HTTP State Management Mechanism
University of California, Berkeley
abarth@eecs.berkeley.eduhttp://www.adambarth.com/httpstateThis document defines the HTTP Cookie and Set-Cookie header fields.
These header fields can be used by HTTP servers to store state (called
cookies) at HTTP user agents, letting the servers maintain a stateful
session over the mostly stateless HTTP protocol. Although cookies have
many historical infelicities that degrade their security and privacy, the
Cookie and Set-Cookie header fields are widely used on the Internet.If you have suggestions for improving this document, please send
email to . Suggestions with
test cases are especially appreciated. Further Working Group information
is available from .This document defines the HTTP Cookie and Set-Cookie header fields.
Using the Set-Cookie header field, an HTTP server can pass name/value
pairs and associated metadata (called cookies) to a user agent. When the
user agent makes subsequent requests to the server, the user agent uses
the metadata and other information to determine whether to return the
name/value pairs in the Cookie header.Although simple on their surface, cookies have a number of
complexities. For example, the server indicates a scope for each cookie
when sending it to the user agent. The scope indicates the maximum
amount of time the user agent should return the cookie, the servers to
which the user agent should return the cookie, and the URI schemes for
which the cookie is applicable.For historical reasons, cookies contain 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
does not provide integrity in the presence of an active network
attacker. 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 via different ports.There are two audiences for this specifications: developers of
cookie-generating servers and developers of cookie-consuming user
agents.To maximize interoperability with user agents, servers SHOULD limit
themselves to the well-behaved profile defined in when generating cookies.User agents MUST implement the more liberal processing rules defined
in , in order to maximize
interoperability with existing servers that do not conform to the
well-behaved profile defined in .This document specifies the syntax and semantics of these headers as
they are actually used on the Internet. In particular, this document
does not create new syntax or semantics beyond those in use today. The
recommendations for cookie generation provided in represent a preferred subset of current
server behavior, and even the more liberal cookie processing algorithm
provided in does not recommend all of
the syntactic and semantic variations in use today. Where some existing
software differs from the recommended protocol in significant ways, the
document contains a note explaining the difference.Prior to this document, there were at least three descriptions of
cookies: the so-called "Netscape cookie specification" , RFC 2109 , and RFC 2965
. However, none of these documents describe how
the Cookie and Set-Cookie headers are actually used on the Internet (see
for historical context). In relation to
previous IETF specifications of HTTP state management mechanisms, this
document requests the following actions:
Change the status of to Historic (it has
already been obsoleted by ).Change the status of to Historic.Indicate that is obsoleted by this
document.
In particular, in moving RFC 2965 to Historic and obsoleting it, this
document deprecates the use of the Cookie2 and Set-Cookie2 header
fields.The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in .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.Conformance requirements phrased as algorithms or specific steps can
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 understand and are not intended to be
performant.This specification uses the Augmented Backus-Naur Form (ABNF)
notation of .The following core rules are included by reference, as defined in
, 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), NUL
(null octet), OCTET (any 8-bit sequence of data except NUL), SP (space),
HTAB (horizontal tab), CHAR (any US-ASCII character), VCHAR (any
visible US-ASCII character), and WSP (whitespace).The OWS (optional whitespace) rule is used where zero or more
linear whitespace characters MAY appear:
OWS SHOULD either not be produced or be produced as a single SP
character.The terms user agent, client, server, proxy, and origin server have
the same meaning as in the HTTP/1.1 specification (, Section 1.3).The request-host is the name of the host, as known by the user
agent, to which the user agent is sending an HTTP request or is
receiving an HTTP response from (i.e., the name of the host to which it
sent the corresponding HTTP request).The term request-uri is defined in Section 5.1.2 of .Two sequences of octets are said to case-insensitively match each
other if and only if they are equivalent under the i;ascii-casemap
collation defined in .The term string means a sequence of non-NUL octets.This section outlines 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 store state, the origin server includes a Set-Cookie header in an
HTTP response. In subsequent requests, the user agent returns a Cookie
request header to the origin server. The Cookie header contains
cookies the user agent received in previous Set-Cookie headers. The
origin server is free to ignore the Cookie header or use its contents
for an application-defined purpose.Origin servers MAY send a Set-Cookie response header with any
response. User agents MAY ignore Set-Cookie headers contained in
responses with 100-level status codes but MUST process Set-Cookie headers
contained in other responses (including responses with 400- and 500-level
status codes). An origin server can include multiple Set-Cookie header
