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RFC 6265
httpstate A. Barth
Internet-Draft U.C. Berkeley
Obsoletes: 2109 (if approved) February 23, 2010
Intended status: Standards Track
Expires: August 27, 2010
HTTP State Management Mechanism
draft-ietf-httpstate-cookie-04
Abstract
This document defines the HTTP Cookie and Set-Cookie headers. These
headers can be used by HTTP servers to store state on HTTP user
agents, letting the servers maintain a stateful session over the
mostly stateless HTTP protocol. The cookie protocol has many
historical infelicities and should be avoided for new applications of
HTTP.
NOTE: If you have suggestions for improving the draft, please send
email to http-state@ietf.org. Suggestions with test cases are
especially appreciated.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
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-
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The list of current Internet-Drafts can be accessed at
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This Internet-Draft will expire on August 27, 2010.
Copyright Notice
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Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Without obtaining an adequate license from the person(s) controlling
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. General Nonsense . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Conformance Criteria . . . . . . . . . . . . . . . . . . . 5
2.2. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 5
2.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. A Well-Behaved Profile . . . . . . . . . . . . . . . . . . . . 9
4.1. Set-Cookie . . . . . . . . . . . . . . . . . . . . . . . . 9
4.1.1. Syntax . . . . . . . . . . . . . . . . . . . . . . . . 9
4.1.2. Semantics (Non-Normative) . . . . . . . . . . . . . . 10
4.2. Cookie . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.2.1. Syntax . . . . . . . . . . . . . . . . . . . . . . . . 12
4.2.2. Semantics . . . . . . . . . . . . . . . . . . . . . . 12
5. The Cookie Protocol . . . . . . . . . . . . . . . . . . . . . 14
5.1. Algorithms . . . . . . . . . . . . . . . . . . . . . . . . 14
5.1.1. Dates . . . . . . . . . . . . . . . . . . . . . . . . 14
5.1.2. Domains . . . . . . . . . . . . . . . . . . . . . . . 16
5.1.3. Paths . . . . . . . . . . . . . . . . . . . . . . . . 16
5.2. The Set-Cookie Header . . . . . . . . . . . . . . . . . . 17
5.2.1. The Max-Age Attribute . . . . . . . . . . . . . . . . 19
5.2.2. The Expires Attribute . . . . . . . . . . . . . . . . 19
5.2.3. The Domain Attribute . . . . . . . . . . . . . . . . . 20
5.2.4. The Path Attribute . . . . . . . . . . . . . . . . . . 20
5.2.5. The Secure Attribute . . . . . . . . . . . . . . . . . 21
5.2.6. The HttpOnly Attribute . . . . . . . . . . . . . . . . 21
5.3. Storage Model . . . . . . . . . . . . . . . . . . . . . . 21
5.4. The Cookie Header . . . . . . . . . . . . . . . . . . . . 24
6. Implementation Considerations . . . . . . . . . . . . . . . . 26
6.1. Limits . . . . . . . . . . . . . . . . . . . . . . . . . . 26
6.2. Application Programmer Interfaces . . . . . . . . . . . . 26
7. Security Considerations . . . . . . . . . . . . . . . . . . . 27
7.1. General Recommendations . . . . . . . . . . . . . . . . . 27
7.2. Ambient Authority . . . . . . . . . . . . . . . . . . . . 27
7.3. Clear Text . . . . . . . . . . . . . . . . . . . . . . . . 27
7.4. Session Identifiers . . . . . . . . . . . . . . . . . . . 28
7.5. Weak Confidentiality . . . . . . . . . . . . . . . . . . . 28
7.6. Weak Integrity . . . . . . . . . . . . . . . . . . . . . . 29
7.7. Reliance on DNS . . . . . . . . . . . . . . . . . . . . . 30
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 31
8.1. Normative References . . . . . . . . . . . . . . . . . . . 31
8.2. Informative References . . . . . . . . . . . . . . . . . . 31
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 32
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 33
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1. Introduction
This document defines the HTTP Cookie and Set-Cookie header. Using
the Set-Cookie header, an HTTP server can store name/value pairs and
associated metadata (called cookies) at the user agent. When the
user agent makes subsequent requests to the server, the user agent
uses the metadata to determine whether to return the name/value pairs
in the Cookie header.
Although simple on its surface, the cookie protocol has a number of
complexities. For example, the server indicates a scope for each
cookie when sending them to the user agent. The scope indicates the
maximum amount of time the user agent should retain the cookie, to
which servers the user agent should return the cookie, and for which
protocols the cookie is applicable.
For historical reasons, the cookie protocol contains a number of
security and privacy infelicities. For example, a server can
indicate that a given cookie is intended for "secure" connections,
but the Secure attribute provides only confidentiality (not
integrity) from active network attackers. Similarly, cookies for a
given host are shared across all the ports on that host, even though
the usual "same-origin policy" used by web browsers isolates content
retrieved from different ports.
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2. General Nonsense
2.1. Conformance Criteria
The key words "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT",
"RECOMMENDED", "MAY", and "OPTIONAL" in document are to be
interpreted as described in [RFC2119].
