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httpstate                                                       A. Barth
Internet-Draft                                             U.C. Berkeley
Obsoletes: 2109 (if approved)                             April 23, 2010
Intended status: Standards Track
Expires: October 25, 2010


                    HTTP State Management Mechanism
                     draft-ietf-httpstate-cookie-08

Abstract

   This document defines the HTTP Cookie and Set-Cookie headers.  These
   headers 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 headers are widely used on the Internet.

































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Editorial Note (To be removed by RFC Editor)

   If you have suggestions for improving this document, please send
   email to http-state@ietf.org.  Suggestions with test cases are
   especially appreciated. https://tools.ietf.org/wg/httpstate/

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on October 25, 2010.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
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   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow



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   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  General Nonsense . . . . . . . . . . . . . . . . . . . . . . .  6
     2.1.  Conformance Criteria . . . . . . . . . . . . . . . . . . .  6
     2.2.  Syntax Notation  . . . . . . . . . . . . . . . . . . . . .  6
     2.3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  7
   3.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     3.1.  Examples . . . . . . . . . . . . . . . . . . . . . . . . .  8
   4.  Server Requirements  . . . . . . . . . . . . . . . . . . . . . 10
     4.1.  Set-Cookie . . . . . . . . . . . . . . . . . . . . . . . . 10
       4.1.1.  Syntax . . . . . . . . . . . . . . . . . . . . . . . . 10
       4.1.2.  Semantics (Non-Normative)  . . . . . . . . . . . . . . 11
     4.2.  Cookie . . . . . . . . . . . . . . . . . . . . . . . . . . 13
       4.2.1.  Syntax . . . . . . . . . . . . . . . . . . . . . . . . 14
       4.2.2.  Semantics  . . . . . . . . . . . . . . . . . . . . . . 14
   5.  User Agent Requirements  . . . . . . . . . . . . . . . . . . . 15
     5.1.  Algorithms . . . . . . . . . . . . . . . . . . . . . . . . 15
       5.1.1.  Dates  . . . . . . . . . . . . . . . . . . . . . . . . 15
       5.1.2.  Domains  . . . . . . . . . . . . . . . . . . . . . . . 17
       5.1.3.  Paths  . . . . . . . . . . . . . . . . . . . . . . . . 17
     5.2.  The Set-Cookie Header  . . . . . . . . . . . . . . . . . . 18
       5.2.1.  The Expires Attribute  . . . . . . . . . . . . . . . . 20
       5.2.2.  The Max-Age Attribute  . . . . . . . . . . . . . . . . 20
       5.2.3.  The Domain Attribute . . . . . . . . . . . . . . . . . 21
       5.2.4.  The Path Attribute . . . . . . . . . . . . . . . . . . 21
       5.2.5.  The Secure Attribute . . . . . . . . . . . . . . . . . 22
       5.2.6.  The HttpOnly Attribute . . . . . . . . . . . . . . . . 22
     5.3.  Storage Model  . . . . . . . . . . . . . . . . . . . . . . 22
     5.4.  The Cookie Header  . . . . . . . . . . . . . . . . . . . . 25
   6.  Implementation Considerations  . . . . . . . . . . . . . . . . 28
     6.1.  Limits . . . . . . . . . . . . . . . . . . . . . . . . . . 28
     6.2.  Application Programmer Interfaces  . . . . . . . . . . . . 28
   7.  Privacy Considerations . . . . . . . . . . . . . . . . . . . . 29
     7.1.  Third-Party Cookies  . . . . . . . . . . . . . . . . . . . 29
     7.2.  User Controls  . . . . . . . . . . . . . . . . . . . . . . 29
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 31
     8.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . 31
     8.2.  Ambient Authority  . . . . . . . . . . . . . . . . . . . . 31



