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Network Working Group                                       Y. Pettersen
Internet-Draft                                        Opera Software ASA
Obsoletes: 2965 (if approved)                             March 14, 2011
Intended status: Standards Track
Expires: September 15, 2011


                   HTTP State Management Mechanism v2
                      draft-pettersen-cookie-v2-06

Abstract

   This document specifies a way to create a stateful session with
   Hypertext Transfer Protocol (HTTP) requests and responses.  It
   describes three HTTP headers, Cookie, Cookie2, and Set-Cookie2, which
   carry state information between participating origin servers and user
   agents.  The method described here differs from both Netscape's
   Cookie proposal [Netscape], and [RFC2965], but it can, provided some
   requirements are met, interoperate with HTTP/1.1 user agents that use
   Netscape's method.  (See the HISTORICAL section.)

   This document defines new rules for how cookies can be shared between
   servers within a domain.  These new rules are intended to address
   security and privacy concerns that are difficult to counter for
   clients implementing Netscape's proposed rules or the rules specified
   by RFC 2965.

   This document reflects implementation experience with RFC 2965 and
   obsoletes it.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any



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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 15, 2011.

Copyright Notice

   Copyright (c) 2011 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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   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
   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.




















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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Description  . . . . . . . . . . . . . . . . . . . . . . . . .  7
     3.1.  Syntax: General  . . . . . . . . . . . . . . . . . . . . .  7
     3.2.  Origin Server Role . . . . . . . . . . . . . . . . . . . .  8
     3.3.  User Agent Role  . . . . . . . . . . . . . . . . . . . . . 12
     3.4.  How an Origin Server Interprets the Cookie Header  . . . . 19
     3.5.  Caching Proxy Role . . . . . . . . . . . . . . . . . . . . 19
   4.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     4.1.  Example 1  . . . . . . . . . . . . . . . . . . . . . . . . 20
     4.2.  Example 2  . . . . . . . . . . . . . . . . . . . . . . . . 22
   5.  Implementation Considerations  . . . . . . . . . . . . . . . . 22
     5.1.  Set-Cookie2 Content  . . . . . . . . . . . . . . . . . . . 23
     5.2.  Stateless Pages  . . . . . . . . . . . . . . . . . . . . . 23
     5.3.  Implementation Limits  . . . . . . . . . . . . . . . . . . 23
     5.4.  Backwards Compatibility  . . . . . . . . . . . . . . . . . 24
   6.  Privacy  . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
     6.1.  User Agent Control . . . . . . . . . . . . . . . . . . . . 24
     6.2.  Origin Server Role . . . . . . . . . . . . . . . . . . . . 25
     6.3.  Clear Text . . . . . . . . . . . . . . . . . . . . . . . . 26
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 26
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 26
     8.1.  Protocol Design  . . . . . . . . . . . . . . . . . . . . . 26
     8.2.  Cookie Spoofing  . . . . . . . . . . . . . . . . . . . . . 27
     8.3.  Unexpected Cookie Sharing  . . . . . . . . . . . . . . . . 27
     8.4.  Cookies for Account Information  . . . . . . . . . . . . . 27
   9.  Historical . . . . . . . . . . . . . . . . . . . . . . . . . . 28
     9.1.  Compatibility with Existing Implementations  . . . . . . . 28
     9.2.  Caching and HTTP/1.0 . . . . . . . . . . . . . . . . . . . 29
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 29
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 29
     11.2. Non-normative References . . . . . . . . . . . . . . . . . 30
   Appendix A.  Open issues . . . . . . . . . . . . . . . . . . . . . 30
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 30














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1.  Introduction

   HTTP cookies are widely used to maintain state across multiple HTTP
   requests in a wide variety of HTTP based applications, such as
   shopping carts for web shops, login credentials, preferences,
   identity information, etc.  While alternatives exists, they are more
   cumbersome than HTTP cookies.  The flexibility and ease of use of
   cookies may therefore have assisted the rapid spread of the World
   Wide Web.

   Unfortunately, some of the flexibility of cookies, specifically how
   cookies are shared among multiple hosts, is causing security and
   privacy concerns.

   [RFC2965] specifies that a cookie may be shared with any server
   within the Reach of the host, that is, the parent domain of the host,
   and the [Netscape] proposal allowed, within certain restrictions,
   even wider sharing to servers in the entire second- or third-level
   domain in which the request-host is part.