Requirements phrased in the imperative as part of algorithms (such as
"strip any leading space characters" or "return false and abort these
steps") are to be interpreted with the meaning of the key word
("MUST", "SHOULD", "MAY", etc) used in introducing the algorithm.
2.2. Syntax Notation
This specification uses the Augmented Backus-Naur Form (ABNF)
notation of [RFC5234].
The following core rules are included by reference, as defined in
[RFC5234], Appendix B.1: ALPHA (letters), CR (carriage return), CRLF
(CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote),
HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
sequence of data), SP (space), HTAB (horizontal tab), VCHAR (any
visible [USASCII] character), and WSP (whitespace).
2.3. Terminology
The terms user agent, client, server, proxy, and origin server have
the same meaning as in the HTTP/1.1 specification ([RFC2616]).
The terms request-host and request-URI refer to the values the user
agent 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.
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3. Overview
We outline here a way for an origin server to send state information
to a user agent, and for the user agent to return the state
information to the origin server.
To initiate a session, the origin server includes a Set-Cookie header
in an HTTP response. (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).
The user agent returns a Cookie request header to the origin server
if it chooses to continue a session. The Cookie header contains a
number of cookies the user agent received in previous Set-Cookie
headers. The origin server MAY ignore the Cookie header or use the
header to determine the current state of the session. The origin
server MAY send the user agent a Set-Cookie response header with the
same or different information, or it MAY send no Set-Cookie header at
all.
Servers MAY return a Set-Cookie response header with any response.
User agents SHOULD send a Cookie request header, subject to other
rules detailed below, with every request.
An origin server MAY include multiple Set-Cookie header fields in a
single response. Note that an intervening gateway MUST NOT fold
multiple Set-Cookie header fields into a single header field.
3.1. Examples
Using the cookie protocol, a server can send the user agent a short
string in an HTTP response that the user agent will return in future
HTTP requests. For example, the server can send the user agent a
"session identifier" named SID with the value 31d4d96e407aad42. The
user agent then returns the session identifier in subsequent
requests.
== Server -> User Agent ==
Set-Cookie: SID=31d4d96e407aad42
== User Agent -> Server ==
Cookie: SID=31d4d96e407aad42
The server can alter the default scope of the cookie using the Path
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and Domain attributes. For example, the server can instruct the user
agent to return the cookie to every path and every subdomain of
example.com.
== Server -> User Agent ==
Set-Cookie: SID=31d4d96e407aad42; Path=/; Domain=.example.com
== User Agent -> Server ==
Cookie: SID=31d4d96e407aad42
The server can store multiple cookies in the user agent. For
example, the server can store a session identifier as well as the
user's preferred language by returning two Set-Cookie response
headers. Notice that the server uses the Secure and HttpOnly
attributes to provide additional security protections for the more-
sensitive session identifier.
== Server -> User Agent ==
Set-Cookie: SID=31d4d96e407aad42; Path=/; Secure, HttpOnly
Set-Cookie: lang=en-US; Path=/; Domain=.example.com
== User Agent -> Server ==
Cookie: SID=31d4d96e407aad42; lang=en-US
If the server wishes the user agent to persist the cookie over
multiple sessions, the server can specify a expiration date in the
Expires attribute. Note that the user agent might might delete the
cookie before the expiration date if the user agent's cookie store
exceeds its quota or if the user manually deletes the server's
cookie.
== Server -> User Agent ==
Set-Cookie: lang=en-US; Expires=Wed, 09 Jun 2021 10:18:14 GMT
== User Agent -> Server ==
Cookie: lang=en-US
Finally, to remove a cookie, the server returns a Set-Cookie header
with an expiration date in the past. The server will be successful
in removing the cookie only if the Path and the Domain attribute in
the Set-Cookie header match the values used when the cookie was
created.
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== Server -> User Agent ==
Set-Cookie: lang=; Expires=Sun, 06 Nov 1994 08:49:37 GMT
== User Agent -> Server ==
(No Cookie header)
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4. A Well-Behaved Profile
This section describes the syntax and semantics of a well-behaved
profile of the protocol. Servers SHOULD use the profile described in
this section, both to maximize interoperability with existing user
agents and because a future version of the cookie protocol could
remove support for some of the most esoteric aspects of the protocol.
User agents, however, MUST implement the full protocol to ensure
interoperability with servers making use of the full protocol.
4.1. Set-Cookie
The Set-Cookie header is used to send cookies from the server to the
user agent.
4.1.1. Syntax
Informally, the Set-Cookie response header comprises the token Set-
Cookie:, followed by a cookie. Each cookie begins with a name-value-
pair, followed by zero or more attribute-value pairs. Servers SHOULD
NOT send Set-Cookie headers that fail to conform to the following
grammar:
set-cookie-header = "Set-Cookie:" OWS set-cookie-string OWS
set-cookie-string = cookie-pair *( ";" cookie-av )
cookie-pair = cookie-name "=" cookie-value
cookie-name = token
cookie-value = token
token = <token, as defined in RFC 2616>
cookie-av = expires-av / domain-av / path-av /
secure-av / httponly-av
expires-av = "Expires" "=" sane-cookie-date
sane-cookie-date = <rfc1123-date, as defined in RFC 2616>
domain-av = "Domain" "=" domain-value
domain-value = token
path-av = "Path" "=" path-value
path-value = <abs_path, as defined in RFC 2616>
secure-av = "Secure"
httponly-av = "HttpOnly"
Servers SHOULD NOT include two attributes with the same name.