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     8.3.  Clear Text . . . . . . . . . . . . . . . . . . . . . . . . 32
     8.4.  Session Identifiers  . . . . . . . . . . . . . . . . . . . 32
     8.5.  Weak Confidentiality . . . . . . . . . . . . . . . . . . . 33
     8.6.  Weak Integrity . . . . . . . . . . . . . . . . . . . . . . 33
     8.7.  Reliance on DNS  . . . . . . . . . . . . . . . . . . . . . 34
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 35
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 35
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 35
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 36
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 37









































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1.  Introduction

   This document defines the HTTP Cookie and Set-Cookie headers.  Using
   the Set-Cookie header, an HTTP server can store name/value pairs and
   associated metadata (called cookies) at the user agent.  When the
   user agent makes subsequent requests to the server, the user agent
   uses the metadata to determine whether to return the name/value pairs
   in the Cookie header.

   Although simple on its surface, cookies have 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 return the cookie, the
   servers to which the user agent should return the cookie, and the
   protocols 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 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.

   Prior to this document, there were at least three descriptions of
   cookies: the so-called "Netscape cookie specification," RFC 2109
   [RFC2109], and RFC 2965 [RFC2965].  However, none of these documents
   describe how the Cookie and Set-Cookie headers are actually used on
   the Internet.  By contrast, this document attempts to specify the
   syntax and semantics of these headers as they are actually used on
   the Internet.



















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2.  General Nonsense

2.1.  Conformance Criteria

   The key words "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT",
   "RECOMMENDED", "MAY", and "OPTIONAL" in document are to be
   interpreted as described in [RFC2119].

   Requirements phrased in the imperative as part of algorithms (such as
   "strip any leading space characters" or "return false and abort these
   steps") are to be interpreted with the meaning of the key word
   ("MUST", "SHOULD", "MAY", etc) used in introducing the algorithm.

   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.

2.2.  Syntax Notation

   This specification uses the Augmented Backus-Naur Form (ABNF)
   notation of [RFC5234].

   The following core rules are included by reference, as defined in
   [RFC5234], Appendix B.1: ALPHA (letters), CR (carriage return), CRLF
   (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote),
   HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
   sequence of data), SP (space), HTAB (horizontal tab), 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            = *( [ obs-fold ] WSP )
                    ; "optional" whitespace
   obs-fold       = CRLF


   OWS SHOULD either not be produced or be produced as a single SP
   character.  Multiple OWS characters that occur within field-content
   SHOULD be replaced with a single SP before interpreting the field
   value or forwarding the message downstream.






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2.3.  Terminology

   The terms user agent, client, server, proxy, and origin server have
   the same meaning as in the HTTP/1.1 specification ([RFC2616]).

   The terms request-host and request-uri refer to the values the user
   agent would send to the server as, respectively, the host (but not
   port) and the absoluteURI (http_URL) of the HTTP Request-Line.











































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3.  Overview

   We outline here a way for an origin server to send state information
   to a user agent and for the user agent to return the state
   information to the origin server.

   To 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 a number of 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.  The
   origin server MAY send the user agent a Set-Cookie response header
   with the same or different information, or it MAY send no Set-Cookie
   header at all.

   Servers MAY return a Set-Cookie response header with any response.
   An origin server MAY include multiple Set-Cookie header fields in a
   single response.  Gateways that wish to be transparent to cookies
   MUST NOT fold multiple Set-Cookie header fields into a single header
   field.

3.1.  Examples

   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.  For example, the server can send the user agent a
   "session identifier" named SID with the value 31d4d96e407aad42.  The
   user agent then returns the session identifier in subsequent
   requests.


   == Server -> User Agent ==
   Set-Cookie: SID=31d4d96e407aad42

   == User Agent -> Server ==
   Cookie: SID=31d4d96e407aad42


   The server can alter the default scope of the cookie using the Path
   and Domain attributes.  For example, the server can instruct the user
   agent to return the cookie to every path and every subdomain of
   example.com.


   == Server -> User Agent ==
   Set-Cookie: SID=31d4d96e407aad42; Path=/; Domain=example.com




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   == User Agent -> Server ==
   Cookie: SID=31d4d96e407aad42


   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.


