http-state Working Group A. Barth
Internet-Draft U.C. Berkeley
Expires: February 16, 2010 August 15, 2009
HTTP State Management Mechanism
draft-abarth-cookie-01
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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 appreciated.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. State and Sessions . . . . . . . . . . . . . . . . . . . . . . 7
4. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.1. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 8
5. Server Conformance . . . . . . . . . . . . . . . . . . . . . . 9
5.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.2. Set-Cookie . . . . . . . . . . . . . . . . . . . . . . . . 9
5.2.1. Syntax . . . . . . . . . . . . . . . . . . . . . . . . 9
5.3. Semantics . . . . . . . . . . . . . . . . . . . . . . . . 10
5.3.1. Cookie Attributes . . . . . . . . . . . . . . . . . . 10
5.4. Cookie . . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.4.1. Syntax . . . . . . . . . . . . . . . . . . . . . . . . 11
5.4.2. Semantics . . . . . . . . . . . . . . . . . . . . . . 12
5.5. Controlling Caching . . . . . . . . . . . . . . . . . . . 12
6. User Agent Conformance . . . . . . . . . . . . . . . . . . . . 14
6.1. Parsing the Set-Cookie Header . . . . . . . . . . . . . . 14
6.2. Parsing Cookie Dates . . . . . . . . . . . . . . . . . . . 15
6.3. Storage Model . . . . . . . . . . . . . . . . . . . . . . 15
6.4. The Cookie Header . . . . . . . . . . . . . . . . . . . . 18
7. Caching Proxy Conformance . . . . . . . . . . . . . . . . . . 21
8. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
9. Implementation Considerations . . . . . . . . . . . . . . . . 23
9.1. Set-Cookie Content . . . . . . . . . . . . . . . . . . . . 23
9.2. Implementation Limits . . . . . . . . . . . . . . . . . . 23
9.2.1. Denial of Service Attacks . . . . . . . . . . . . . . 24
10. Privacy . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
10.1. User Agent Control . . . . . . . . . . . . . . . . . . . . 25
10.2. Protocol Design . . . . . . . . . . . . . . . . . . . . . 26
11. Security Considerations . . . . . . . . . . . . . . . . . . . 27
11.1. Clear Text . . . . . . . . . . . . . . . . . . . . . . . . 27
11.2. Cookie Spoofing . . . . . . . . . . . . . . . . . . . . . 27
11.3. Unexpected Cookie Sharing . . . . . . . . . . . . . . . . 27
12. Other, Similar, Proposals . . . . . . . . . . . . . . . . . . 28
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 29
Appendix B. Tabled Items . . . . . . . . . . . . . . . . . . . . 30
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 31
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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. Overview
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 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.
4.1. Examples
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5. Server Conformance
5.1. 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
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 MUST NOT fold multiple
Set-Cookie headers into a single header. [TODO: Investigate how UAs
cope with folded headers.]
5.2. Set-Cookie
5.2.1. Syntax
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 NAME=VALUE
attribute-value pair must come first in each cookie.
set-cookie-header = "Set-Cookie:" name-value-pairs
name-value-pairs = name-value-pair *(";" name-value-pair)
name-value-pair = name ["=" value] ; optional value
name = token
value = token
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[TODO: Investigate what token actually means.]
Attributes names are case-insensitive. White space is permitted
between tokens. Note that although the above syntax description
shows value as optional, some attributes require values.
The cookie-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 who examines the Set-Cookie header.
NOTE: The syntax above allows whitespace between the attribute and
the = sign. Servers wishing to interoperate with some legacy user
agents might wish to elide this extra white space to maximize
compatibility.
5.3. Semantics
When the user agent receives a Set-Cookie header, the user agent
stores the cookie in its cookie store. When the user agent makes
another HTTP request to the origin server, the user agent will return
the cookie in the Cookie header.
The server can override the default handling of cookies by specifying
a number of cookie attributes. User agents ignore unrecognized
cookie attributes.
5.3.1. Cookie Attributes
This section describes the semantics of a number of cookie
attributes.
5.3.1.1. Max-Age
Syntax A sequence of ASCII numerals.
Semantics The value of the Max-Age attribute represents the maximum
lifetime of the cookie, measured in seconds from the moment the
user agent receives the cookie. If the server does not supply an
Expires or a Max-Age attribute, the lifetime of the cookie is
limited to the current session (as defined by the user agent).
5.3.1.2. Expires
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Syntax An RFC 1123 date [cite]. (User agents use a very forgiving
date parers; see Section [TODO]).
Semantics The value of the Expires attribute represents the maximum
lifetime of the cookie, represented as the point in time at which
the cookie expires. If the server does not supply an Expires or a
Max-Age attribute, the lifetime of the cookie is limited to the
current session (as defined by the user agent).
5.3.1.3. Domain
[TODO: Test Domain.] The Domain attribute specifies the domain for
which the cookie is valid. The leading dot isn't required. If there
is no Domain attribute, the default is to return the cookie only to
the origin server. [TODO: You can only set cookies for related
domains.]