   In some domain hierarchies, such as the generic Top Level Domain
   (TLD) dotCOM domain this will work well, but in many TLDs such as the
   Country-Code TLD (ccTLD) dotUK, this kind of sharing can cause
   problems, unless severely restricted, because it makes assumptions
   about what control and authorization has actually been granted the
   request-host.  The dotUK TLD and many other ccTLDs have numerous
   subdomains that are treated as actual TLDs or registry like domains,
   similar to dotCOM, dotNet and dotORG, such as the co.uk, org.uk and
   ac.uk domains that are used to group otherwise unrelated domains into
   categories based on their intended usage (e.g. commercial, non-
   commercial, governmental, academic).

   Permitting cookies to be shared across such registry-like domains may
   result in undesirable datasharing, denial of service problems, even
   security related problems.

   The original rules in Netscape's proposal, one internal dot in domain
   in the generic TLDs and two in domain name for non-generic TLDs,
   turned out not to be good enough, nor were they properly implemented
   in any client as they severely limited legitimate use of cookies;
   RFC2965's one level up rule restricted the problem somewhat, but not
   enough, as it was still possible to bypass the restrictions.  Clients
   have implemented or proposed various heuristics to limit the impact
   of the problem, some by using a blacklist of second level domains
   that the client is not permitted to accept cookies for, others use
   DNS IP address lookups of the Set-Cookie header field's Domain
   attribute as a heuristic method to determine the appropriateness of
   permitting a cookie to be set, and a large database of domains that



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   should not be able to accept cookies have also been proposed.

   Similarly, but less serious, the ability to set cookies to a parent
   path can, under some circumstances, cause interference between
   different applications in a given environment.  In single application
   environments such sharing is not dangerous, but could be problematic
   when multiple independent administrators share the same service, such
   as in shared hosting environments where all users are hosted in their
   own path on the same server.  In such environments a malicious user
   can set a cookie that is shared by many users, and since most version
   0 [Netscape] implementations do not enforce a prefix path restriction
   it is also possible to limit the cookies to a path not controlled by
   the user, but not visible to all the other users on the host.  Such
   cookies can interfere with the function of other applications on the
   host or within the domain.

   This document presents an alternative method for reducing these
   problems by

   o  Removing the Domain attribute that permitted cookies to be
      specified for the parent domain, and instead introduces the
      SubDomain attribute that will permit servers to share cookies, but
      only with servers whose name domain-matches the name of the
      request-host that set the cookie, and not parent domains.

   o  Removing the Path attribute, replacing it by the SubPath attribute
      that may be used to specify which resources under the request-path
      will be allowed to receive the cookie, instead of specifying which
      parent path is allowed to send the cookie.

   This specification will not be able to accept cookies for hosts that
   are using domain specifications for parent domains as defined by the
   previous cookie specifications, but implementations using the older
   specification will be able to accept cookies from hosts following
   this specification.

   This document also introduces new requirements for the contents of
   the Cookie header field, specifically that the $Domain and $Path
   attributes must always be sent, even when no domain or path has been
   specified, as this will allow request-hosts to verify the domain of
   the cookies even for cookies received from hosts using the older
   specifications.


2.  Terminology

   The terms user agent, client, server, proxy, origin server, and
   header field, and the tokens DIGIT, token, http_URL, and quoted-



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   string have the same meaning as in the HTTP/1.1 specification
   [RFC2616].  The terms abs_path and absoluteURI have the same meaning
   as in the URI Syntax specification [RFC3986].

   The grammar uses the notation from the HTTP/1.1 specification
   [RFC2616] to describe the syntax of the HTTP header fields.

   Host name (HN) means either the host domain name (HDN) or the numeric
   Internet Protocol (IP) address or IP-literal of a host, as defined by
   [RFC3986].  The fully qualified domain name is preferred; use of
   numeric IP addresses or IP-literals is strongly discouraged.

   The terms request-host and request-URI refer to the values the client
   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.  Note that request-host is a HN.

   The term effective host name is related to host name.  If a host name
   is a host domain name and contains no dots, the effective host name
   is that name with the string .local appended to it.  Otherwise the
   effective host name is the same as the host name.  Note that all
   effective host names contain at least one dot.

   The term request-port refers to the port portion of the absoluteURI
   (http_URL) of the HTTP request line.  If the absoluteURI has no
   explicit port, the request-port is the HTTP default, 80, for HTTP
   URIs, and the HTTPS default, 443, for HTTPS URIs [RFC2818].  The
   request-port of a cookie is the request-port of the request in which
   a Set-Cookie2 response header field was returned to the user agent.

   Host names can be specified either as an IP address, IP-literal or an
   HDN string.  Sometimes we compare one host name with another.  (Such
   comparisons SHALL be case-insensitive.)  Host A's name domain-matches
   host B's if

   o  their host name strings string-compare equal; or

   o  A is a HDN string and has the form NB, where N is a non-empty name
      string, B has the form .B', and B' is a HDN string.  (So, x.y.com
      domain-matches .Y.com but not Y.com.)