   == Server -> User Agent ==
   Set-Cookie: SID=31d4d96e407aad42; Path=/; Secure; HttpOnly
   Set-Cookie: lang=en-US; Path=/; Domain=example.com

   == User Agent -> Server ==
   Cookie: SID=31d4d96e407aad42; lang=en-US


   If the server wishes the user agent to persist the cookie over
   multiple sessions, the server can specify a expiration date in the
   Expires attribute.  Note that the user agent might delete the cookie
   before the expiration date if the user agent's cookie store exceeds
   its quota or if the user manually deletes the server's cookie.


   == Server -> User Agent ==
   Set-Cookie: lang=en-US; Expires=Wed, 09 Jun 2021 10:18:14 GMT

   == User Agent -> Server ==
   Cookie: SID=31d4d96e407aad42; lang=en-US


   Finally, to remove a cookie, the server returns a Set-Cookie header
   with an expiration date in the past.  The server will be successful
   in removing the cookie only if the Path and the Domain attribute in
   the Set-Cookie header match the values used when the cookie was
   created.


   == Server -> User Agent ==
   Set-Cookie: lang=; Expires=Sun, 06 Nov 1994 08:49:37 GMT

   == User Agent -> Server ==
   Cookie: SID=31d4d96e407aad42





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4.  Server Requirements

   This section describes the syntax and semantics of a well-behaved
   profile of the Cookie and Set-Cookie headers.  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
   or Set-Cookie headers could remove support for some of the most
   esoteric semantics.  User agents, however, MUST implement the full
   semantics to ensure interoperability with servers making use of the
   full semantics.

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 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:


   set-cookie-header = "Set-Cookie:" SP set-cookie-string
   set-cookie-string = cookie-pair *( ";" SP 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 / max-age-av / domain-av /
                       path-av / secure-av / httponly-av /
                       extension-av
   expires-av        = "Expires=" sane-cookie-date
   sane-cookie-date  = <rfc1123-date, as defined in RFC 2616>
   max-age-av        = "Max-Age=" 1*DIGIT
   domain-av         = "Domain=" domain-value
   domain-value      = <subdomain, as defined in RFC 1034>
   path-av           = "Path=" path-value
   path-value        = <abs_path, except those containing ";">
   secure-av         = "Secure"
   httponly-av       = "HttpOnly"
   extension-av      = <any CHAR except CTLs or ";">


   Servers SHOULD NOT include two attributes with the same name.



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   Servers SHOULD NOT include two Set-Cookie header fields in the same
   response with the same cookie-name.

   The cookie-value is opaque to the user agent and MAY be anything the
   origin server chooses to send.  "Opaque" implies that the content is
   of interest and relevance only to the origin server.  The content is,
   in fact, readable by anyone who examines the Set-Cookie header.

   To maximize compatibility with user agents, servers that wish to
   store non-ASCII data in a cookie-value SHOULD encode that data using
   a printable ASCII encoding.

   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 user agents represent dates using 32-bit UNIX time_t
   values.  Some of these user agents might contain bugs that cause them
   process dates after the year 2038 incorrectly.  Servers wishing to
   interoperate with these user agents might wish to use dates before
   2038.

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 cookies.  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.  Subsequently, when the user
   agent makes an HTTP request, the user agent consults its cookie store
   and 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 already exists in its
   cookie store, the existing cookie is evicted from the cookie store
   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 it expires at the end of the
   current session (as defined by the user agent).  User agents ignore
   unrecognized cookie attributes.




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4.1.2.1.  The Expires Attribute

   The Expires attribute indicates the maximum lifetime of the cookie,
   represented as the date and time at which the cookie expires.  The
   user agent is not required to retain the cookie until the specified
   date has passed.  In fact, user agents often evict cookies from the
   cookie store due to memory pressure or privacy concerns.

4.1.2.2.  The Max-Age Attribute

   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 until the specified
   date has passed.  In fact, user agents often evict cookies from the
   cookie store due to memory pressure or privacy concerns.