5.3.1.4. Path
[TODO: Test path.] The Path attribute specifies the subset of URLs
to which this cookie applies.
5.3.1.5. Secure
Syntax The empty string.
Semantics The user agent SHOULD protect the confidentiality of
cookies with the Secure attribute.
5.3.1.6. HttpOnly
Syntax The empty string.
Semantics The user agent SHOULD protect confidentiality of cookies
with the HttpOnly attribute by including HttpOnly cookies only
when generating cookie strings for use in HTTP requests.
5.4. Cookie
5.4.1. Syntax
The user agent returns stored cookies to the origin server in the
cookie header. The Cookie header shares a common syntax with the
Set-Cookie header, but the semantics of the header differ
dramatically.
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cookie-header = "Cookie:" name-value-pairs
name-value-pairs = name-value-pair *(";" name-value-pair)
name-value-pair = name "=" value
name = token
value = token
NOTE: If the server supplies a Set-Cookie header that does not
conform to the grammar in Section TODO, the user agent might not
supply a Cookie header that conforms to the grammar in this Section.
5.4.2. Semantics
Each name-value-pair represents a cookie stored by the user agent.
The cookie name is returned in as the name and the cookie value is
returned as the value.
5.5. Controlling Caching
[TODO: Should we go into this much detail here? This seems redundant
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.
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o To allow caching of a document and require that it be validated
before returning it to the client: Cache-Control: must-revalidate.
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.
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6. User Agent Conformance
Not all origin servers conform to the behavior specified in the
previous section. To ensure interoperability, user agents MUST
process cookies in a manner that is "black-box" indistinguishable
from the requirements in this section.
6.1. Parsing the Set-Cookie Header
Let an LWS character be either a U+20 (SPACE) or a U+09 (TAB)
character.
A user agent MUST use the following algorithm to parse the Set-Cookie
header:
1. [TODO: Deal with ',' characters.]
2. If the header contains a ';' character:
the name-value string is characters up to, but not including,
the first ';', and the unparsed-cookie-attributes are the
remainder of the header (including the ';' in question).
Otherwise:
the name-value string is all the character contained in the
header, and the unparsed-cookie-attributes is the empty
string.
3. If the first non-LWS character of the name-value string is '=',
remove it.
4. If the name-value string contains a '=' character:
the name string is the characters up to, but not including,
the first '=' character, and the value string is the
characters after the first '=' character .
Otherwise:
the name string is empty, and the value string is the entire
name-value string.
5. Remove any leading or trailing space from the name string.
6. Remove any leading or trailing space from the value string.
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7. The cookie-name is the name string.
8. The cookie-value is the value string.
The user agent MUST use the following algorithm to parse the
unparsed-attributes:
1. [TODO: Figure out how to parse cookie attributes.]
[TODO: Can parsing a cookie ever fail?]
[TODO: Convert Max-Age to a date during parsing.]
When the user agent finishes parsing the Set-Cookie header, the user
agent *receives a cookie* from the origin server with name cookie-
name, value cookie-value, and attributes cookie-attributes.
6.2. Parsing Cookie Dates
Basically, cookie dates are a mess for historical reasons.
To be compatible with legacy servers, however, user agents should
accept dates formated according to this grammar:
cookie-date = rfc1123-like-date / mystery-date
rfc1123-like-date = weekday "," SP rfc1123-like-dmy SP time SP "GMT"
weekday = "Monday" / "Mon" / "Tuesday" / "Tue" / ...
rfc1123-like-dmy = day dmy-div month dmy-div year
dmy-div = SP / "-"
day = 2DIGIT / *1SP DIGIT
month = "Jan" / "Feb" / ...
year = 2DIGIT / 4DIGIT
time = 2DIGIT ":" 2DIGIT ":" 2DIGIT
mystery-date = *CHAR ; see below
[TODO: More information about mystery-date.]
6.3. Storage Model
When the user agent receives a cookie, the user agent SHOULD record
the cookie in its cookie store as follows.
A user agent MAY ignore received cookies in their entirety if the
user agent is configured to block receiving cookie for a particular
response. For example, the user agent might wish to block receiving
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cookies from "third-party" responses.