   Note that domain-match is not a commutative operation: a.b.c.com
   domain-matches .c.com, but not the reverse.

   The reach R of a host name H is defined as follows:

   o  If




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      *  H is the host domain name of a host; and,

      *  H has the form A.B; and

      *  A has no embedded (that is, interior) dots; and

      *  B has at least one embedded dot, or B is the string "local".
         then the reach of H is .B.

   o  Otherwise, the reach of H is H.

   For two strings that represent paths, P1 and P2, P1 path-matches P2
   if P2 is a prefix of P1 (including the case where P1 and P2 string-
   compare equal).  Thus, the string /tec/waldo path-matches /tec.

   Because it was used in Netscape's original implementation of state
   management, we will use the term cookie to refer to the state
   information that passes between an origin server and user agent, and
   that gets stored by the user agent.


3.  Description

   We describe 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 goal is to have a minimal
   impact on HTTP and user agents.

3.1.  Syntax: General

   The two state management header fields, Set-Cookie2 and Cookie, have
   common syntactic properties involving attribute-value pairs.

      av-pairs    =     av-pair *(";" av-pair)
      av-pair     =     attr ["=" value] ; optional value
      attr        =     token
      value       =     token | quoted-string

   Attributes (names) (attr) are case-insensitive.  White space is
   permitted between tokens.  Note that while the above syntax
   description shows value as optional, most attrs require them.  Note
   also that, unless prohibited, all values in the grammar below can be
   represented as quoted string, even if the grammar does not directly
   indicate it.

   NOTE: The syntax above allows whitespace between the attribute name
   and the = sign.




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3.2.  Origin Server Role

3.2.1.  General

   The origin server initiates a session, if it so desires.  To do so,
   it returns an extra HTTP response header to the client, Set-Cookie2,
   described below.

   A user agent returns a Cookie request header field (see below) to the
   origin server if it chooses to continue the session.  The origin
   server MAY ignore it or use it to determine the current state of the
   session.  It MAY send back to the client a Set-Cookie2 response
   header field with the same or different information, or it MAY send
   no Set-Cookie2 header at all.  The origin server effectively ends a
   session by sending the client a Set-Cookie2 header field with Max-
   Age=0.

   Servers MAY return Set-Cookie2 response header fields with any
   response.  User agents SHOULD send Cookie request header fields,
   subject to other rules detailed below, with every request.

   An origin server MAY include multiple Set-Cookie2 header fields in a
   response.  Note that an intervening gateway could fold such multiple
   headers into a single header, as described by [RFC2616].

3.2.2.  Set-Cookie2 Syntax

   The syntax for the Set-Cookie2 response header field is

      set-cookie      =       "Set-Cookie2:" cookies
      cookies         =       1#cookie
      cookie          =       NAME "=" VALUE *(";" set-cookie-av)
      NAME            =       attr
      VALUE           =       value
      set-cookie-av   =       "Comment" "=" value
                      |       "CommentURL" "=" <"> http_URL <">
                      |       "Discard"
                      |       "SubDomain"
                      |       "Max-Age" "=" value
                      |       "SubPath" "=" value
                      |       "Port" [ "=" <"> portlist <"> ]
                      |       "Unsecure"
                      |       "Version" "=" 1*DIGIT
                      |       "HttpOnly"
      portlist        =       1#portnum
      portnum         =       1*DIGIT

   Informally, the Set-Cookie2 response header comprises the token "Set-



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   Cookie2:", followed by a comma-separated list of one or more cookies.
   Each cookie begins with a NAME=VALUE pair, followed by zero or more
   semi-colon-separated attribute-value pairs.  The syntax for
   attribute-value pairs was shown earlier.  The specific attributes and
   the semantics of their values follows.  The NAME=VALUE attribute-
   value pair MUST come first in each cookie.  The others, if present,
   can occur in any order.  If an attribute appears more than once in a
   cookie, the client SHALL use only the value associated with the first
   appearance of the attribute; a client MUST ignore values after the
   first.

   The NAME of a cookie MAY be the same as one of the attributes in this
   specification.  However, because the cookie's NAME must come first in
   a Set-Cookie2 response header field, the NAME and its VALUE cannot be
   confused with an attribute-value pair.

   NAME=VALUE  REQUIRED.  The name of the state information ("cookie")
      is NAME, and its value is VALUE.  NAMEs that begin with $ are
      reserved and MUST NOT be used by applications.  The 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 interest and
      relevance only to the origin server.  The content may, in fact, be
      readable by anyone that examines the Set-Cookie2 header field.

