Internet DRAFT - draft-abarth-cookie
draft-abarth-cookie
httpstate A. Barth
Internet-Draft U.C. Berkeley
Obsoletes: 2109 (if approved) December 30, 2009
Intended status: Standards Track
Expires: July 3, 2010
HTTP State Management Mechanism
draft-abarth-cookie-07
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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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 5
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 7
4. A Well-Behaved Profile . . . . . . . . . . . . . . . . . . . . 8
4.1. Set-Cookie . . . . . . . . . . . . . . . . . . . . . . . . 8
4.1.1. Syntax . . . . . . . . . . . . . . . . . . . . . . . . 8
4.1.2. Semantics (Non-Normative) . . . . . . . . . . . . . . 9
4.2. Cookie . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.2.1. Syntax . . . . . . . . . . . . . . . . . . . . . . . . 11
4.2.2. Semantics . . . . . . . . . . . . . . . . . . . . . . 11
5. The Cookie Protocol . . . . . . . . . . . . . . . . . . . . . 12
5.1. Algorithms . . . . . . . . . . . . . . . . . . . . . . . . 12
5.1.1. Dates . . . . . . . . . . . . . . . . . . . . . . . . 12
5.1.2. Domains . . . . . . . . . . . . . . . . . . . . . . . 14
5.1.3. Paths . . . . . . . . . . . . . . . . . . . . . . . . 14
5.2. The Set-Cookie Header . . . . . . . . . . . . . . . . . . 15
5.2.1. The Max-Age Attribute . . . . . . . . . . . . . . . . 16
5.2.2. The Expires Attribute . . . . . . . . . . . . . . . . 17
5.2.3. The Domain Attribute . . . . . . . . . . . . . . . . . 18
5.2.4. The Path Attribute . . . . . . . . . . . . . . . . . . 18
5.2.5. The Secure Attribute . . . . . . . . . . . . . . . . . 19
5.2.6. The HttpOnly Attribute . . . . . . . . . . . . . . . . 19
5.3. Storage Model . . . . . . . . . . . . . . . . . . . . . . 19
5.4. The Cookie Header . . . . . . . . . . . . . . . . . . . . 21
6. Implementation Limits . . . . . . . . . . . . . . . . . . . . 23
7. Security Considerations . . . . . . . . . . . . . . . . . . . 24
7.1. Clear Text . . . . . . . . . . . . . . . . . . . . . . . . 24
7.2. Weak Confidentiality . . . . . . . . . . . . . . . . . . . 24
7.3. Weak Integrity . . . . . . . . . . . . . . . . . . . . . . 24
8. Normative References . . . . . . . . . . . . . . . . . . . . . 26
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 27
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 28
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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
(called cookies) at the user agent. When the user agent makes
subsequent requests to the server, the user agent will 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 persist 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.
1.1. 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).
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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.
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 the user agent, and for the user agent to return the state
information to the origin server.
The origin server initiates a session, if it so desires, by including
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 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 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
[TODO: Put some examples here.
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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 semi-colon-separated attribute-value
pairs. Servers SHOULD NOT send Set-Cookie headers that fail to
conform to the following grammar:
set-cookie-header = "Set-Cookie:" OWS a-cookie OWS
a-cookie = 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" "=" cookie-date
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, possibly in a server-selected
printable ASCII encoding. "Opaque" implies that the content is of
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interest and relevance only to the origin server. The content is, in
fact, be readable by anyone who examines the Set-Cookie header.
NOTE: The syntax above allows whitespace between the attribute and
the U+3D ("=") 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 setting their values to the empty
string or by including an Expires attribute with a value in the past.
By default, cookies are returned only to the origin server and expire
at a the end of the current session (as defined by the user agent).
The server can override the default handling of cookies by specifying
various cookie attributes. User agents ignore unrecognized cookie
attributes.
4.1.2.1. Expires
The Expires attribute represent the maximum lifetime of the cookie,
represented as the date and time at which the cookie expires. The
user agent is not required to persist 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 the hosts for which the cookie is
applicable. For example, if the domain attribute contains the value
".example.com", the use agent will include the cookie in the Cookie
header when making HTTPS requests to example.com, www.example.com,
and www.corp.example.com. (Note that the leading U+2E (".") is
meaningless and not required.) If the server omits the Domain
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attribute, the user agent will return the cookie only to the origin
server
The user agent will reject cookies less 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 a response from
foo.example.com, but the user agent will not accept a Domain
attribute of ".bar.example.com" or ".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+2F ("/") 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, a
response to a request for /foo/bar.html can include a Set-Cookie
header with a Path attribute of "/baz". 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
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 [RFC5234]).
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
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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 = "Set-Cookie:" OWS cookie-pair *( ";" cookie-pair) OWS
cookie-pair = cookie-name "=" cookie-value
cookie-name = token
cookie-value = token
token = <token, as defined in Section 2.2 of RFC 2616>
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 is marked Secure or
HttpOnly.
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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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 precision to enable a user agent that implement
the protocol precisely as specified to interoperate with existing
servers.