The cookie-value is opaque to the user agent and MAY be anything the
origin server chooses to send. "Opaque" implies that the content is
of interest and relevance only to the origin server. The content is,
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in fact, readable by anyone who examines the Set-Cookie header.
To maximize compatibility with user agents, servers that wish to
store non-ASCII data in a cookie-value SHOULD encode that data using
a printable ASCII encoding, such as base64.
NOTE: The syntax above allows whitespace between the attribute and
the U+003D ("=") character. Servers wishing to interoperate with
some legacy user agents might wish to elide this whitespace.
4.1.2. Semantics (Non-Normative)
This section describes a simplified semantics of the Set-Cookie
header. These semantics are detailed enough to be useful for
understanding the most common uses of the cookie protocol. The full
semantics are described in Section 5.
When the user agent receives a Set-Cookie header, the user agent
stores the cookie in its cookie store. When the user agent
subsequently makes an HTTP request, the user agent consults its
cookie store and includes the applicable, non-expired cookies in the
Cookie header.
If the cookie store already contains a cookie with the same cookie-
name, domain-value, and path-value, the existing cookie is evicted
from the cookie store and replaced with the new value. Notice that
servers can delete cookies by including an Expires attribute with a
value in the past.
Unless the cookie's attributes indicate otherwise, the cookie is
returned only to the origin server, and it expires at the end of the
current session (as defined by the user agent). User agents ignore
unrecognized cookie attributes.
4.1.2.1. Expires
The Expires attribute indicates the maximum lifetime of the cookie,
represented as the date and time at which the cookie expires. The
user agent is not required to retain the cookie until the specified
date has passed. In fact, user agents often evict cookies from the
cookie store due to memory pressure or privacy concerns.
4.1.2.2. Domain
The Domain attribute specifies those hosts for which the cookie will
be sent. For example, if the Domain attribute contains the value
".example.com", the user agent will include the cookie in the Cookie
header when making HTTP requests to example.com, www.example.com, and
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www.corp.example.com. (Note that a leading U+002E ("."), if present,
is ignored.) If the server omits the Domain attribute, the user
agent will return the cookie only to the origin server.
WARNING: Some legacy user agents treat an absent Domain attribute
as if the Domain attribute were present and contained the current
host name. For example, if example.com returns a Set-Cookie
header without a Domain attribute, these user agents will send the
cookie to www.example.com.
The user agent will reject cookies (refuse to store them in the
cookie store) unless the Domain attribute specifies a scope for the
cookie that would include the origin server. For example, the user
agent will accept a Domain attribute of ".example.com" or of
".foo.example.com" from foo.example.com, but the user agent will not
accept a Domain attribute of ".bar.example.com" or of
".baz.foo.example.com".
NOTE: For security reasons, some user agents are configured to reject
Domain attributes that do not correspond to a "registry controlled"
domain (or a subdomain of a registry controlled domain). For
example, some user agents will reject Domain attributes of ".com".
4.1.2.3. Path
The Path attribute limits the scope of the cookie to a set of paths.
When a cookie has a Path attribute, the user agent will include the
cookie in an HTTP request only if the path portion of the Request-URI
matches (or is a subdirectory of) the cookie's Path attribute, where
the U+002F ("/") character is interpreted as a directory separator.
If the server omits the Path attribute, the user agent will use the
directory of the Request-URI's path component as the default value.
Although seemingly useful for isolating cookies between different
paths within a given domain, the Path attribute cannot be relied upon
for security for two reasons: First, user agents do not prevent one
path from overwriting the cookies for another path. For example, if
a response to a request for /foo/bar.html attempts to set a cookie
with a Path attribute of "/baz" the user agent will store that cookie
in the cookie store. Second, the "same-origin" policy implemented by
many user agents does not isolate different paths within an origin.
For example, /foo/bar.html can read cookies with a Path attribute of
"/baz" because they are within the "same origin".
4.1.2.4. Secure
The Secure attribute limits the scope of the cookie to "secure"
channels (where "secure" is defined by the user agent). When a
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cookie has the Secure attribute, the user agent will include the
cookie in an HTTP request only if the request is transmitted over a
secure channel (typically TLS [RFC5246]).
Although seemingly useful for protecting cookies from active network
attackers, the Secure attribute protects only the cookie's
confidentiality. An active network attacker can overwrite Secure
cookies from an insecure channel, disrupting the integrity of the
cookies.
4.1.2.5. HttpOnly
The HttpOnly attribute limits the scope of the cookie to HTTP
requests. In particular, the attribute instructs the user agent to
elide the cookie when providing access to its cookie store via "non-
HTTP" APIs (as defined by the user agent).
4.2. Cookie
4.2.1. Syntax
The user agent returns stored cookies to the origin server in the
Cookie header. If the server conforms to the requirements in this
section, the requirements in the next section will cause the user
agent to return a Cookie header that conforms to the following
grammar:
cookie-header = "Cookie:" OWS cookie-string OWS
cookie-string = cookie-pair *( ";" cookie-pair )
4.2.2. Semantics
Each cookie-pair represents a cookie stored by the user agent. The
cookie-name and the cookie-value are returned verbatim from the
corresponding parts of the Set-Cookie header.