fields in a single response. The presence of a Cookie or a Set-Cookie
header field does not preclude HTTP caches from storing and reusing a
response.Origin servers SHOULD NOT fold multiple Set-Cookie header fields into
a single header field. The usual mechanism for folding HTTP headers
fields (i.e., as defined in ) might change the
semantics of the Set-Cookie header field because the %x2C (",") character
is used by Set-Cookie in a way that conflicts with such folding.Using the Set-Cookie header, a server can send the user agent a
short string in an HTTP response that the user agent will return in
future HTTP requests that are within the scope of the cookie. 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.The server can alter the default scope
of the cookie using the Path 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.As shown in the next example, the server can store multiple cookies
at 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 header fields. Notice that the server uses the Secure and
HttpOnly attributes to provide additional security protections for the
more-sensitive session identifier (see .)Notice that the Cookie header above contains two cookies, one named
SID and one named lang. If the server wishes the user agent to persist
the cookie over multiple "sessions" (e.g., user agent restarts), the
server can specify an expiration date in the Expires attribute. Note
that the user agent 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.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.This section describes the syntax and semantics of a well-behaved
profile of the Cookie and Set-Cookie headers.The Set-Cookie HTTP response header is used to send cookies from the
server to the user agent.Informally, the Set-Cookie response header contains the header
name "Set-Cookie" followed by a ":" and 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:Note that some of the grammatical terms above reference documents
that use different grammatical notations than this document (which
uses ABNF from ).The semantics of the cookie-value are not defined by this
document.To maximize compatibility with user agents, servers that wish to
store arbitrary data in a cookie-value SHOULD encode that data, for
example, using Base64 .The portions of the set-cookie-string produced by the cookie-av
term are known as attributes. To maximize compatibility with user
agents, servers SHOULD NOT produce two attributes with the same name
in the same set-cookie-string. (See
for how user agents handle this case.)Servers SHOULD NOT include more than one Set-Cookie header field
in the same response with the same cookie-name. (See for how user agents handle this case.)If a server sends multiple responses containing Set-Cookie headers
concurrently to the user agent (e.g., when communicating with the
user agent over multiple sockets), these responses create a "race
condition" that can lead to unpredictable behavior.NOTE: Some existing user agents differ on their interpretation of
two-digit years. To avoid compatibility issues, servers SHOULD use
the rfc1123-date format, which requires a four-digit year.NOTE: Some user agents store and process dates in cookies as
32-bit UNIX time_t values. Implementation bugs in the libraries
supporting time_t processing on some systems might cause such user
agents to process dates after the year 2038 incorrectly.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 cookies by servers. The full
semantics are described in .When the user agent receives a Set-Cookie header, the user agent
stores the cookie together with its attributes. Subsequently, when
the user agent makes an HTTP request, the user agent includes the
applicable, non-expired cookies in the Cookie header.If the user agent receives a new cookie with the same cookie-name,
domain-value, and path-value as a cookie that it has already stored,
the existing cookie is evicted and replaced with the new cookie.
Notice that servers can delete cookies by sending the user agent a
new cookie with 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 not, e.g., to any
subdomains), and it expires at the end of the current session (as
defined by the user agent). User agents ignore unrecognized cookie
attributes (but not the entire cookie).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 due to memory pressure or privacy concerns.The Max-Age attribute indicates the maximum lifetime of the
cookie, represented as the number of seconds until the cookie
expires. The user agent is not required to retain the cookie for
the specified duration. In fact, user agents often evict
cookies from due to memory pressure or privacy concerns.
NOTE: Some existing user agents do not support the Max-Age
attribute. User agents that do not support the Max-Age attribute
ignore the attribute.If a cookie has both the Max-Age and the Expires attribute, the
Max-Age attribute has precedence and controls the expiration date
of the cookie. If a cookie has neither the Max-Age nor the Expires
attribute, the user agent will retain the cookie until "the current
session is over" (as defined by the user agent).The Domain attribute specifies those hosts to which the cookie
will be sent. For example, if the value of the Domain attribute
is "example.com", the user agent will include the cookie in
the Cookie header when making HTTP requests to example.com,
www.example.com, and www.corp.example.com. (Note that a leading
%x2E ("."), if present, is ignored even though that character is
not permitted, but a trailing %x2E ("."), if present, will cause
the user agent to ignore the attribute.) If the server omits the
Domain attribute, the user agent will return the cookie only to the
origin server.