      WARNING: Not all user agents support the Max-Age attribute.  User
      agents that do not support the Max-Age attribute will 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).

4.1.2.3.  The Domain Attribute

   The Domain attribute specifies those hosts to which the cookie will
   be sent.  For example, if the Domain attribute contains the value
   "example.com", the user agent will include the cookie in the Cookie
   header when making HTTP requests to example.com, www.example.com, and
   www.corp.example.com.  (Note that a leading U+002E ("."), if present,
   is ignored even though that character is not permitted by the
   subdomain production in [RFC1034].)  If the server omits the Domain
   attribute, the user agent will return the cookie only to the origin
   server.

      WARNING: Some legacy user agents treat an absent Domain attribute
      as if the Domain attribute were present and contained the current
      host name.  For example, if example.com returns a Set-Cookie
      header without a Domain attribute, these user agents will
      erroneously send the cookie to www.example.com as well.

   The user agent will reject cookies (refuse to store them in the
   cookie store) unless the Domain attribute specifies a scope for the
   cookie that would include the origin server.  For example, the user
   agent will accept a Domain attribute of "example.com" or of



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   "foo.example.com" from foo.example.com, but the user agent will not
   accept a Domain attribute of "bar.example.com" or of
   "baz.foo.example.com".

   NOTE: For security reasons, some user agents are configured to reject
   Domain attributes that correspond to "public suffixes."  For example,
   some user agents will reject Domain attributes of "com" or "co.uk".

4.1.2.4.  The Path Attribute

   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.

   The user agent will include the cookie in an HTTP request only if the
   path portion of the request-uri matches (or is a subdirectory of) the
   cookie's Path attribute, where the U+002F ("/") character is
   interpreted as a directory separator.

   Although seemingly useful for isolating cookies between different
   paths within a given domain, the Path attribute cannot be relied upon
   for security (see Section 8).

4.1.2.5.  The Secure Attribute

   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 [RFC5246]).

   Although seemingly useful for protecting cookies from active network
   attackers, the Secure attribute protects only the cookie's
   confidentiality.  An active network attacker can overwrite Secure
   cookies from an insecure channel, disrupting its integrity.

4.1.2.6.  The HttpOnly Attribute

   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 its cookie store via "non-
   HTTP" APIs (such as HTML's document.cookie API).

4.2.  Cookie






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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
   Section 4.1, the requirements in the Section 5 will cause the user
   agent to return a Cookie header that conforms to the following
   grammar:


   cookie-header = "Cookie:" OWS cookie-string OWS
   cookie-string = cookie-pair *( ";" SP cookie-pair )


4.2.2.  Semantics

   Each cookie-pair represents a cookie stored by the user agent.  The
   cookie-name and the cookie-value are returned from the corresponding
   parts of the Set-Cookie header.

   Notice that the cookie attributes are not returned.  In particular,
   the server cannot determine from the Cookie header alone when a
   cookie will expire, for which domains the cookie is valid, for which
   paths the cookie is valid, or whether the cookie was set with the
   Secure or HttpOnly attributes.

   The semantics of individual cookies in the Cookie header is not
   defined by this document.  Servers are expected to imbue these
   cookies with 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.,
   with different Path or Domain attributes), servers SHOULD NOT rely
   upon the order in which these cookies appear in the header.

















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5.  User Agent Requirements

   For historical reasons, the full semantics of cookies contains a
   number of exotic quirks.  This section is intended to specify the
   Cookie and Set-Cookie headers in sufficient detail to allow a user
   agent implementing these requirements precisely to interoperate with
   existing servers.

5.1.  Algorithms

   This section defines a number of algorithms used by user agents to
   process the Cookie and Set-Cookie headers.

