DRAFT Bell Laboratories, Lucent Technologies
Lou Montulli
Netscape Communications
<draft-ietf-http-state-mgmt-02.txt>
June 13, 1996 Expires December 13, 1996
Proposed HTTP State Management Mechanism
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are
working documents of the Internet Engineering Task Force
(IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six
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``work in progress.''
To learn the current status of any Internet-Draft, please
check the ``1id-abstracts.txt'' listing contained in the
Internet- Drafts Shadow Directories on ftp.is.co.za (Africa),
nic.nordu.net (Europe), munnari.oz.au (Pacific Rim),
ds.internic.net (US East Coast), or ftp.isi.edu (US West
Coast).
This is authors' draft 2.25.
1. ABSTRACT
This proposal 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 numeric IP address of
a host, or its full Internet domain name, resolved to a top-level domain
such as .com or .uk (fully qualified domain name, FQDN). The fully
qualified domain name is preferred.
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The terms request-host and request-URI refer to the values the client
would send to the server as, respectively, the host (but not port) and
abs_path portions of the absoluteURI (http_URL) of the HTTP request
line. Note that request-host must be a FQHN.
Hosts names can be specified either as an IP address or a FQHN string.
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 proposal 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 proposed 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, may 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.
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4. The session is implicit in the exchange of state information.
4. PROPOSAL OUTLINE
The proposal we outline here defines 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 of the proposal 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
uses the notation and tokens DIGIT (decimal digits), token (informally,
a sequence of non-special, non-white space characters), and word
(informally, a "-quoted string or token) from the HTTP/1.1 specification
to describe their syntax.
av-pairs = av-pair *(";" av-pair)
av-pair = attr 0,1*("=" value) ; optional value
attr = token
value = word
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 space 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 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
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Set-Cookie header that has an immediate expiration time.
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 = "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.
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.
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
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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.
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 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.
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* 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.
Because HTTP/1.0 caches ignore Cache-control directives, it might also
be appropriate to send Expires:<a date in the past> (example: Expires:
Thu, 01 Jan 1980 00:00:00 GMT) to avoid improper caching in older
caches.
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.
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* 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.
* 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=VALUE, Domain, and Path attribute values exactly match
those of a pre-existing cookie, the new one 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.
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;
* the request-URI;
* the cookie's age.
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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.
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.
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 proposal
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
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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.''
NB: Many current user agents already provide an acceptable review
option that would render many links verifiable:
* When the mouse is over a link, the user agents display the link
that would be followed.
* By letting a user view source, or save and examine source, the user
agents let a user examine the link that would be followed when a
form's submit button is selected.
* When automatic image loading is disabled, users can review the
links that correspond to images by using one of the above
techniques. However, if automatic image loading is enabled, those
links would be unverifiable.
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"; $Path="/acme"
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$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.
* 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
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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.
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
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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
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.
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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 into a server-side database. Of course, using a
database creates some problems that the state management proposal 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)
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* 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.
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.) The state management
proposal 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.
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* 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 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.'' Any retained state information would then 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
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origin server should be vigilant to prevent a bad Cookie value from
causing failures.
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
proposal 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.
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. Cookies are
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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.
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.
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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.
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: dmk@bell-labs.com
Lou Montulli
Netscape Communications Corp.
501 E. Middlefield Rd.
Mountain View, CA 94043
Phone: (415) 528-2600
Email: montulli@netscape.com
Expires December 13, 1996
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