Notice that the cookie attributes are not returned. In particular,
the server cannot determine from the Cookie header alone when a
cookie will expire, for which domains the cookie is valid, for which
paths the cookie is valid, or whether the cookie was set with the
Secure or HttpOnly attributes.
The semantics of individual cookies in the Cookie header is not
defined by this document. Servers are expected to imbue these
cookies with server-specific semantics.
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Although cookies are serialized linearly in the Cookie header,
servers SHOULD NOT rely upon the serialization order. In particular,
if the Cookie header contains two cookies with the same name, servers
SHOULD NOT rely upon the order in which these cookies appear in the
header.
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5. The Cookie Protocol
For historical reasons, the full cookie protocol contains a number of
exotic quirks. This section is intended to specify the cookie
protocol in enough detail to enable a user agent that implements the
protocol precisely as specified to interoperate with existing
servers.
Conformance requirements phrased as algorithms or specific steps may
be implemented in any manner, so long as the end result is
equivalent. (In particular, the algorithms defined in this
specification are intended to be easy to follow, and not intended to
be performant.)
5.1. Algorithms
This section defines a number of algorithms used by the cookie
protocol.
5.1.1. Dates
The user agent MUST use the following algorithm to *parse a cookie-
date*:
1. Using the grammar below, divide the cookie-date into date-tokens.
cookie-date = *delimiter date-token-list *delimiter
date-token-list = date-token *( 1*delimiter date-token )
delimiter = %x09 / %x20 / %x21 / %x22 / %x23 / %x24 /
%x25 / %x26 / %x27 / %x28 / %x29 / %x2A /
%x2B / %x2C / %x2D / %x2E / %x2F / %x3B /
%x3C / %x3D / %x3E / %x3F / %x40 / %x5B /
%x5C / %x5D / %x5E / %x5F / %x60 / %x7B /
%x7C / %x7D / %x7E
date-token = day-of-month / month / year / time / mystery
day-of-month = 2DIGIT / DIGIT
month = "jan" [ mystery ] / "feb" [ mystery ] /
"mar" [ mystery ] / "apr" [ mystery ] /
"may" [ mystery ] / "jun" [ mystery ] /
"jul" [ mystery ] / "aug" [ mystery ] /
"sep" [ mystery ] / "oct" [ mystery ] /
"nov" [ mystery ] / "dec" [ mystery ]
year = 5DIGIT / 4DIGIT / 3DIGIT / 2DIGIT / DIGIT
time = 2DIGIT ":" 2DIGIT ":" 2DIGIT
mystery = <anything except a delimiter>
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2. Process each date-token sequentially in the order the date-tokens
appear in the cookie-date:
1. If the found-day-of-month flag is not set and the date-token
matches the day-of-month production, set the found-day-of-
month flag and set the day-of-month-value to the number
denoted by the date-token. Skip the remaining sub-steps and
continue to the next date-token.
2. If the found-month flag is not set and the date-token matches
the month production, set the found-month flag and set the
month-value to the month denoted by the date-token. Skip the
remaining sub-steps and continue to the next date-token.
3. If the found-year flag is not set and the date-token matches
the year production, set the found-year flag and set the
year-value to the number denoted by the date-token. Skip the
remaining sub-steps and continue to the next date-token.
4. If the found-time flag is not set and the token matches the
time production, set the found-time flag and set the hour-
value, minute-value, and second-value to the numbers denoted
by the digits in the date-token, respectively. Skip the
remaining sub-steps and continue to the next date-token.
3. Abort these steps and *fail to parse* if
* at least one of the found-day-of-month, found-month, found-
year, or found-time flags is not set,
* the day-of-month-value is less than 1 or greater than 31,
* the year-value is less than 1601 or greater than 30827,
* the hour-value is greater than 23,
* the minute-value is greater than 59, or
* the second-value is greater than 59.
4. If the year-value is greater than 68 and less than 100, increment
the year-value by 1900.
5. If the year-value is greater than or equal to 0 and less than 69,
increment the year-value by 2000.
6. Let the parsed-cookie-date be the date whose day-of-month, month,
year, hour, minute, and second (in GMT) are the day-of-month-
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value, the month-value, the year-value, the hour-value, the
minute-value, and the second-value, respectively.
7. Return the parsed-cookie-date as the result of this algorithm.
5.1.2. Domains
A *canonicalized* host-name is the host-name converted to lower case.
A request-host *domain-matches* a cookie-domain if at least one of
the following conditions hold:
o The cookie-domain and the canonicalized request-host are
identical.
o The cookie-domain is a suffix of the canonicalized request-host,
the last character of the canonicalized request-host that is not
included in the cookie-domain is a U+002E (".") character, and
request-host is a host name (i.e., not an IP address). [TODO: Is
this the right way to spec this???]
5.1.3. Paths
The user agent MUST use the following algorithm to compute the
*default-path* of a cookie:
1. Let uri-path be the path portion of the Request-URI.
2. If the first character of the uri-path is not a U+002F ("/")
character, output U+002F ("/") and skip the remaining steps.