WARNING: Some existing 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 erroneously send the cookie to www.example.com as well.The user agent will reject cookies unless the Domain attribute
specifies a scope for the cookie that would include the origin
server. For example, the user agent will accept a cookie with a
Domain attribute of "example.com" or of "foo.example.com" from
foo.example.com, but the user agent will not accept a cookie with
a Domain attribute of "bar.example.com" or of
"baz.foo.example.com".NOTE: For security reasons, many user agents are configured to
reject Domain attributes that correspond to "public suffixes". For
example, some user agents will reject Domain attributes of "com" or
"co.uk". (See for more
information.)The scope of each cookie is limited to a set of paths,
controlled by the Path attribute. 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. (See for more details.)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 %x2F
("/") character is interpreted as a directory separator.Although seemingly useful for isolating cookies between
different paths within a given host, the Path attribute cannot
be relied upon for security (see ).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 HTTP over Transport Layer Security (TLS)
).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 their integrity (see
for more details).The HttpOnly attribute limits the scope of the cookie to HTTP
requests. In particular, the attribute instructs the user agent to
omit the cookie when providing access to cookies via "non-HTTP"
APIs (such as a web browser API that exposes cookies to
scripts).Note that the HttpOnly attribute is independent of the Secure
attribute: a cookie can have both the HttpOnly and the Secure
attribute.The user agent sends stored cookies to the origin server in the
Cookie header. If the server conforms to the requirements in (and the user agent conforms to the
requirements in ), the user
agent will send a Cookie header that conforms to the following
grammar:Each cookie-pair represents a cookie stored by the user agent.
The cookie-pair contains the cookie-name and cookie-value the user
agent received in 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 hosts 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 are not
defined by this document. Servers are expected to imbue these cookies
with application-specific semantics.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 (e.g.,
that were set with different Path or Domain attributes), servers
SHOULD NOT rely upon the order in which these cookies appear in the
header.This section specifies the Cookie and Set-Cookie headers in sufficient
detail that a user agent implementing these requirements precisely can
interoperate with existing servers (even those that do not conform to the
well-behaved profile described in ).A user agent could enforce more restrictions than those specified
herein (e.g., for the sake of improved security); however, experiments
have shown that such strictness reduces the likelihood that a user agent
will be able to interoperate with existing servers.This section defines some algorithms used by user agents to process
specific subcomponents of the Cookie and Set-Cookie headers.The user agent MUST use an algorithm equivalent to the following
algorithm to parse a cookie-date. Note that the various boolean
flags defined as a part of the algorithm (i.e., found-time,
found-day-of-month, found-month, found-year) are initially "not set".
Using the grammar below, divide the cookie-date into
date-tokens.
Process each date-token sequentially in the order the
date-tokens appear in the cookie-date:
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.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.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.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.If the year-value is greater than or equal to 70 and less than
or equal to 99, increment the year-value by 1900.If the year-value is greater than or equal to 0 and less than
or equal to 69, increment the year-value by 2000.
NOTE: Some existing user agents interpret two-digit years
differently.Abort these steps and fail to parse the cookie-date 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,the hour-value is greater than 23,the minute-value is greater than 59, orthe second-value is greater than 59.
(Note that leap seconds cannot be represented in this syntax.)
Let the parsed-cookie-date be the date whose day-of-month,
month, year, hour, minute, and second (in UTC) are the
day-of-month-value, the month-value, the year-value, the
hour-value, the minute-value, and the second-value,
respectively. If no such date exists, abort these steps and fail
to parse the cookie-date.Return the parsed-cookie-date as the result of this
algorithm.A canonicalized host name is the string generated by the following algorithm:
Convert the host name to a sequence of individual domain name
labels.Convert each label that is not a NR-LDH label, to a A-label
(see Section 2.3.2.1 of for the fomer and
latter), or to a "punycode label" (a label resulting from the
"ToASCII" conversion in Section 4 of ), as
appropriate (see Section 6.3 of this specification).Concatentate the resulting labels, separated by a %x2E (".")