Although some parts of the cookie protocol is specified
algorithmically, user agents are free to implement the cookie
protocol in any manner as long as their resultant behavior is "black-
box" indistinguishable from a user agent that implements the protocol
as described.
5.1. Algorithms
The cookie protocol uses a number of self-contained algorithms, which
are described in this section.
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 = 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 the cookie-domain
is a suffix of the canonicalized request-host and at least one of the
following conditions hold:
o The cookie-domain and the canonicalized request-host are
identical.
o The last character of the canonicalized request-host that is not
included in the cookie-domain is a U+2E (".") 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+2F ("/")
character, output U+2F ("/") and skip the remaining steps.
3. If the uri-path contains only a single U+2F ("/") character,
output U+2F ("/") and skip the remaining steps.
4. Output the characters of the uri-path from the first character up
to, and but not including, the right-most U+2F ("/").
A request-path *path-matches* a cookie-path if the cookie-path is a
prefix of the request-path and at least one of the following
conditions hold: [TODO: This isn't exactly what IE does.]
o The cookie-path and the request-path are identical.
o The last character of the cookie-path is U+2F ("/").
o The first character of the request-path that is not included in
the cookie-path is a U+2F ("/") character.
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5.2. The Set-Cookie Header
When a user agent receives an 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 entirely.
2. If the header contains a U+3B (";") character:
The name-value-pair string is characters up to, but not
including, the first U+3B (";"), and the unparsed-attributes
are the remainder of the header (including the U+3B (";") in
question).
Otherwise:
The name-value-pair string is all the character contained in
the header, and the unparsed-attributes is the empty string.
3. If the name-value-pair string contains a U+3D ("=") character:
The (possibly empty) name string is the characters up to, but
not including, the first U+3D ("=") character, and the
(possibly empty) value string is the characters after the
first U+3D ("=") character.
Otherwise:
The name string is empty, and the value string is 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:
1. If the unparsed-attributes string is empty, skip the rest of
these steps.
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2. Consume the first character of the unparsed-attributes (which
will be a U+3B (";") character).
3. If the remaining unparsed-attributes contains a U+3B (";")
character:
Consume the characters of the unparsed-attributes up to, but
not including, the first U+3B (";") 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+3D ("=") character:
The (possibly empty) attribute-name string is the characters
up to, but not including, the first U+3D ("=") character, and
the (possibly empty) attribute-value string is the characters
after the first U+3D ("=") character.
Otherwise:
The attribute-name string is 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 header,
the user agent *receives a cookie* from the Request-URI with name
cookie-name, value cookie-value, and attributes cookie-attribute-
list.
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 "-"
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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 time-delta be the number of seconds between the server-date
and the parsed-cookie-date.
Let the expiry-time be the current date and time plus delta-
seconds seconds.
Otherwise:
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
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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+2E ("."):
Let cookie-domain be the attribute-value with the leading U+2E
(".") character.
Otherwise:
Let cookie-domain be the entire attribute-value.
[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 canonicalized 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+2F ("/"):
Let cookie-path be the default-path. [TODO: We need more tests
for this, including with " characters and with multiple Path
attributes.]
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.
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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 Secure 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.
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:
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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".
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.
4. 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.
5. 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.
6. 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.
7. Remove from the cookie store all cookies that have the 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.]
8. Insert the newly created cookie into the cookie store unless the
cookie's name and value are both empty.
The user agent MUST evict a cookie from the cookie store if a cookie
exists in the cookie store with an expiry date in the past.
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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).
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 more than a predetermined
number of other cookies.
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 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 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:
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The cookie's host-only-flag is false and the request-host
domain-matches cookie's domain.
* 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 an "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 include the
cookie unless the cookie-string is begin generated for a "non-
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+3D ("=") character.
2. Output the cookie's value.
3. If there is an unprocessed cookie in the cookie-list, output
the characters U+3B and U+20 ("; ").
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6. Implementation 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.
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7. Security Considerations
7.1. Clear Text
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.
3. A malicious client could alter the Cookie header before
transmission, with unpredictable results.
Servers SHOULD encrypt and sign their cookies. However, encrypting
and signing cookies does not prevent an attacker from transplanting a
cookie from one user agent to another.
In addition to encrypting and signing the the contents of every
cookie, servers that require a higher level of security SHOULD use
the cookie protocol only over a secure channel.
7.2. Weak Confidentiality
Cookies do provide isolation by port. If a cookie is readable by a
service running on one port, the cookie is also readable by a 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 machine 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
are also available to other schemes, such as ftp and gopher. This
lack of isolation is most easily seen when a user agent retrieves a
URI with a gopher scheme via HTTP, but the lack of isolation by
scheme is also apparent via non-HTTP APIs that permit access to
cookies, such as HTML's document.cookie API.
7.3. Weak Integrity
Cookies do not 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
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Domain attribute of ".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.
Similarly, an active network attacker can inject cookies into the
Cookie header sent to https://example.com/ by impersonating a
response from 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 their cookies. However, using cryptography does not fully
ameliorate the issue 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.
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8. Normative References
[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.
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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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