   Comment=value  OPTIONAL.  Because cookies can be used to derive or
      store private information about a user, the value of the Comment
      attribute allows an origin server to document how it intends to
      use the cookie.  The user can inspect the information to decide
      whether to initiate or continue a session with this cookie.
      Characters in value MUST be in UTF-8 encoding.  [RFC3629]

   CommentURL="http_URL"  OPTIONAL.  Because cookies can be used to
      derive or store private information about a user, the CommentURL
      attribute allows an origin server to document how it intends to
      use the cookie.  The user can inspect the information identified
      by the URL to decide whether to initiate or continue a session
      with this cookie.

   Discard  OPTIONAL.  The Discard attribute instructs the user agent to
      discard the cookie unconditionally when the user agent terminates.

   SubDomain  OPTIONAL.  The SubDomain attribute specifies that the user
      agent should share the cookie with any hosts that domain-matches
      the name of the host sending the cookie






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   Max-Age=value  OPTIONAL.  The value of the Max-Age attribute is
      delta-seconds, the lifetime of the cookie in seconds, a decimal
      non-negative integer.  To handle cached cookies correctly, a
      client SHOULD calculate the age of the cookie according to the age
      calculation rules in the HTTP/1.1 specification [RFC2616].  When
      the age is greater than delta-seconds seconds, the client SHOULD
      discard the cookie.  A value of zero means the cookie SHOULD be
      discarded immediately.

   SubPath=value  OPTIONAL.  The value of the SubPath attribute
      specifies the subset of URLs within the default path on the origin
      server to which this cookie applies.

   Port[="portlist"]  OPTIONAL.  The Port attribute restricts the port
      to which a cookie may be returned in a Cookie request header
      field.  Note that the syntax REQUIREs quotes around the OPTIONAL
      portlist even if there is only one portnum in portlist.

   Unsecure  OPTIONAL.  The Unsecure attribute (with no value) is only
      used for cookies sent over secure connections and directs the user
      agent that the associated cookie can also be sent over an unsecure
      connection, not just to over (unspecified) secure connections, to
      the origin server whenever it sends back this cookie.  The default
      for cookies sent over a secure connection is to protect the
      confidentiality and authenticity of the information in the cookie,
      so the client MUST NOT send a cookie over an unsecure connection
      if that cookie was received over a secure connection, but only
      send it over a connection at least as secure as the one it was
      received over unless the Unsecure flag was set for that cookie.

   Version=value  REQUIRED.  The value of the Version attribute, a
      decimal integer, identifies the version of the state management
      specification to which the cookie conforms.  For this
      specification, Version=2 applies.

   HttpOnly  Cookies with this flag set MUST NOT be provided to non-HTTP
      requestors, such as scripts.  Additionally, non-HTTP updates that
      would overwrite such cookies, or that includes this flag, MUST be
      refused.

3.2.3.  Controlling Caching

   An origin server must be cognizant of the effect of possible caching
   of both the returned resource and the Set-Cookie2 header field.
   Caching "public" documents is desirable.  For example, if the origin
   server wants to use a public document such as a "front door" page as
   a sentinel to indicate the beginning of a session for which a Set-
   Cookie2 response header field must be generated, the page SHOULD be



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   stored in caches "pre-expired" so that the origin server will see
   further requests.  "Private documents", for example those that
   contain information strictly private to a session, SHOULD NOT be
   cached in shared caches.

   If the cookie is intended for use by a single user, the Set-Cookie2
   header field SHOULD NOT be cached.  A Set-Cookie2 header field that
   is intended to be shared by multiple users MAY be cached.

   The origin server SHOULD send the following additional HTTP/1.1
   response header fields, depending on circumstances:

   o  To suppress caching of the Set-Cookie2 header field:

       Cache-control: no-cache="set-cookie2"

   and one of the following:

   o  To suppress caching of a private document in shared caches:

       Cache-control: private

   o  To allow caching of a document and require that it be validated
      before returning it to the client:

       Cache-Control: must-revalidate, max-age=0

   o  To allow caching of a document, but to require that proxy caches
      (not user agent caches) validate it before returning it to the
      client:

       Cache-Control: proxy-revalidate, max-age=0

   o  To allow caching of a document and request that it be validated
      before returning it to the client (by "pre-expiring" it):

       Cache-control: max-age=0
      Not all caches will revalidate the document in every case.