character.A string domain-matches a given domain string if at least one of
the following conditions hold:
The domain string and the string are identical. (Note that both
the domain string and the string will have been canonicalized to
lower case at this point.)All of the following conditions hold:
The domain string is a suffix of the string.The last character of the string that is not included in the
domain string is a %x2E (".") character.The string is a host name (i.e., not an IP address).The user agent MUST use an algorithm equivalent to the following
algorithm to compute the default-path of a cookie:
Let uri-path be the path portion of the request-uri if such a
portion exists (and empty otherwise). For example, if the
request-uri contains just a path (and optional query string), then
the uri-path is that path (without the %x3F ("?") character or
query string), and if the request-uri contains a full absoluteURI,
the uri-path is the path component of that URI.If the uri-path is empty or if the first character of the
uri-path is not a %x2F ("/") character, output %x2F ("/") and skip
the remaining steps.If the uri-path contains only a single %x2F ("/") character,
output %x2F ("/") and skip the remaining steps.Output the characters of the uri-path from the first character
up to, but not including, the right-most %x2F ("/").A request-path path-matches a given cookie-path if at least one of
the following conditions hold:
The cookie-path and the request-path are identical.The cookie-path is a prefix of the request-path and the last
character of the cookie-path is %x2F ("/").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-path is a %x2F ("/") character.When a user agent receives a Set-Cookie header field in an HTTP
response, the user agent MAY ignore the Set-Cookie header field in its
entirety. For example, the user agent might wish to block responses to
"third-party" requests from setting cookies (See ).If the user agent does not ignore the Set-Cookie header field in its
entirety, the user agent MUST parse the field-value of the Set-Cookie
header field as a set-cookie-string (defined below).NOTE: The algorithm below is more permissive than the grammar in
. For example, the algorithm strips
leading and trailing whitespace from the cookie name and value (but
maintains internal whitespace), whereas the grammar in forbids whitespace in these positions.
User agents use this algorithm so as to interoperate with servers that
do not follow the recommendations in .A user agent MUST use an algorithm equivalent to the following
algorithm to parse a "set-cookie-string":
If the set-cookie-string contains a %x3B (";") character:
The name-value-pair string consists of the characters up to,
but not including, the first %x3B (";"), and the
unparsed-attributes consist of the remainder of the
set-cookie-string (including the %x3B (";") 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.If the name-value-pair string lacks a %x3D ("=") character,
ignore the set-cookie-string entirely.The (possibly empty) name string consists of the characters up to,
but not including, the first %x3D ("=") character, and the
(possibly empty) value string consists of the characters after the
first %x3D ("=") character.Remove any leading or trailing WSP characters from the name
string and the value string.If the name string is empty, ignore the set-cookie-string
entirely.The cookie-name is the name string, and the cookie-value is the
value string.The user agent MUST use an algorithm equivalent to the following
algorithm to parse the unparsed-attributes:
If the unparsed-attributes string is empty, skip the rest of
these steps.Discard the first character of the unparsed-attributes (which
will be a %x3B (";") character).If the remaining unparsed-attributes contains a %x3B (";")
character:
Consume the characters of the unparsed-attributes up to, but
not including, the first %x3B (";") character.
Otherwise:
Consume the remainder of the unparsed-attributes.
Let the cookie-av string be the characters consumed in this
step.If the cookie-av string contains a %x3D ("=") character:
The (possibly empty) attribute-name string consists of the
characters up to, but not including, the first %x3D ("=")
character, and the (possibly empty) attribute-value string
consists of the characters after the first %x3D ("=")
character.
Otherwise:
The attribute-name string consists of the entire cookie-av
string, and the attribute-value string is empty.Remove any leading or trailing WSP characters from the
attribute-name string and the attribute-value string.Process the attribute-name and attribute-value according to the
requirements in the following subsections. (Notice that attributes
with unrecognized attribute-names are ignored.)Return to Step 1 of this algorithm.When the user agent finishes parsing the set-cookie-string, the user
agent is said to "receive a cookie" from the request-uri with name
cookie-name, value cookie-value, and attributes cookie-attribute-list.
(See for additional requirements
triggered by receiving a cookie.)If the attribute-name case-insensitively matches the string
"Expires", the user agent MUST process the cookie-av as follows.Let the expiry-time be the result of parsing the attribute-value
as cookie-date (see ).If the attribute-value failed to parse as a cookie date, ignore
the cookie-av.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 earliest date the user
agent can represent, the user agent MAY replace the expiry-time with
the earliest representable date.Append an attribute to the cookie-attribute-list with an
attribute-name of Expires and an attribute-value of expiry-time.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 earliest representable 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 Max-Age and an attribute-value of expiry-time.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 %x2E
("."):
Let cookie-domain be the attribute-value without the leading
%x2E (".") character.