3. If the uri-path contains only a single U+002F ("/") character,
output U+002F ("/") and skip the remaining steps.
4. Output the characters of the uri-path from the first character up
to, but not including, the right-most U+002F ("/").
A request-path *path-matches* a cookie-path if at least one of the
following conditions hold: [TODO: This isn't exactly what IE or
Firefox does.]
o The cookie-path and the request-path are identical.
o The cookie-path is a prefix of the request-path and the last
character of the cookie-path is U+002F ("/").
o The cookie-path is a prefix of the request-path and the first
character of the request-path that is not included in the cookie-
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path is a U+002F ("/") character.
5.2. The Set-Cookie Header
When a user agent receives a Set-Cookie header in an HTTP response,
the user agent *receives a set-cookie-string* consisting of the value
of the header.
A user agent MUST use the following algorithm to parse set-cookie-
strings:
1. If the set-cookie-string is empty or consists entirely of WSP
characters, the user agent MAY ignore the set-cookie-string
entirely.
2. If the set-cookie-string contains a U+003B (";") character:
The name-value-pair string consists of the characters up to,
but not including, the first U+003B (";"), and the unparsed-
attributes consist of the remainder of the set-cookie-string
(including the U+003B (";") in question).
Otherwise:
The name-value-pair string consists of all the characters
contained in the set-cookie-string, and the unparsed-
attributes is the empty string.
3. If the name-value-pair string contains a U+003D ("=") character:
The (possibly empty) name string consists of the characters up
to, but not including, the first U+003D ("=") character, and
the (possibly empty) value string consists of the characters
after the first U+003D ("=") character.
Otherwise:
The name string is empty, and the value string consists of the
entire name-value-pair string.
4. Remove any leading or trailing WSP characters from the name
string and the value string.
5. The cookie-name is the name string, and the cookie-value is the
value string.
The user agent MUST use the following algorithm to parse the
unparsed-attributes:
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1. If the unparsed-attributes string is empty, skip the rest of
these steps.
2. Consume the first character of the unparsed-attributes (which
will be a U+003B (";") character).
3. If the remaining unparsed-attributes contains a U+003B (";")
character:
Consume the characters of the unparsed-attributes up to, but
not including, the first U+003B (";") character.
Otherwise:
Consume the remainder of the unparsed-attributes.
Let the cookie-av string be the characters consumed in this step.
4. If the cookie-av string contains a U+003D ("=") character:
The (possibly empty) attribute-name string consists of the
characters up to, but not including, the first U+003D ("=")
character, and the (possibly empty) attribute-value string
consists of the characters after the first U+003D ("=")
character.
Otherwise:
The attribute-name string consists of the entire cookie-av
string, and the attribute-value string is empty. (Note that
this step differs from the analogous step when parsing the
name-value-pair string.)
5. Remove any leading or trailing WSP characters from the attribute-
name string and the attribute-value string.
6. Process the attribute-name and attribute-value according to the
requirements in the following subsections.
7. Return to Step 1.
When the user agent finishes parsing the set-cookie-string, the user
agent *receives a cookie* from the Request-URI with name cookie-name,
value cookie-value, and attributes cookie-attribute-list.
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5.2.1. The Max-Age Attribute
If the attribute-name case-insensitively matches the string "Max-
Age", the user agent MUST process the cookie-av as follows.
If the first character of the attribute-value is not a DIGIT or a "-"
character, ignore the cookie-av.
If the remainder of attribute-value contains a non-DIGIT character,
ignore the cookie-av.
Let delta-seconds be the attribute-value converted to an integer.
If delta-seconds is less than or equal to zero (0), let expiry-time
be the current date and time. Otherwise, let the expiry-time be the
current date and time plus delta-seconds seconds.
Append an attribute to the cookie-attribute-list with an attribute-
name of Expires (note the name conversion) and an attribute-value of
expiry-time.
5.2.2. The Expires Attribute
If the attribute-name case-insensitively matches the string
"Expires", the user agent MUST process the cookie-av as follows.
Let the parsed-cookie-date be the result of parsing the attribute-
value as cookie-date.
If the attribute-value failed to parse as a cookie date, ignore the
cookie-av.
If the user agent received the set-cookie-string from an HTTP
response that contains a Date header field and the contents of the
last Date header field successfully parse as a cookie-date:
Let server-date be the date obtained by parsing the contents of
the last Date header field as a cookie-date.
Let delta-seconds be the number of seconds between the server-date
and the parsed-cookie-date (i.e., parsed-cookie-date - server-
date).
Let the expiry-time be the current date and time plus delta-
seconds seconds.
Otherwise:
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Let the expiry-time be the parsed-cookie-date.
If the expiry-time is later than the last date the user agent can
represent, the user agent MAY replace the expiry-time with the last
representable date.
If the expiry-time is earlier than the first date the user agent can
represent, the user agent MAY replace the expiry-time with the first
representable date.
Append an attribute to the cookie-attribute-list with an attribute-
name of Expires and an attribute-value of expiry-time.
5.2.3. The Domain Attribute
If the attribute-name case-insensitively matches the string "Domain",
the user agent MUST process the cookie-av as follows.