5.1.1.  Dates

   The user agent MUST use an algorithm equivalent to the following
   algorithm to parse a cookie-date:

   1.  Using the grammar below, divide the cookie-date into date-tokens.


   cookie-date     = *delimiter date-token-list *delimiter
   date-token-list = date-token *( 1*delimiter date-token )
   delimiter       = %x09 / %x20 / %x21 / %x22 / %x23 / %x24 /
                     %x25 / %x26 / %x27 / %x28 / %x29 / %x2A /
                     %x2B / %x2C / %x2D / %x2E / %x2F / %x3B /
                     %x3C / %x3D / %x3E / %x3F / %x40 / %x5B /
                     %x5C / %x5D / %x5E / %x5F / %x60 / %x7B /
                     %x7C / %x7D / %x7E
   date-token      = day-of-month / month / year / time / mystery
   day-of-month    = 2DIGIT / DIGIT
   month           = "jan" [ mystery ] / "feb" [ mystery ] /
                     "mar" [ mystery ] / "apr" [ mystery ] /
                     "may" [ mystery ] / "jun" [ mystery ] /
                     "jul" [ mystery ] / "aug" [ mystery ] /
                     "sep" [ mystery ] / "oct" [ mystery ] /
                     "nov" [ mystery ] / "dec" [ mystery ]
   year            = 4DIGIT / 3DIGIT / 2DIGIT / DIGIT
   time            = time-field ":" time-field ":" time-field
   time-field      = 2DIGIT / DIGIT
   CTLwoHTAB       = %x00-08 / %x0A-1F / %x7F
                     ; CTL except HTAB
   mystery         = CTLwoHTAB / ":" / ALPHA / DIGIT / %x80-FF
                     ; any OCTET 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.  If the year-value is greater than 68 and less than 100, increment
       the year-value by 1900.

   4.  If the year-value is greater than or equal to 0 and less than 69,
       increment the year-value by 2000.

   5.  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,

       *  the hour-value is greater than 23,

       *  the minute-value is greater than 59, or

       *  the second-value is greater than 59.






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   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-
       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
   and converted to ASCII according to the IDNA specification [RFC3490].

   A host-name domain-matches a cookie-domain if at least one of the
   following conditions hold:

   o  The cookie-domain and the host-name are identical.

   o  All of the following conditions hold:

      *  The cookie-domain is a suffix of the host-name.

      *  The last character of the host-name that is not included in the
         cookie-domain is a U+002E (".") character.

      *  The host-name is a host name (i.e., not an IP address).

5.1.3.  Paths

   The user agent MUST use an algorithm equivalent to the following
   algorithm to compute the default-path of a cookie:

   1.  Let uri-path be the path portion of the request-uri.  That is, if
       the request-uri contains just a path (and optional query string),
       then the uri-path is that path (without the U+003F ("?")
       character or query string), and if the request-uri contains a
       full absoluteURI, the uri-path is the abs_path component of that
       URI.

   2.  If the uri-path is empty or if first character of the uri-path is
       not a U+002F ("/") character, output U+002F ("/") and skip the
       remaining steps.

   3.  If the uri-path contains only a single U+002F ("/") character,
       output U+002F ("/") and skip the remaining steps.

   4.  Output the characters of the uri-path from the first character up
       to, but not including, the right-most U+002F ("/").




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   A request-path path-matches a cookie-path if at least one of the
   following conditions hold:

   o  The cookie-path and the request-path are identical.

   o  The cookie-path is a prefix of the request-path and the last
      character of the cookie-path is U+002F ("/").

   o  The cookie-path is a prefix of the request-path and the first
      character of the request-path that is not included in the cookie-
      path is a U+002F ("/") character.

5.2.  The Set-Cookie Header

   When a user agent receives a Set-Cookie header in an HTTP response,
   the user agent receives a set-cookie-string consisting of the value
   of the header.

   A user agent MUST use an algorithm equivalent to the following
   algorithm to parse set-cookie-strings:

   1.  If the set-cookie-string contains a U+003B (";") character:

          The name-value-pair string consists of the characters up to,
          but not including, the first U+003B (";"), and the unparsed-
          attributes consist of the remainder of the set-cookie-string
          (including the U+003B (";") in question).

       Otherwise:

          The name-value-pair string consists of all the characters
          contained in the set-cookie-string, and the unparsed-
          attributes is the empty string.