The user agent stores the following fields about each cookie:
o name (a sequence of bytes)
o value (a sequence of bytes)
o expiry (a date)
o domain (a cookie-domain)
o path (a cookie-path)
o creation (a date)
o last-access (a date)
o persistent (a Boolean)
o host-only (a Boolean)
o secure-only (a Boolean)
o http-only (a Boolean)
When the user agent receives a cookie, the user agent MUST follow the
following algorithm:
1. Create a new cookie based on the parsed Set-Cookie header:
1. Create a new cookie with the following default field values:
+ name = the cookie-name
+ value = the cookie-value
+ expiry = the latest representable date
+ domain = the request-host
+ path = 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]
+ last-access = the date and time the cookie was received
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+ last-access = the date and time the cookie was received
+ persistent = false
+ host-only = true
+ secure-only = false
+ http-only = false
2. Update the default field values according to the cookie-
attributes:
expiry If the cookie-attributes contains at least one
Expires or a Max-Age attribute, store the value of the
[TODO: first] such attribute in the expiry field. Store
the value true in the persistent field.
domain If the cookie-attributes contains at least one Domain
attribute, store the value of the [TODO: first] such
attribute in the domain field. Store the value false in
the host-only field. [TODO: Reject cookies for unrelated
domains.] [TODO: If the URL's host is an IP address, let
Domain to be an IP address if it matches the URL's host
exactly, but set the host-only flag. ]
path If the cookie-attributes contains at least one Path
attribute, store the value of the [TODO: first] such
attribute in the path field.
secure-only If the cookie-attributes contains at least one
Secure attribute, store the value true in the secure-only
field.
http-only If the cookie-attributes contains at least one
HttpOnly attribute, store the value true in the http-only
field.
2. Remove from the cookie store all cookies that have the share the
same name, domain, path, and host-only fields as the newly
created cookie. [TODO: Valiate this list!] [TODO: There's some
funny business around http-only here.]
3. Insert the newly created cookie into the cookie store.
The user agent MUST evict a cookie from the cookie store if either of
the following conditions are met:
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o A cookie exists in the cookie store with an expiry date in the
past.
o More than 50 cookies exist in the cookie store with the same
domain field.
The user agent MAY evict cookies from the cookie store if the cookie
store exceeds some maximum storage bound (such as 3000 cookies).
When the user agent evicts cookies from the cookie store, the user
agent MUST evict cookies in the following priority order:
1. A cookie with an expiry date in the past.
2. A cookie that shares a domain field with more than 50 other
cookies in the cookie store.
3. All other cookies.
If two cookies have the same removal priority, the user agent MUST
evict the cookie with the least recent last-access date first.
When the user agent exits, the user agent MUST remove from the cookie
store all cookies with the persistent field set to false.
6.4. The Cookie Header
When the user agent generates an HTTP request for a particular URI,
the user agent SHOULD attach exactly one HTTP named Cookie if the
cookie-string (defined below) for that URI is non-empty.
A user agent MAY elide the Cookie header in its entirety if the user
agent is configured to block sending cookie for a particular request.
For example, the user agent might wish to block sending cookies
during "third-party" requests.
When generating a cookie-string from a URI with a "secure" scheme,
the user agent MUST set the SECURE flag to true. Otherwise, the user
agent MUST set the SECURE flag to false.
NOTE: The notion of an "secure" scheme is not defined by this
document. Typically, user agents consider a scheme secure if the
scheme refers to a protocol that makes use of transport-layer
security, such as TLS. For example, most user agents consider
"https" to be a secure scheme.
When generating a cookie-string for use in an HTTP request, the user
agent MUST set the HTTP flag to true. Otherwise, the user agent MUST
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set the HTTP flag to false.
The user agent MUST use the following algorithm to compute the
cookie-string from a cookie store and from a URI:
1. Let cookie-list be the set of cookies from the cookie store that
meet the following requirements:
* The cookie's domain field must domain-match the URI's host.
[TODO: Spec me]
* The cookie's path field must path-match the URI's path.
[TODO: Spec me]
* If the cookie's host-only flag is set, the cookie's domain
field must denote exactly the same FQDN as the URI's host.
[TODO: Internet Explorer does not implement this requirement
but most other major implementations do.]
* If the cookie's secure-only field is true, then the SECURE
flag must be true.
* If the cookie's http-only field is true, then the HTTP flag
must be true.
NOTE: The Cookie header will not contain any expired cookies
because cookies past their expiry date are removed from the
cookie store immediately.
2. Sort the cookie-list in the following order:
* Cookies with longer path fields are listed before cookies with
shorter path field.
* Among cookies that have equal length path fields, cookies with
earlier creation dates are listed before cookies with later
creation dates.
3. Update the last-access field of each cookie in the cookie-list to
the current date.
4. Serialize the cookie-list into a cookie-string by processing each
cookie in the cookie-list in order:
1. Output the cookie's name field.
2. Output the character U+3D ("=")
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3. Output the cookie's value field.
4. If there is an unprocessed cookie in the cookie-list, output
the characters U+3B and U+20 ("; ")
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7. Caching Proxy Conformance
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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8. Examples
[TODO: Write sensible examples.]
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9. Implementation Considerations
Here we speculate on likely or desirable details for an origin server
that implements state management.
9.1. 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.]
9.2. 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 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)
User agents created for specific purposes or for limited-capacity
devices should provide at least 50 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.
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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.]
9.2.1. 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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10. Privacy
10.1. 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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10.2. 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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11. Security Considerations
11.1. 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.
11.2. 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.]
11.3. 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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12. 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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Appendix B. Tabled Items
Tabled items:
o Public suffix.
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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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