   HTTP/1.1 servers MUST send Expires: old-date (where old-date is a
   date long in the past) on responses containing Set-Cookie2 response
   header fields unless they know for certain (by out of band means)
   that there are no HTTP/1.0 proxies in the response chain.  HTTP/1.1
   servers MAY send other Cache-Control directives that permit caching
   by HTTP/1.1 proxies in addition to the Expires: old-date directive;
   the Cache-Control directive will override the Expires: old-date for
   HTTP/1.1 proxies.




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3.3.  User Agent Role

3.3.1.  Interpreting Set-Cookie2

   The user agent keeps separate track of state information that arrives
   via Set-Cookie2 response header fields from each origin server (as
   distinguished by name or IP address and port).  The user agent MUST
   ignore attribute-value pairs whose attribute it does not recognize or
   that contain invalid data, and if necessary ignore the entire header
   field.  The user agent applies these defaults for optional attributes
   that are missing:

   Discard  The default behavior is dictated by the presence or absence
      of a Max-Age attribute.

   Domain  Defaults to the effective request-host.  (Note that because
      there is no dot at the beginning of effective request-host, the
      default Domain can only domain-match itself.)

   Max-Age  The default behavior is to discard the cookie when the user
      agent exits.

   Path  Defaults to the path of the request URL that generated the Set-
      Cookie2 response, up to and including the right-most /.

   Port  The default behavior is that a cookie MAY be returned to any
      request-port.

   Unsecure  Only for cookies received over a secure connection: If
      absent, the user agent MUST NOT send the cookie over an unsecure
      channel.  (Cookies received over an unsecure connection can be
      sent to secure connections)

   HttpOnly  The default behavior is that a cookie can be included in
      all listings of cookies for a given URL, also those requested by
      non-HTTP requestors, e.g scripts.

   The user agent MUST ignore the SubDomain attribute if the effective
   request-host is an IP-address or IP-literal.

   If the SubDomain attribute is present the state attribute Domain
   becomes .H where H is the effective request-host.

   If the SubPath attribute is present the state attribute Path becomes
   Px where P is the default path, up to and including the right-most /
   and x is the value of the attribute.





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3.3.2.  Rejecting Cookies

   To prevent possible security or privacy violations, a user agent
   rejects a cookie according to rules below.  The goal of the rules is
   to try to limit the set of servers for which a cookie is valid, based
   on the values of the Path, Domain, and Port attributes and the
   request-URI, request-host and request-port.

   A user agent rejects (SHALL NOT store its information) if the Version
   attribute is missing, or contains a value of 1 or higher.  Moreover,
   a user agent rejects (SHALL NOT store its information) if any of the
   following is true of the attributes explicitly present in the Set-
   Cookie2 response header field:

   o  The value for the SubPath attribute appended to the default path
      is not a prefix of the request-URI.

   o  The Port attribute has a "port-list", and the request-port was not
      in the list.

   o  The source of the cookie is non-HTTP, e.g. a script, that either
      include the HttpOnly attribute, or would overwrite a cookie with
      the HttpOnly attribute.

   Examples:

   o  A Set-Cookie2 with Port="80,8000" will be accepted if the request
      was made to port 80 or 8000 and will be rejected otherwise.

   o  A Set-Cookie2 from a path /example1/example1 for SubPath=exam will
      be accepted for the path /example1/exam

   o  A Set-Cookie2 from a path /example1/example1 for SubPath=exor will
      be rejected because exor is not a prefix of example1.

3.3.3.  Cookie Management

   If a user agent receives a Set-Cookie2 response header field whose
   NAME is the same as that of a cookie it has previously stored, the
   new cookie supersedes the old when: the old and new Domain attribute
   values compare equal, using a case-insensitive string-compare; and,
   the old and new Path attribute values string-compare equal (case-
   sensitive).  However, if the Set-Cookie2 has a value for Max-Age of
   zero, the (old and new) cookie is discarded.  Otherwise a cookie
   persists (resources permitting) until whichever happens first, then
   gets discarded: its Max-Age lifetime is exceeded; or, if the Discard
   attribute is set, the user agent terminates the session.




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   Because user agents have finite space in which to store cookies, they
   MAY also discard older cookies to make space for newer ones, using,
   for example, a least-recently-used algorithm, along with constraints
   on the maximum number of cookies that each origin server may set.

   If a Set-Cookie2 response header field includes a Comment attribute,
   the user agent SHOULD store that information in a human-readable form
   with the cookie and SHOULD display the comment text as part of a
   cookie inspection user interface.

   If a Set-Cookie2 response header field includes a CommentURL
   attribute, the user agent SHOULD store that information in a human-
   readable form with the cookie, or, preferably, SHOULD allow the user
   to follow the http_URL link as part of a cookie inspection user
   interface.