Otherwise:
Let cookie-domain be the entire attribute-value.Convert the cookie-domain to lower case.Append an attribute to the cookie-attribute-list with an
attribute-name of Domain and an attribute-value of
cookie-domain.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 %x2F ("/"):
Let cookie-path be the default-path.
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.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.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.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:
A user agent MAY ignore a received cookie in its entirety. For
example, the user agent might wish to block receiving cookies from
"third-party" responses or the user agent might not wish to store
cookies that exceed some size.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.If the cookie-attribute-list contains an attribute with an
attribute-name of "Max-Age":
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
"Max-Age".
Otherwise, if the cookie-attribute-list contains an attribute with
an attribute-name of "Expires" (and does not contain an attribute
with an attribute-name of "Max-Age"):
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".
Otherwise:
Set the cookie's persistent-flag to false.Set the cookie's expiry-time to the latest representable
date.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.If the user agent is configured to reject "public suffixes" and
the domain-attribute is a public suffix:
If the domain-attribute is identical to the canonicalized
request-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 .If the domain-attribute is non-empty:
If the canonicalized request-host does not domain-match the
domain-attribute:
Ignore the cookie entirely and abort these steps.
Otherwise:
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 canonicalized request-host.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.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.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.If the cookie was received from a "non-HTTP" API and the
cookie's http-only-flag is set, abort these steps and ignore the
cookie entirely.If the cookie store contains a cookie with the same name, domain,
and path as the newly created cookie:
Let old-cookie be the existing cookie with the same name,
domain, and path as the newly created cookie. (Notice that this
algorithm maintains the invariant that there is at most one such
cookie.)If the newly created cookie was received from a "non-HTTP"
API and the old-cookie's http-only-flag is set, abort these
steps and ignore the newly created cookie entirely.Update the creation-time of the newly created cookie to match
the creation-time of the old-cookie.Remove the old-cookie from the cookie store.Insert the newly created cookie into the cookie store.A cookie is "expired" if the cookie has an expiry date in the
past.The user agent MUST evict all expired cookies from the cookie store
if, at any time, an expired cookie exists in the cookie store.At any time, the user agent MAY "remove excess cookies" from the
cookie store if the number of cookies sharing a domain field exceeds
some implementation defined upper bound (such as 50 cookies).At any time, the user agent MAY "remove excess cookies" from the
cookie store if the cookie store exceeds some predetermined upper bound
(such as 3000 cookies).When the user agent removes excess cookies from the cookie store,
the user agent MUST evict cookies in the following priority order:
Expired cookies.Cookies that share a domain field with more than a predetermined
number of other cookies.All cookies.If two cookies have the same removal priority, the user agent MUST
evict the cookie with the earliest 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.The user agent includes stored cookies in the Cookie HTTP request
header.When the user agent generates an HTTP request, the user agent MUST
NOT attach more than one Cookie header field.A user agent MAY omit the Cookie header in its entirety. For
example, the user agent might wish to block sending cookies during
"third-party" requests from setting cookies (See ).If the user agent does attach a Cookie header field to an HTTP
request, the user agent MUST send the cookie-string (defined below) as
the value of the header field.The user agent MUST use an algorithm equivalent to the following
algorithm to compute the "cookie-string" from a cookie store and a
request-uri:
Let cookie-list be the set of cookies from the cookie store
that meet all of the following requirements:
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 canonicalized
request-host domain-matches cookie's domain.The request-uri's path path-matches cookie's path.If the cookie's secure-only-flag 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 SSL or TLS. For example, most user agents consider
"https" to be a scheme that denotes a secure protocol.If the cookie's http-only-flag is true, then exclude the
cookie if the cookie-string is being generated for a
"non-HTTP" API (as defined by the user agent).The user agent SHOULD 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.
NOTE: Not all user agents sort the cookie-list in this order, but
this order reflects common practice when this document was written,
and, historically, there have been servers that (erroneously)
depended on this order.Update the last-access-time of each cookie in the cookie-list
to the current date and time.Serialize the cookie-list into a cookie-string by processing each
cookie in the cookie-list in order:
Output the cookie's name, the %x3D ("=") character, and the
cookie's value.If there is an unprocessed cookie in the cookie-list, output
the characters %x3B and %x20 ("; ").