If the attribute-value is empty, the behavior is undefined. However,
user agent SHOULD ignore the cookie-av entirely.
If the first character of the attribute-value string is U+002E ("."):
Let cookie-domain be the attribute-value without the leading
U+002E (".") character.
Otherwise:
Let cookie-domain be the entire attribute-value.
Convert the cookie-domain to lower case.
[TODO: Test ".127.0.0.1" and "127.0.0.1"]
Append an attribute to the cookie-attribute-list with an attribute-
name of Domain and an attribute-value of cookie-domain.
5.2.4. The Path Attribute
If the attribute-name case-insensitively matches the string "Path",
the user agent MUST process the cookie-av as follows.
If the attribute-value is empty or if the first character of the
attribute-value is not U+002F ("/"):
Let cookie-path be the default-path. [TODO: We need more tests
for this, including with " characters and with multiple Path
attributes.]
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Otherwise:
Let cookie-path be the attribute-value.
Append an attribute to the cookie-attribute-list with an attribute-
name of Path and an attribute-value of cookie-path.
5.2.5. The Secure Attribute
If the attribute-name case-insensitively matches the string "Secure",
the user agent MUST append an attribute to the cookie-attribute-list
with an attribute-name of Secure and an empty attribute-value.
5.2.6. The HttpOnly Attribute
If the attribute-name case-insensitively matches the string
"HttpOnly", the user agent MUST append an attribute to the cookie-
attribute-list with an attribute-name of HttpOnly and an empty
attribute-value.
5.3. 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 a received cookie in its entirety if the user
agent is configured to block receiving cookies. For example, the
user agent might wish to block receiving cookies from "third-party"
responses.
The user agent stores the following fields about each cookie: name,
value, expiry-time, domain, path, creation-time, last-access-time,
persistent-flag, host-only-flag, secure-only-flag, and http-only-
flag.
When the user agent receives a cookie from a Request-URI with name
cookie-name, value cookie-value, and attributes cookie-attribute-
list, the user agent MUST process the cookie as follows:
1. Create a new cookie with name cookie-name, value cookie-value.
Set the creation-time and the last-access-time to the current
date and time.
2. If the cookie-attribute-list contains an attribute with an
attribute-name of "Expires":
Set the cookie's persistent-flag to true.
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Set the cookie's expiry-time to attribute-value of the last
attribute in the cookie-attribute-list with an attribute-name
of "Expires". [TODO: Test that this really works when mixing
Max-Age and Expires.]
Otherwise:
Set the cookie's persistent-flag to false.
Set the cookie's expiry-time to the latest representable
date.
3. If the cookie-attribute-list contains an attribute with an
attribute-name of "Domain":
Let the domain-attribute be the attribute-value of the last
attribute in the cookie-attribute-list with an attribute-name
of "Domain".
Otherwise:
Let the domain-attribute be the empty string.
4. If the user agent is configured to use a "public suffix" list
and the domain-attribute is a public suffix:
If the domain-attribute is identical to the canonicalized
Request-URI's host:
Let the domain-attribute be the empty string.
Otherwise:
Ignore the cookie entirely and abort these steps
NOTE: A "public suffix" is a domain that is controlled by a
public registry, such as "com", "co.uk", and "pvt.k12.wy.us".
This step is essential for preventing attacker.com from
disrupting the integrity of example.com by setting a cookie
with a Domain attribute of "com". Unfortunately, the set of
public suffixes (also known as "registry controlled domains")
changes over time. If feasible, user agents SHOULD use an
up-to-date public suffix list, such as the one maintained by
the Mozilla project at http://publicsuffix.org/.
5. If the domain-attribute is non-empty:
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If the Request-URI's host does not domain-match the domain-
attribute, ignore the cookie entirely and abort these steps.
Set the cookie's host-only-flag to false.
Set the cookie's domain to the domain-attribute.
Otherwise:
Set the cookie's host-only-flag to true.
Set the cookie's domain to the host of the Request-URI.
6. If the cookie-attribute-list contains an attribute with an
attribute-name of "Path", set the cookie's path to attribute-
value of the last attribute in the cookie-attribute-list with an
attribute-name of "Path". Otherwise, set cookie's path to the
default-path of the Request-URI.
7. If the cookie-attribute-list contains an attribute with an
attribute-name of "Secure", set the cookie's secure-only-flag to
true. Otherwise, set cookie's secure-only-flag to false.
8. If the cookie-attribute-list contains an attribute with an
attribute-name of "HttpOnly", set the cookie's http-only-flag to
true. Otherwise, set cookie's http-only-flag to false.
9. Remove from the cookie store all cookies that share the same
name, domain, path, and host-only-flag as the newly created
cookie. [TODO: Validate this list!] [TODO: There's some funny
business around http-only here.]
10. If the cookie's name and value are both empty, abort these
steps.
11. If the cookie's expiry-time is not in the future, abort these
steps.
12. Insert the newly created cookie into the cookie store.
The user agent MUST evict a cookie from the cookie store if, at any
time, a cookie exists in the cookie store with an expiry date in the
past.
The user agent MAY evict a cookie from the cookie store if the number
of cookies sharing a domain field exceeds some predetermined upper
bound (such as 50 cookies).