   2.  If the name-value-pair string lacks a U+003D ("=") character,
       ignore the set-cookie-string entirely.

   3.  The (possibly empty) name string consists of the characters up
       to, but not including, the first U+003D ("=") character, and the
       (possibly empty) value string consists of the characters after
       the first U+003D ("=") character.

   4.  Remove any leading or trailing WSP characters from the name
       string and the value string.

   5.  If the name string is empty, ignore the set-cookie-string
       entirely.




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   6.  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:

   1.  If the unparsed-attributes string is empty, skip the rest of
       these steps.

   2.  Discard the first character of the unparsed-attributes (which
       will be a U+003B (";") character).

   3.  If the remaining unparsed-attributes contains a U+003B (";")
       character:

          Consume the characters of the unparsed-attributes up to, but
          not including, the first U+003B (";") character.

       Otherwise:

          Consume the remainder of the unparsed-attributes.

       Let the cookie-av string be the characters consumed in this step.

   4.  If the cookie-av string contains a U+003D ("=") character:

          The (possibly empty) attribute-name string consists of the
          characters up to, but not including, the first U+003D ("=")
          character, and the (possibly empty) attribute-value string
          consists of the characters after the first U+003D ("=")
          character.

       Otherwise:

          The attribute-name string consists of the entire cookie-av
          string, and the attribute-value string is empty.

   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.  (Notice that
       attributes with unrecognizeed attribute-names are ignored.)

   7.  Return to Step 1.

   When the user agent finishes parsing the set-cookie-string, the user
   agent receives a cookie from the request-uri with name cookie-name,



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   value cookie-value, and attributes cookie-attribute-list.

5.2.1.  The Expires Attribute

   If the attribute-name case-insensitively matches the string
   "Expires", the user agent MUST process the cookie-av as follows.

   Let the parsed-cookie-date be the result of parsing the attribute-
   value as cookie-date.

   If the attribute-value failed to parse as a cookie date, ignore the
   cookie-av.

   If the user agent received the set-cookie-string from an HTTP
   response that contains a Date header field and the contents of the
   last Date header field successfully parse as a cookie-date:

      Let server-date be the date obtained by parsing the contents of
      the last Date header field as a cookie-date.

      Let delta-seconds be the number of seconds between the server-date
      and the parsed-cookie-date (i.e., parsed-cookie-date - server-
      date).

      Let the expiry-time be the current date and time plus delta-
      seconds seconds.

   Otherwise:

      Let the expiry-time be the parsed-cookie-date.

   If the expiry-time is later than the last date the user agent can
   represent, the user agent MAY replace the expiry-time with the last
   representable date.

   If the expiry-time is earlier than the first date the user agent can
   represent, the user agent MAY replace the expiry-time with the first
   representable date.

   Append an attribute to the cookie-attribute-list with an attribute-
   name of Expires and an attribute-value of expiry-time.

5.2.2.  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 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.

5.2.3.  The Domain Attribute

   If the attribute-name case-insensitively matches the string "Domain",
   the user agent MUST process the cookie-av as follows.

   If the attribute-value is empty, the behavior is undefined.  However,
   user agent SHOULD ignore the cookie-av entirely.

   If the first character of the attribute-value string is U+002E ("."):

      Let cookie-domain be the attribute-value without the leading
      U+002E (".") character.

   Otherwise:

      Let cookie-domain be the entire attribute-value.

   Convert the cookie-domain to lower case.

   Append an attribute to the cookie-attribute-list with an attribute-
   name of Domain and an attribute-value of cookie-domain.

5.2.4.  The Path Attribute

   If the attribute-name case-insensitively matches the string "Path",
   the user agent MUST process the cookie-av as follows.

   If the attribute-value is empty or if the first character of the
   attribute-value is not U+002F ("/"):

      Let cookie-path be the default-path.

   Otherwise:




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      Let cookie-path be the attribute-value.

