http-state Working Group A. Barth
Internet-Draft U.C. Berkeley
Expires: February 8, 2010 August 7, 2009
HTTP State Management Mechanism
draft-abarth-cookie-00
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Abstract
This document defines the HTTP Cookie and Set-Cookie headers.
NOTE:
This document is currently a "straw-man" cookie proposal. Much of
the text herein is completely wrong. If you have suggestions for
improving the draft, please send email to http-state@ietf.org.
Suggestions with test cases are especially appriciated.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. State and Sessions . . . . . . . . . . . . . . . . . . . . . . 5
4. Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Syntax: General . . . . . . . . . . . . . . . . . . . . . 6
4.2. Origin Server Role . . . . . . . . . . . . . . . . . . . . 6
4.2.1. General . . . . . . . . . . . . . . . . . . . . . . . 6
4.2.2. Set-Cookie Syntax . . . . . . . . . . . . . . . . . . 7
4.2.3. Controlling Caching . . . . . . . . . . . . . . . . . 9
4.3. User Agent Role . . . . . . . . . . . . . . . . . . . . . 10
4.3.1. Interpreting Set-Cookie . . . . . . . . . . . . . . . 10
4.3.2. Rejecting Cookies . . . . . . . . . . . . . . . . . . 11
4.3.3. Cookie Management . . . . . . . . . . . . . . . . . . 12
4.3.4. Sending Cookies to the Origin Server . . . . . . . . . 12
4.3.5. Sending Cookies in Unverifiable Transactions . . . . . 14
4.4. How an Origin Server Interprets the Cookie Header . . . . 14
4.5. Caching Proxy Role . . . . . . . . . . . . . . . . . . . . 14
5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
5.1. Example 1 . . . . . . . . . . . . . . . . . . . . . . . . 15
5.2. Example 2 . . . . . . . . . . . . . . . . . . . . . . . . 16
6. Implementation Considerations . . . . . . . . . . . . . . . . 18
6.1. Set-Cookie Content . . . . . . . . . . . . . . . . . . . . 18
6.2. Implementation Limits . . . . . . . . . . . . . . . . . . 18
6.2.1. Denial of Service Attacks . . . . . . . . . . . . . . 19
7. Privacy . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
7.1. User Agent Control . . . . . . . . . . . . . . . . . . . . 20
7.2. Protocol Design . . . . . . . . . . . . . . . . . . . . . 21
8. Security Considerations . . . . . . . . . . . . . . . . . . . 22
8.1. Clear Text . . . . . . . . . . . . . . . . . . . . . . . . 22
8.2. Cookie Spoofing . . . . . . . . . . . . . . . . . . . . . 22
8.3. Unexpected Cookie Sharing . . . . . . . . . . . . . . . . 22
9. Other, Similar, Proposals . . . . . . . . . . . . . . . . . . 23
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 24
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 25
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1. Introduction
This document defines the HTTP Cookie and Set-Cookie header.
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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. [TODO: What does "strongly discouraged" mean?]
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
o both host names are IP addresses and their host name strings match
exactly; or
o both host names are FQDN strings and their host name strings match
exactly; or
o 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.
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3. State and Sessions
This document describes a way to create stateful sessions with HTTP
requests and responses. 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.
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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.
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.
[TODO: Test this grammar. I think there are many, many issue with
this grammer. For example, this grammar seems to permit whitespace
around the "=", but I don't think that actually works.]
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. [TODO: This is probably wrong, however.]
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 the client a Set-Cookie response header with
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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. [TODO: Need
to say something about Expires here.]
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. [TODO: Investigate how UAs cope with
such folded headers.]
4.2.2. Set-Cookie Syntax
The syntax for the Set-Cookie response header is
[TODO: Valdiate this syntax.]
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
[TODO: Expires is clearly missing.]
| "Path" "=" value
| "Secure"
[TODO: HTTPOnly is also missing.]
| "Version" "=" 1*DIGIT
[TODO: Version is likely a fantasy.]
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 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. [TODO: Test what happens when
attributes are multiply defined.]
NAME=VALUE
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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.
[TODO: I suspect the $ rule is a fantasy.] 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. [TODO: Does this actually exist?]
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. [TODO: Test what happens without 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. [TODO: Test negative integers.] 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. [TODO:
We should give better implementation advice than this.]
The user agent (possibly under the user's control) may
determine what level of security it considers appropriate for
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"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
confidentiality of the cookie's value.
Version=version
Required [TODO: Unlikely]. The Version attribute, a decimal
integer, identifies to which version of the state management
specification the cookie conforms. For this specification,
Version=1 applies. [TODO: Remove this attribute.]
4.2.3. Controlling Caching
[TODO: Should we go into this much detail here? This seems redudant
with the HTTP specs.]
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: [TODO: Is this good
advice?]
o To suppress caching of the Set-Cookie header: Cache-control: no-
cache="set-cookie".
and one of the following:
o To suppress caching of a private document in shared caches: Cache-
Control: private.
o 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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o 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.
o 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.
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:
Version Defaults to "old cookie" behavior as originally specified by
Netscape. See the HISTORICAL section. [TODO: Unlikely.]