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The user agent MAY evict a cookie from the cookie store if the cookie
store exceeds some predetermined upper bound (such as 3000 cookies).
When the user agent evicts a cookie from the cookie store, the user
agent MUST evict cookies in the following priority order:
1. Cookies with an expiry date in the past.
2. Cookies that share a domain field with more than a predetermined
number of other cookies.
3. All 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 current session is over" (as defined by the user agent),
the user agent MUST remove from the cookie store all cookies with the
persistent-flag set to false.
5.4. The Cookie Header
When the user agent generates an HTTP request, the user agent SHOULD
attach exactly one HTTP header named Cookie if the cookie-string
(defined below) for the Request-URI is non-empty.
A user agent MAY elide the Cookie header in its entirety if the user
agent is configured to block sending cookies. For example, the user
agent might wish to block sending cookies during "third-party"
requests.
The user agent MUST use the following algorithm to compute the
cookie-string from a cookie store and a Request-URI:
1. Let cookie-list be the set of cookies from the cookie store that
meet all of the following requirements:
* Let request-host be the Request-URI's host. Either:
The cookie's host-only-flag is true and the canonicalized
request-host is identical to the cookie's domain.
Or:
The cookie's host-only-flag is false and the request-host
domain-matches cookie's domain.
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* The Request-URI's path patch-matches cookie's path.
* If the cookie's secure-only field is true, then the Request-
URI's scheme must denote a "secure" protocol (as defined by
the user agent).
NOTE: The notion of a "secure" protocol is not defined by
this document. Typically, user agents consider a protocol
secure if the protocol makes use of transport-layer
security, such as TLS. For example, most user agents
consider "https" to be a scheme that denotes a secure
protocol.
* If the cookie's http-only field is true, then exclude the
cookie unless the cookie-string is being generated for an
"HTTP" API (as defined by the user agent).
2. Sort the cookie-list in the following order:
* Cookies with longer paths are listed before cookies with
shorter paths.
* Among cookies that have equal length path fields, cookies with
earlier creation-times are listed before cookies with later
creation-times.
3. Update the last-access-time of each cookie in the cookie-list to
the current date and time.
4. Serialize the cookie-list into a cookie-string by processing each
cookie in the cookie-list in order:
1. If the cookie's name is non-empty, output the cookie's name
followed by the U+003D ("=") character.
2. Output the cookie's value.
3. If there is an unprocessed cookie in the cookie-list, output
the characters U+003B and U+0020 ("; ").
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6. Implementation Considerations
6.1. Limits
Practical user agent implementations have limits on the number and
size of cookies that they can store. General-use user agents SHOULD
provide each of the following minimum capabilities:
o At least 4096 bytes per cookie (as measured by the sum of the
length of the cookie's name, value, and attributes).
o At least 50 cookies per domain.
o At least 3000 cookies total.
Servers SHOULD use as few and as small cookies as possible to avoid
reaching these implementation limits and to avoid network latency due
to the Cookie header being included in every request.
Servers should gracefully degrade if the user agent fails to return
one or more cookies in the Cookie header because the user agent might
evict any cookie at any time on orders from the user.
6.2. Application Programmer Interfaces
One reason the cookie protocol uses such an esoteric syntax is
because many platforms (both in servers and user agents) provide
string-based application programmer interfaces (APIs), requiring
application-layer programmers to generate and parse the syntax used
by the cookie protocol.
Instead of providing string-based APIs to the cookie protocols,
implementations would be well-served by providing more semantic APIs.
It is beyond the scope of this document to recommend specific API
designs, but there are clear benefits to accepting a abstract "Date"
object instead of a serialized date string.
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7. Security Considerations
7.1. General Recommendations
The cookie protocol is NOT RECOMMENDED for new applications.
For applications that do use the cookie protocol, servers SHOULD NOT
rely upon cookies for security.
7.2. Ambient Authority
A server that uses cookies to authenticate users can suffer security
vulnerabilities because some user agents let remote parties issue
HTTP requests from the user agent (e.g., via HTTP redirects and HTML
forms). When issuing those requests, user agent attaches cookies
even if the entity does not know the contents of the cookies,
possibly letting the remote entity exercise authority at an unwary
server.
Although this security concern goes by a number of names (e.g.,
cross-site request forgery, confused deputy), the issue stems from
cookies being a form of ambient authority. Cookies encourage server
operators to separate designation (in the form of URLs) from
authorization (in the form of cookies). Consequently, the user agent
might supply the authorization for a resource designated by the
attacker, possibly causing the server or its clients to undertake
actions designated by the attacker as though they were authorized by
the user.
Instead of using cookies for authorization, server operators might
wish to consider entangling designation and authorization by treating
URLs as capabilities. Instead of storing secrets in cookies, this
approach stores secrets in URLs, requiring the remote entity to
supply the secret itself. Although this approach is not a panacea,
judicious use of these principles can lead to more robust security.
7.3. Clear Text
Unless sent over a secure channel (such as TLS), the information in
the Set-Cookie and Cookie headers is transmitted in the clear.
1. All sensitive information conveyed in these headers is exposed to
an eavesdropper.
2. A malicious intermediary could alter the headers as they travel
in either direction, with unpredictable results.