   Append an attribute to the cookie-attribute-list with an attribute-
   name of Path and an attribute-value of cookie-path.

5.2.5.  The Secure Attribute

   If the attribute-name case-insensitively matches the string "Secure",
   the user agent MUST append an attribute to the cookie-attribute-list
   with an attribute-name of Secure and an empty attribute-value.

5.2.6.  The HttpOnly Attribute

   If the attribute-name case-insensitively matches the string
   "HttpOnly", the user agent MUST append an attribute to the cookie-
   attribute-list with an attribute-name of HttpOnly and an empty
   attribute-value.

5.3.  Storage Model

   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.   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 cookie that exceed some size.

   2.   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.

   3.   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



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        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.

   4.   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.

   5.   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 http://publicsuffix.org/.



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   6.   If the domain-attribute is non-empty:

           If the cannonicalized request-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 canonicalized request-host.

   7.   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.

   8.   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.

   9.   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.

   10.  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.

   11.  If the cookie store contains a cookie with the same name,
        domain, and path as the newly created cookie:

        1.  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.)

        2.  If the newly created cookie was received from an non-HTTP
            API and the old-cookie's http-only-flag is set, abort these
            steps and ignore the newly created cookie entirely.

        3.  Update the creation-time of the newly created cookie to
            match the creation-time of the old-cookie.



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        4.  Remove the old-cookie from the cookie store.

   12.  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 predetermined upper bound (such as 50 cookies).

   At any time, the user agent MAY "remove excess cookies" form 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:

   1.  Expired cookies.

   2.  Cookies that share a domain field with more than a predetermined
       number of other cookies.

   3.  All cookies.

   If two cookies have the same removal priority, the user agent MUST
   evict the cookie with the least recent last-access date first.

   When "the current session is over" (as defined by the user agent),
   the user agent MUST remove from the cookie store all cookies with the
   persistent-flag set to false.

5.4.  The Cookie Header

   When the user agent generates an HTTP request, the user agent SHOULD
   attach exactly one HTTP header named Cookie if the cookie-string
   (defined below) for the request-uri is non-empty.

   A user agent MAY omit the Cookie header in its entirety.  For
   example, the user agent might wish to block sending cookies during
   "third-party" requests.

   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:




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   1.  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 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 unless the cookie-string is being generated for an
          "HTTP" API (as defined by the user agent).

   2.  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.

   3.  Update the last-access-time of each cookie in the cookie-list to
       the current date and time.




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   4.  Serialize the cookie-list into a cookie-string by processing each
       cookie in the cookie-list in order:

       1.  Output the cookie's name, the U+003D ("=") character, and the
           cookie's value.

       2.  If there is an unprocessed cookie in the cookie-list, output
           the characters U+003B and U+0020 ("; ").

   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 SHOULD use the UTF-8
   character encoding [RFC3629].





































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6.  Implementation Considerations

6.1.  Limits

   Practical user agent implementations have limits on the number and
   size of cookies that they can store.  General-use user agents SHOULD
   provide each of the following minimum capabilities:

   o  At least 4096 bytes per cookie (as measured by the sum of the
      length of the cookie's name, value, and attributes).

   o  At least 50 cookies per domain.

   o  At least 3000 cookies total.

   Servers SHOULD use as few and as small cookies as possible to avoid
   reaching these implementation limits and to 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.

6.2.  Application Programmer Interfaces

   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 programmer 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 a abstract "Date" object instead of a
   serialized date string.













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7.  Privacy Considerations

   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 domains.

7.1.  Third-Party Cookies

   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.

   Some user agents restrict how third-party cookies behave.  For
   example, some user agents refuse to send the Cookie header in third-
   party requests.  Other user agents 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.

7.2.  User Controls

   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 agent 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



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   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 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.











































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8.  Security Considerations

8.1.  Overview

   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.

8.2.  Ambient Authority

   A server that uses cookies to authenticate users can suffer security
   vulnerabilities because some user agents let remote parties issue
   HTTP requests from the user agent (e.g., via HTTP redirects 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 use of these principles can lead to more robust security.