   The cookie inspection user interface may include a facility whereby a
   user can decide, at the time the user agent receives the Set-Cookie2
   response header field, whether or not to accept the cookie.  A
   potentially confusing situation could arise if the following sequence
   occurs:

   o  the user agent receives a cookie that contains a CommentURL
      attribute;

   o  the user agent's cookie inspection interface is configured so that
      it presents a dialog to the user before the user agent accepts the
      cookie;

   o  the dialog allows the user to follow the CommentURL link when the
      user agent receives the cookie; and,

   o  when the user follows the CommentURL link, the origin server (or
      another server, via other links in the returned content) returns
      another cookie.

   The user agent SHOULD NOT send any cookies in this context.  The user
   agent MAY discard any cookie it receives in this context that the
   user has not, through some user agent mechanism, deemed acceptable.

   User agents SHOULD allow the user to control cookie destruction, but
   they MUST NOT extend the cookie's lifetime beyond that controlled by
   the Discard and Max-Age attributes.  An infrequently-used cookie may
   function as a "preferences file" for network applications, and a user
   may wish to keep it even if it is the least-recently-used cookie.
   One possible implementation would be an interface that allows the
   permanent storage of a cookie through a checkbox (or, conversely, its
   immediate destruction).



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   Privacy considerations dictate that the user have considerable
   control over cookie management.  The PRIVACY section contains more
   information.

3.3.4.  Sending Cookies to the Origin Server

   When it sends a request to an origin server, the user agent includes
   a Cookie request header field if it has stored cookies that are
   applicable to the request, based on

   o  the request-host and request-port;

   o  the request-URI;

   o  the cookie's age.

   The syntax for the header field is:

cookie          =  "Cookie:" cookie-version 1*((";" | ",") cookie-value)
cookie-value    =  NAME "=" VALUE ";" cookie-path ";" cookie-domain
                   [";" cookie-port] [";" cookie-origin]

cookie-version  =  "$Version" "=" value
NAME            =  attr
VALUE           =  value
cookie-path     =  "$Path" "=" value
cookie-domain   =  "$Domain" "=" value
cookie-port     =  "$Port" [ "=" <"> port_list <"> ]
cookie-origin   =  "$Origin" "=" [ <"> http_URL <"> ]


   cookie-version  The value of the cookie-version attribute MUST be the
      value from the Version attribute of the corresponding Set-Cookie2
      response header field.  Otherwise the value for cookie-version is
      0.

   name  This is the verbatim name from the original Set-Cookie or Set-
      Cookie2 header field setting the cookie item.

   cookie-value  This is a verbatim copy of the value from the original
      Set-Cookie or Set-Cookie2 header field setting the cookie item.
      Quotes MUST be included if they were originally used, and MUST NOT
      be used if the original value did not contain them.

   cookie-path  The value for the cookie-path attribute MUST be the
      value from the cookie's Path state attribute, as determined when
      the corresponding Set-Cookie2 response header field was parsed.
      If the cookie was set using a previous specification this value



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      MUST be the value of the Path attribute of the corresponding
      response header field or, if the Path attribute was absent, the
      default path of the URI used to set the cookie.

   cookie-domain  The value for the cookie-domain attribute MUST be the
      value from the cookie's Domain state attribute, as determined when
      the corresponding Set-Cookie2 response header field was parsed.
      If the response header field used the SubDomain attribute the
      domain value MUST be prefixed by a ".", if the Domain attribute is
      a default domain the domain value MUST NOT be prefixed by a ".".
      If the cookie was set by host supporting a previous version this
      value MUST be the Domain attribute from the correponding header
      field, including a preceding "." if the Domain attribute was
      present; if it was not present the domain value must be the name
      of the host setting the cookie, without being prefixed with a ".".

   cookie-port  The cookie-port attribute of the Cookie request header
      field MUST be exactly the value of the Port attribute, if one was
      present, in the corresponding Set-Cookie2 response header field.
      That is, the port attribute MUST be present if the Port attribute
      was present in the Set-Cookie2 header field, and it MUST have the
      same value (the exact character sequence), if any.  Otherwise, if
      the Port attribute was absent from the Set-Cookie2 header field,
      the attribute likewise MUST be omitted from the Cookie request
      header field.

   cookie-origin  The cookie-origin attribute of the Cookie header field
      MUST be sent for all cookies that were set according to [Netscape]
      and [RFC2965], and the value MUST be at least the scheme,
      authority and default path portion (as defined for the cookie path
      attribute) of the URI that originally set the cookie.  If the URI
      is not known then the URI part of this attribute MUST be empty.
      This attribute can be used by the server receiving the cookie to
      determine if it is willing to trust the cookie, based on which
      server (and path) originally set the cookie.  The basic
      information of this attribute, except the scheme, is implied by
      the cookie-domain and cookie-path attributes when the cookie is
      set according to this specification, and the client SHOULD NOT
      send this attribute with cookies set according to this
      specification.