Domain Defaults to the request-host. (Note that there is no dot at
the beginning of request-host.) [TODO: This is important to
test!]
Max-Age The default behavior is to discard the cookie when the user
agent exits. [TODO: Interaction with Expires.]
Expires The default behavior is to discard the cookie when the user
agent exits. [TODO: Interaction with Max-Age.]
Path Defaults to the path of the request URL that generated the Set-
Cookie response, up to, but not including, the right-most /.
[TODO: Test! This seems wrong for paths that are just a single
slash]
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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 must
reject a cookie (shall not store its information) if any of the
following is true:
o The value of the Path attribute is not a prefix of the request-
URI. [TODO: This is a lie.]
o The value for the Domain attribute contains no embedded dots or
does not start with a dot.
o The value for the request-host does not domain-match the Domain
attribute. [TODO: Test whether you can set a cookie for a
subdomain of yourself.]
o 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. [TODO: I don't think this is
right. foo.bar.baz.com can set a cookie for .baz.com]
o [TODO: Need to interact with public suffix list!]
Examples:
o 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. [TODO: I
don't think this is right.]
o A Set-Cookie from request-host x.foo.com for Domain=.foo.com would
be accepted.
o A Set-Cookie with Domain=.com or Domain=.com., will be rejected,
because there is no embedded dot.
o A Set-Cookie with Domain=foo.com will be rejected because the
value for Domain does not begin with a dot. [TODO: This seems
unlikely, but test!]
o A Set-Cookie with Domain=.co.uk will be rejected because .co.uk is
a public suffix.
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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. [TODO: Do cookies
really accumulate like this? Also, need to talk about Expires]
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.
[TODO: Consider recommending a cookie eviction strategy that works in
practice.]
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. [TODO: I think the Comment
attribute is a fantasy.]
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). [TODO:
Remove?]
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
o the request-host,
o the request-URI, and
o 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
[TODO: This syntax is entirely wrong.]
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. [TODO: Need a more complex algorithm here
involving the / character.]
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. [TODO: Figure out the
correct ordering.]
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.
[TODO: Test whether servers actually do this.]
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4.3.5. Sending Cookies in Unverifiable Transactions
[TODO: This entire section seems like a fantasy.]
[TODO: Consider explaining how third-party cookie blocking works.]
4.4. How an Origin Server Interprets the Cookie Header
[TODO: This section appears to be nonsense.]
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 [TODO: If they're already in the HTTP
specification, aren't they redundant here?]:
o Honor requests from the cache, if possible, based on cache
validity rules.
o Pass along a Cookie request header in any request that the proxy
must make of another server.
o Return the response to the client. Include any Set-Cookie
response header.
o Cache the received response subject to the control of the usual
headers, such as Expires, Cache-Control: no-cache, and Cache-
Control: private.
o 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).
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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. [TODO: This is insecure.]
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
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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.
[TODO: This example is really silly. We shouldn't be recommending
this at all.]
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"
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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 to a server-side resource. [TODO: Describe
briefly how to generate a decent session key.]
[TODO: We could recommend that servers encrypt and mac their cookie
data.]
[TODO: Mention issues that arise from having multiple concurrent
sessions.]
6.2. 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. [TODO: Why not?] 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: [TODO: Where do these numbers come from?]
o at least 300 cookies
o 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)
o 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.
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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. [TODO: Could
mention latency issues that arise from having tons of cookies.]
6.2.1. 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. [TODO: These minima still let an attacker exhaust the
entire cookie store. There's not much we can do about it though.]
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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
o to completely disable the sending and saving of cookies,
o to determine whether a stateful session is in progress, and
o 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
o 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,
o to display a visual indication that a stateful session is in
progress,
o to let the user decide which cookies, if any, should be saved when
the user concludes a window or user agent session, or
o 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. If the user
chooses to retain state information, it would be restored the next
time the user agent runs.
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7.2. Protocol Design
The restrictions on the value of the Domain attribute 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.
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8. Security Considerations
8.1. Clear Text
The information in the Set-Cookie and Cookie headers is transmitted
in the clear. Three consequences are:
1. Any sensitive information that is conveyed in in the headers is
exposed to an easedropper.
2. A malicious intermediary could alter the headers as they travel
in either direction, with unpredictable results.
3. A malicious client could alter the Cookie header before
transmission, with unpredictable results.
These facts imply that information of a personal and/or financial
nature should 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.
8.2. Cookie Spoofing
[TODO: Mention integrity issue where a sibling domain can inject
cookies.]
[TODO: Mention integrity issue where a HTTP can inject cookies into
HTTPS.]
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 host b.com. User agent
implementors are strongly encouraged to prevent this sort of exchange
whenever possible. [TODO: How are they supposed to do this? This
section makes little sense.]
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9. Other, Similar, Proposals
[TODO: Describe relation to the Netscape Cookie Spec, RFC 2109, RFC
2629, and cookie-v2.]
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Appendix A. Acknowledgements
This document borrows heavily from RFC 2109. [TODO: Figure out the
proper way to credit the authors of RFC 2109.]
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Author's Address
Adam Barth
University of California, Berkeley
Email: abarth@eecs.berkeley.edu
URI: http://www.adambarth.com/
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