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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

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.  This document may contain material
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Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.














































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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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