NOTE: Despite its name, the cookie-string is actually a sequence of
octets, not a sequence of characters. To convert the cookie-string (or
components thereof) into a sequence of characters (e.g., for
presentation to the user), the user agent might wish to try using the
UTF-8 character encoding to decode the octet
sequence. This decoding might fail, however, because not every
sequence of octets is valid UTF-8.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:
At least 4096 bytes per cookie (as measured by the sum of the
length of the cookie's name, value, and attributes).At least 50 cookies per domain.At least 3000 cookies total.Servers SHOULD use as few and as small cookies as possible to avoid
reaching these implementation limits and to minimize network bandwidth
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.One reason the Cookie and Set-Cookie headers uses such esoteric
syntax is because many platforms (both in servers and user agents)
provide a string-based application programing interface (API) to
cookies, requiring application-layer programmers to generate and parse
the syntax used by the Cookie and Set-Cookie headers, which many
programmers have done incorrectly, resulting in interoperability
problems.Instead of providing string-based APIs to cookies, platforms 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 an abstract "Date" object instead of a
serialized date string.IDNA2008 supersedes IDNA2003 . However, there are differences between the two
specifications, and thus there can be differences in processing (e.g.
converting) domain name labels that have been registered under one from
those registered under the other. There will be a transition period of
some time during which IDNA2003-based domain name labels will exist in
the wild. User agents SHOULD implement IDNA2008 and MAY implement or in order to facilitate their IDNA transition. If a user agent does
not implement IDNA2008, the user agent MUST implement IDNA2003 .Cookies are often criticized for letting servers track
users. For example, a number of "web analytics" companies use cookies
to recognize when a user returns to a web site or visits another web
site. Although cookies are not the only mechanism servers can use to
track users across HTTP requests, cookies facilitate tracking because
they are persistent across user agent sessions and can be shared
between hosts.Particularly worrisome are so-called "third-party" cookies. In
rendering an HTML document, a user agent often requests resources
from other servers (such as advertising networks). These third-party
servers can use cookies to track the user even if the user never
visits the server directly. For example, if a user visits a site
that contains content from a third party and then later visits
another site that contains content from the same third party, the
third party can track the user between the two sites.Some user agents restrict how third-party cookies behave. For
example, some of these user agents refuse to send the Cookie
header in third-party requests. Others refuse to process the
Set-Cookie header in responses to third-party requests. User
agents vary widely in their third-party cookie policies. This
document grants user agents wide latitude to experiment with
third-party cookie policies that balance the privacy and
compatibility needs of their users. However, this document does
not endorse any particular third-party cookie policy.Third-party cookie blocking policies are often ineffective at
achieving their privacy goals if servers attempt to work around
their restrictions to track users. In particular, two collaborating
servers can often track users without using cookies at all by
injecting identifying information into dynamic URLs.User agents SHOULD provide users with a mechanism for managing
the cookies stored in the cookie store. For example, a user agent
might let users delete all cookies received during a specified time
period or all the cookies related to a particular domain. In
addition, many user agents include a user interface element that lets
users examine the cookies stored in their cookie store.User agents SHOULD provide users with a mechanism for disabling
cookies. When cookies are disabled, the user agent MUST NOT include
a Cookie header in outbound HTTP requests and the user agent MUST
NOT process Set-Cookie headers in inbound HTTP responses.Some user agents provide users the option of preventing
persistent storage of cookies across sessions. When configured
thusly, user agents MUST treat all received cookies as if the
persistent-flag were set to false. Some popular user agents expose
this functionality via "private browsing" mode Some user agents provide users with the ability to approve
individual writes to the cookie store. In many common usage
scenarios, these controls generate a large number of prompts.
However, some privacy-conscious users find these controls useful
nonetheless.Although servers can set the expiration date for cookies to the
distant future, most user agents do not actually retain cookies for
multiple decades. Rather than chosing gratiously long expiration
periods, servers SHOULD promote user privacy by selecting reasonable
cookie expiration periods based on the purpose of the cookie. For
example, a typical session identifier might reasonably be set to
expire in two weeks.Cookies have a number of security pitfalls. This section overviews
a few of the more salient issues.In particular, cookies encourage developers to rely on ambient
authority for authentication, often becoming vulnerable to attacks
such as cross-site request forgery . Also, when
storing session identifiers in cookies, developers often create session
fixation vulnerabilities.Transport-layer encryption, such as that employed in HTTPS, is
insufficient to prevent a network attacker from obtaining or altering
a victim's cookies because the cookie protocol itself has various
vulnerabilities (see "Weak Confidentiality" and "Weak Integrity",
below). In addition, by default, cookies do not provide
confidentiality or integrity from network attackers, even when used in
conjunction with HTTPS.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 or
HTML forms). When issuing those requests, user agents attach cookies
even if the remote party does not know the contents of the cookies,
potentially letting the remote party 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
application of these principles can lead to more robust security.Unless sent over a secure channel (such as TLS), the information in
the Cookie and Set-Cookie headers is transmitted in the clear.