   Note that there is neither a Comment nor a CommentURL attribute in
   the Cookie request header field corresponding to the ones in the Set-
   Cookie2 response header field.  The user agent does not return the
   comment information to the origin server.

   The user agent applies the following rules to choose applicable
   cookie-values to send in a Cookie request header field from among all



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   the cookies it has received:

   Domain Selection  The origin server's effective host name MUST
      domain-match the Domain state attribute of the cookie.

   Port Selection  There are three possible behaviors, depending on the
      Port attribute in the Set-Cookie2 response header field:

      1.  By default (no Port attribute), the cookie MAY be sent to any
          port.

      2.  If the attribute is present but has no value (e.g., Port), the
          cookie MUST only be sent to the request-port it was received
          from.

      3.  If the attribute has a port-list, the cookie MUST only be
          returned if the new request-port is one of those listed in
          port-list.

   Path Selection  The request-URI MUST path-match the Path state
      attribute of the cookie.

   Max-Age Selection  Cookies that have expired should have been
      discarded and thus are not forwarded to an origin server.

   HttpOnly  Cookies with the HttpOnly attribute MUST NOT be returned to
      non-HTTP requestors, e.g.  Javascript.

   If multiple cookies satisfy the criteria above, they are ordered in
   the Cookie header field such that those with more specific Path
   attributes precede those with less specific.  Ordering with respect
   to other attributes (e.g., Domain) is unspecified.

   Note: For backward compatibility, the separator in the Cookie header
   field is semi-colon (;) everywhere.  A server SHOULD also accept
   comma (",") as the separator between cookie-values for future
   compatibility.

   A client MAY split a Cookie header field into multiple Cookie header
   fields, but SHOULD NOT do this unless comma is used as the separator,
   and the receiving server is expected to handle a multi-header Cookie
   value.

3.3.5.  Identifying What Version is Understood:  Cookie2

   The Cookie2 request header field facilitates interoperation between
   clients and servers that understand different versions of the cookie
   specification.  When the client sends one or more cookies to an



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   origin server, if at least one of those cookies contains a $Version
   attribute whose value is different from the version that the client
   understands, then the client MUST also send a Cookie2 request header
   field, the syntax for which is

      cookie2 =       "Cookie2:" cookie-version

   Here the value for cookie-version is the highest version of cookie
   specification (currently 2) that the client understands.  The client
   MUST only send at most one such request header field per request.

3.3.6.  Sending Cookies in Unverifiable Transactions

   Users MUST have control over sessions in order to ensure privacy.
   (See PRIVACY section below.)  To simplify implementation and to
   prevent an additional layer of complexity where adequate safeguards
   exist, however, this document distinguishes between transactions that
   are verifiable and those that are unverifiable.  A transaction is
   verifiable if the user, or a user-designated agent, has the option to
   review the request-URI prior to its use in the transaction.  A
   transaction is unverifiable if the user does not have that option.
   Unverifiable transactions typically arise when a user agent
   automatically requests inlined or embedded entities or when it
   resolves redirection (3xx) responses from an origin server.
   Typically the origin transaction, the transaction that the user
   initiates, is verifiable, and that transaction may directly or
   indirectly induce the user agent to make unverifiable transactions.

   An unverifiable transaction is to a third-party host if its request-
   host U does not domain-match the reach R of the request-host O in the
   origin transaction.

   When it makes an unverifiable transaction, a user agent MUST disable
   all cookie processing (i.e., MUST NOT send cookies, and MUST NOT
   accept any received cookies) if the transaction is to a third-party
   host.

   This restriction prevents a malicious service author from using
   unverifiable transactions to induce a user agent to start or continue
   a session with a server in a different domain.  The starting or
   continuation of such sessions could be contrary to the privacy
   expectations of the user, and could also be a security problem.

   User agents MAY offer configurable options that allow the user agent,
   or any autonomous programs that the user agent executes, to ignore
   the above rule, so long as these override options default to "off".

   (NOTE: Mechanisms may be proposed that will automate overriding the



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   third-party restrictions under controlled conditions.)