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3. A malicious client could alter the Cookie header before
transmission, with unpredictable results.
Servers SHOULD encrypt and sign the contents of cookies when
transmitting them to the user agent (even when sending the cookies
over a secure channel). However, encrypting and signing cookie
contents does not prevent an attacker from transplanting a cookie
from one user agent to another or from replaying the cookie at a
later time.
In addition to encrypting and signing the contents of every cookie,
servers that require a higher level of security SHOULD use the cookie
protocol only over a secure channel. When using the cookie protocol
over a secure channel, servers SHOULD set the Secure attribute in
every cookie. If a server does not set the Secure attribute, the
protection provided by the secure channel will be largely moot.
7.4. Session Identifiers
Instead of storing session information directly in a cookie (where it
might be exposed to or replayed by an attacker), servers commonly
store a nonce (or "session identifier") in a cookie. When the server
receives an HTTP request with a nonce, the server can look up state
information associated with the cookie using the nonce as a key.
Using session identifier cookies limits the damage an attacker can
cause if the attacker learns the contents of a cookie because the
nonce is useful only for interacting with the server (unlike non-
nonce cookie content, which might itself be sensitive). Furthermore,
using a single nonce prevents an attacker from "splicing" together
cookie content from two interactions with the server, which could
cause the server to behave unexpectedly.
Using session identifiers is not without risk. For example, the
server SHOULD take care to avoid "session fixation" vulnerabilities.
A session fixation attack proceeds in three steps. First, the
attacker transplants a session identifier from his or her user agent
to the victim's user agent. Second, the victim uses that session
identifier to interact with the server, possibly imbuing the session
identifier with the user's credentials or confidential information.
Third, the attacker uses the session identifier to interact with
server directly, possibly obtaining the user's authority or
confidential information.
7.5. Weak Confidentiality
Cookies do not provide isolation by port. If a cookie is readable by
a service running on one port, the cookie is also readable by a
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service running on another port of the same server. If a cookie is
writable by a service on one port, the cookie is also writable by a
service running on another port of the same server. For this reason,
servers SHOULD NOT both run mutually distrusting services on
different ports of the same host and use cookies to store security-
sensitive information.
Cookies do not provide isolation by scheme. Although most commonly
used with the http and https schemes, the cookies for a given host
might also available to other schemes, such as ftp and gopher.
Although this lack of isolation by scheme is most apparent in via
non-HTTP APIs that permit access to cookies (e.g., HTML's
document.cookie API), the lack of isolation by scheme is actually
present in the cookie protocol itself (e.g., consider retrieving a
URI with the gopher scheme via HTTP).
Cookies do not always provide isolation by path. Although the
network-level protocol does not send cookie stored for one path to
another, some user agents expose cookies via non-HTTP APIs, such as
HTML's document.cookie API. Because some of these user agents (e.g.,
web browsers) do not isolate resources received from different paths,
a resource retrieved from one path might be able to access cookies
stored for another path.
7.6. Weak Integrity
Cookies do not provide integrity guarantees for sibling domains (and
their subdomains). For example, consider foo.example.com and
bar.example.com. The foo.example.com server can set a cookie with a
Domain attribute of ".example.com" (possibly overwriting an existing
".example.com" cookie set by bar.example.com), and the user agent
will include that cookie in HTTP requests to bar.example.com. In the
worst case, bar.example.com will be unable to distinguish this cookie
from a cookie it set itself. The foo.example.com server might be
able to leverage this ability to mount an attack against
bar.example.com.
Even though the cookie protocol supports the Path attribute, the Path
attribute does not provide any integrity protection because the user
agent with accept an arbitrary Path attribute in a Set-Cookie header.
For example, an HTTP response to a request for
http://example.com/foo/bar can set a cookie with a Path attribute of
"/qux". Consequently, servers SHOULD NOT both run mutually
distrusting services on different paths of the same host and use
cookies store security sensitive information.
An active network attacker can also inject cookies into the Cookie
header sent to https://example.com/ by impersonating a response from
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http://example.com/ and injecting a Set-Cookie header. The HTTPS
server at example.com will be unable to distinguish these cookies
from cookies that it set itself in an HTTPS response. An active
network attacker might be able to leverage this ability to mount an
attack against example.com even if example.com uses HTTPS
exclusively.
Servers can partially mitigate these attacks by encrypting and
signing the contents of their cookies. However, using cryptography
does not mitigate the issue completely because an attacker can replay
a cookie he or she received from the authentic example.com server in
the user's session, with unpredictable results.
Finally, an attacker might be able to force the user agent to delete
cookies by storing large number of cookies. Once the user agent
reaches its storage limit, the user agent will be forced to evict
some cookies. Servers SHOULD NOT rely upon user agents retaining
cookies.
7.7. Reliance on DNS
The cookie protocol relies upon the Domain Name System (DNS) for
security. If the DNS is partially or fully compromised, the cookie
protocol might fail to provide the security properties required by
applications.
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8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
8.2. Informative References
[RFC2109] Kristol, D. and L. Montulli, "HTTP State Management
Mechanism", RFC 2109, February 1997.
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Appendix A. Acknowledgements
This document borrows heavily from RFC 2109 [RFC2109]. [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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