All sensitive information conveyed in these headers is exposed to
an eavesdropper.A malicious intermediary could alter the headers as they travel
in either direction, with unpredictable results.A malicious client could alter the Cookie header before
transmission, with unpredictable results.Servers SHOULD encrypt and sign the contents of cookies (using
whatever format the server desires) 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 and Set-Cookie headers only over a secure channel. When using
cookies over a secure channel, servers SHOULD set the Secure
attribute (see ) for every cookie. If a
server does not set the Secure attribute, the protection provided by
the secure channel will be largely moot.For example, consider a webmail server that stores a session
identifier in a cookie and is typically accessed over HTTPS. If the
server does not set the Secure attribute on its cookies, an active
network attacker can intercept any outbound HTTP request from the user
agent and redirect that request to the webmail server over HTTP. Even
if the webmail server is not listening for HTTP connections, the user
agent will still include cookies in the request. The active network
attacker can intercept these cookies, replay them against the server,
and learn the contents of the user's email. If, instead, the server
had set the Secure attribute on its cookies, the user agent would not
have included the cookies in the clear-text request.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.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
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 be available to other schemes, such as ftp and gopher.
Although this lack of isolation by scheme is most apparent in
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 requirements for processing cookies themselves (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 cookies 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.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 Set-Cookie header supports the Path attribute, the
Path attribute does not provide any integrity protection because the
user agent will 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 to 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 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 a 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.Cookies rely 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.The permanent message header field registry (see ) should be updated with the following
registrations:Header field name: CookieApplicable protocol: httpStatus: standardAuthor/Change controller: IETFSpecification document: this specification ()Header field name: Set-CookieApplicable protocol: httpStatus: standardAuthor/Change controller: IETFSpecification document: this specification ()Header field name: Cookie2Applicable protocol: httpStatus: obsoletedAuthor/Change controller: IETFSpecification document: Header field name: Set-Cookie2Applicable protocol: httpStatus: obsoletedAuthor/Change controller: IETFSpecification document: Domain names - concepts
and facilitiesInformation Sciences Institute (ISI)Requirements for Internet Hosts - Application and
SupportUniversity of Southern California (USC), Information
Sciences Institute4676 Admiralty WayMarina del ReyCA90292-6695US+1 213 822 1511Braden@ISI.EDU
Key words for use in RFCs to Indicate Requirement Levels
Harvard University1350 Mass. Ave.CambridgeMA 02138- +1 617 495 3864sob@harvard.edu
General
keywordIn many standards track documents several words are used to
signify the requirements in the specification. These words are
often capitalized. This document defines these words as they
should be interpreted in IETF documents. Authors who follow these
guidelines should incorporate this phrase near the beginning of
their document:
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described
in RFC 2119.Note that the force of these words is modified by the
requirement level of the document in which they are used.Hypertext Transfer Protocol -- HTTP/1.1University of California, Irvinefielding@ics.uci.eduW3Cjg@w3.orgCompaq Computer Corporationmogul@wrl.dec.comMIT Laboratory for Computer Sciencefrystyk@w3.orgXerox Corporationmasinter@parc.xerox.comMicrosoft Corporationpaulle@microsoft.comW3Ctimbl@w3.orgInternationalizing Domain Names in Applications (IDNA)Until now, there has been no standard method for domain names
to use characters outside the ASCII repertoire. This document
defines internationalized domain names (IDNs) and a mechanism
called Internationalizing Domain Names in Applications (IDNA) for
handling them in a standard fashion. IDNs use characters drawn
from a large repertoire (Unicode), but IDNA allows the non-ASCII
characters to be represented using only the ASCII characters
already allowed in so-called host names today. This
backward-compatible representation is required in existing
protocols like DNS, so that IDNs can be introduced with no
changes to the existing infrastructure. IDNA is only meant for
processing domain names, not free text. [STANDARDS TRACK]
See for an explanation why the
normative reference to an obsoleted specification is needed.