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8.3.  Clear Text

   Unless sent over a secure channel (such as TLS), the information in
   the Cookie and Set-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 the contents of cookies when
   transmitting them to the user agent (even when sending the cookies
   over a secure channel).  However, encrypting and signing cookie
   contents does not prevent an attacker from transplanting a cookie
   from one user agent to another or from replaying the cookie at a
   later time.

   In addition to encrypting and signing the contents of every cookie,
   servers that require a higher level of security SHOULD use the Cookie
   and Set-Cookie headers only over a secure channel.  When using
   cookies over a secure channel, servers SHOULD set the Secure
   attribute for every cookie.  If a server does not set the Secure
   attribute, the protection provided by the secure channel will be
   largely moot.

8.4.  Session Identifiers

   Instead of storing session information directly in a cookie (where it
   might be exposed to or replayed by an attacker), servers commonly
   store a nonce (or "session identifier") in a cookie.  When the server
   receives an HTTP request with a nonce, the server can look up state
   information associated with the cookie using the nonce as a key.

   Using session identifier cookies limits the damage an attacker can
   cause if the attacker learns the contents of a cookie because the
   nonce is useful only for interacting with the server (unlike non-
   nonce cookie content, which might itself be sensitive).  Furthermore,
   using a single nonce prevents an attacker from "splicing" together
   cookie content from two interactions with the server, which could
   cause the server to behave unexpectedly.

   Using session identifiers is not without risk.  For example, the
   server SHOULD take care to avoid "session fixation" vulnerabilities.
   A session fixation attack proceeds in three steps.  First, the



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   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.

8.5.  Weak Confidentiality

   Cookies do not provide isolation by port.  If a cookie is readable by
   a service running on one port, the cookie is also readable by a
   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 via
   non-HTTP APIs that permit access to cookies (e.g., HTML's
   document.cookie API), the lack of isolation by scheme is actually
   present in 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.

8.6.  Weak Integrity

   Cookies do not provide integrity guarantees for sibling domains (and
   their subdomains).  For example, consider foo.example.com and
   bar.example.com.  The foo.example.com server can set a cookie with a
   Domain attribute of "example.com" (possibly overwriting an existing
   "example.com" cookie set by bar.example.com), and the user agent will
   include that cookie in HTTP requests to bar.example.com.  In the
   worst case, bar.example.com will be unable to distinguish this cookie
   from a cookie it set itself.  The foo.example.com server might be
   able to leverage this ability to mount an attack against
   bar.example.com.



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   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 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.

8.7.  Reliance on DNS

   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.
















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9.  References

9.1.  Normative References

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, November 1987.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC3490]  Faltstrom, P., Hoffman, P., and A. Costello,
              "Internationalizing Domain Names in Applications (IDNA)",
              RFC 3490, March 2003.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

   [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.

9.2.  Informative References

   [RFC2109]  Kristol, D. and L. Montulli, "HTTP State Management
              Mechanism", RFC 2109, February 1997.

   [RFC2965]  Kristol, D. and L. Montulli, "HTTP State Management
              Mechanism", RFC 2965, October 2000.

















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Appendix A.  Acknowledgements

   This document borrows heavily from RFC 2109 [RFC2109].  We are
   indebted to David M. Kristol and Lou Montulli for their efforts to
   specify the cookie protocol.  David M. Kristol, in particular,
   provided invaluable advice on navigating the IETF process.  We would
   also like to thank Thomas Broyer, Tyler Close, Bil Corry, corvid,
   Lisa Dusseault, Roy T. Fielding, Blake Frantz, Eran Hammer-Lahav,
   Jeff Hodges, Achim Hoffmann, Georg Koppen, Dean McNamee, Mark Miller,
   Mark Pauley, Yngve N. Pettersen, Julian Reschke, Mark Seaborn, Maciej
   Stachowiak, Daniel Stenberg, David Wagner, Dan Winship, and Dan Witte
   for their valuable feedback on this document.







































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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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