Internet Application Protocol Collation RegistryMany Internet application protocols include string-based
lookup, searching, or sorting operations. However, the problem
space for searching and sorting international strings is large,
not fully explored, and is outside the area of expertise for the
Internet Engineering Task Force (IETF). Rather than attempt to
solve such a large problem, this specification creates an
abstraction framework so that application protocols can precisely
identify a comparison function, and the repertoire of comparison
functions can be extended in the future.
Augmented BNF for Syntax Specifications: ABNF
Brandenburg InternetWorkingdcrocker@bbiw.netTHUS plc.paul.overell@thus.net
The Transport Layer Security (TLS) Protocol Version 1.2
Internationalized Domain Names for Applications (IDNA):
Definitions and Document FrameworkThis document is one of a collection that, together, describe
the protocol and usage context for a revision of Internationalized
Domain Names for Applications (IDNA), superseding the earlier
version. It describes the document collection and provides
definitions and other material that are common to the set.
[STANDARDS TRACK]HTTP State Management MechanismBell Laboratories, Lucent
Technologiesdmk@bell-labs.comNetscape Communications Corp.montulli@netscape.comHTTP State Management MechanismBell Laboratories, Lucent
Technologiesdmk@bell-labs.comEpinions.com, Inc.lou@montulli.orgHTTP Over TLSThis memo describes how to use Transport Layer Security (TLS)
to secure Hypertext Transfer Protocol (HTTP) connections over the
Internet. This memo provides information for the Internet
community.Persistent Client State -- HTTP CookiesNetscape Communications Corp.HTTP Cookies: Standards, Privacy, and PoliticsUTF-8, a transformation format of ISO 10646ISO/IEC 10646-1 defines a large character set called the
Universal Character Set (UCS) which encompasses most of the
world's writing systems. The originally proposed encodings of
the UCS, however, were not compatible with many current
applications and protocols, and this has led to the development
of UTF-8, the object of this memo. UTF-8 has the characteristic
of preserving the full US-ASCII range, providing compatibility
with file systems, parsers and other software that rely on
US-ASCII values but are transparent to other values. This memo
obsoletes and replaces RFC 2279.The Base16, Base32, and Base64 Data EncodingsThis document describes the commonly used base 64, base 32, and
base 16 encoding schemes. It also discusses the use of line-feeds
in encoded data, use of padding in encoded data, use of
non-alphabet characters in encoded data, use of different encoding
alphabets, and canonical encodings.Registration Procedures for Message Header FieldsMapping Characters for Internationalized Domain Names in
Applications (IDNA) 2008In the original version of the Internationalized Domain Names in
Applications (IDNA) protocol, any Unicode code points taken from
user input were mapped into a set of Unicode code points that "made
sense", and then encoded and passed to the domain name system
(DNS). The IDNA2008 protocol (described in RFCs 5890, 5891, 5892,
and 5893) presumes that the input to the protocol comes from a set
of "permitted" code points, which it then encodes and passes to the
DNS, but does not specify what to do with the result of user input.
This document describes the actions that can be taken by an
implementation between receiving user input and passing permitted
code points to the new IDNA protocol. This document is not an
Internet Standards Track specification; it is published for
informational purposes.Unicode IDNA Compatibility ProcessingRobust Defenses for Cross-Site Request ForgeryAn Analysis of Private Browsing Modes in Modern BrowsersThis document borrows heavily from RFC 2109 .
We are indebted to David M. Kristol and Lou Montulli for their efforts to
specify cookies. David M. Kristol, in particular, provided invaluable
advice on navigating the IETF process. We would also like to thank Thomas
Broyer, Tyler Close, Alissa Cooper, Bil Corry, corvid, Lisa Dusseault,
Roy T. Fielding, Blake Frantz, Anne van Kesteren, Eran Hammer-Lahav, Jeff
Hodges, Bjoern Hoehrmann, Achim Hoffmann, Georg Koppen, Dean McNamee,
Alexey Melnikov, Mark Miller, Mark Pauley, Yngve N. Pettersen, Julian
Reschke, Peter Saint-Andre, Mark Seaborn, Maciej Stachowiak, Daniel
Stenberg, Tatsuhiro Tsujikawa, David Wagner, Dan Winship, and Dan Witte
for their valuable feedback on this document.