   Many current user agents already provide a review option that would
   render many links verifiable.  For instance, some user agents display
   the URL that would be referenced for a particular link when the mouse
   pointer is placed over that link.  The user can therefore determine
   whether to visit that site before causing the browser to do so.
   (Though not implemented on current user agents, a similar technique
   could be used for a button used to submit a form -- the user agent
   could display the action to be taken if the user were to select that
   button.)  However, even this would not make all links verifiable; for
   example, links to automatically loaded images would not normally be
   subject to "mouse pointer" verification.

   Many user agents also provide the option for a user to view the HTML
   source of a document, or to save the source to an external file where
   it can be viewed by another application.  While such an option does
   provide a crude review mechanism, some users might not consider it
   acceptable for this purpose.

3.4.  How an Origin Server Interprets the Cookie Header

   A user agent returns much of the information in the Set-Cookie2
   header field to the origin server when the request-URI path-matches
   the Path attribute of the cookie.  When it receives a Cookie header
   field, the origin server SHOULD treat cookies with NAMEs whose prefix
   is $ specially, as an attribute for the cookie.

3.5.  Caching Proxy Role

   One reason for separating state information from both a URL and
   document content is to facilitate the scaling that caching permits.
   To support cookies, a caching proxy MUST obey these rules already in
   the HTTP specification:

   o  Honor requests from the cache, if possible, based on cache
      validity rules.

   o  Pass along a Cookie request header field in any request that the
      proxy must make of another server.

   o  Return the response to the client.  Include any Set-Cookie2
      response header field.

   o  Cache the received response subject to the control of the usual
      header fields, such as Expires,

      Cache-control: no-cache



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      and

      Cache-control: private

   o  Cache the Set-Cookie2 subject to the control of the usual header
      field,

      Cache-control: no-cache="set-cookie2"
      (The Set-Cookie2 header field should usually not be cached.)

   Proxies MUST NOT introduce Set-Cookie2 (Cookie) header fields of
   their own in proxy responses (requests).


4.  Examples

4.1.  Example 1

   Most detail of request and response header fields has been omitted.
   Assume the user agent has no stored cookies, and that the hostname is
   www.example.com

   1.  User Agent -> Server

       POST /acme/login HTTP/1.1
       [form data]

   User identifies self via a form.

   2.  Server -> User Agent

       HTTP/1.1 200 OK
       Set-Cookie2: Customer="WILE_E_COYOTE"; Version="2";

   Cookie reflects user's identity.

   3.  User Agent -> Server

       POST /acme/pickitem HTTP/1.1
       Cookie: $Version="2"; Customer="WILE_E_COYOTE";
               $Domain="www.example.com"; $Path="/acme/"
       [form data]

   User selects an item for "shopping basket".

   4.  Server -> User Agent

       HTTP/1.1 200 OK



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       Set-Cookie2: Part_Number="Rocket_Launcher_0001"; Version="2"

   Shopping basket contains an item.

   5.  User Agent -> Server

       POST /acme/shipping HTTP/1.1
       Cookie: $Version="2";
               Customer="WILE_E_COYOTE";
                        $Domain="www.example.com"; $Path="/acme/";
               Part_Number="Rocket_Launcher_0001";
                        $Domain="www.example.com"; $Path="/acme/"
       [form data]

   User selects shipping method from form.

   6.  Server -> User Agent

       HTTP/1.1 200 OK
       Set-Cookie2: Shipping="FedEx"; Version="2"

   New cookie reflects shipping method.

   7.  User Agent -> Server

       POST /acme/process HTTP/1.1
       Cookie: $Version="2";
               Customer="WILE_E_COYOTE";
                        $Domain="www.example.com"; $Path="/acme/";
               Part_Number="Rocket_Launcher_0001";
                        $Domain="www.example.com"; $Path="/acme/";
               Shipping="FedEx";
                        $Domain="www.example.com"; $Path="/acme/"
       [form data]

   User chooses to process order.

   8.  Server -> User Agent

       HTTP/1.1 200 OK

   Transaction is complete.

   The user agent makes a series of requests on the origin server, after
   each of which it receives a new cookie.  All the cookies have the
   same Path attribute and (default) domain.  Because the request-URIs
   all path-match /acme/, the Path attribute of each cookie, each
   request contains all the cookies received so far.



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4.2.  Example 2

   This example illustrates the effect of the Path attribute.  All
   detail of request and response header fields has been omitted.
   Assume the user agent has no stored cookies.

   Imagine the user agent has received, in response to earlier requests,
   the response header fields

      Set-Cookie2: Part_Number="Rocket_Launcher_0001"; Version="2"

   and

      Set-Cookie2: Part_Number="Riding_Rocket_0023"; Version="2";
              SubPath="ammo"

   A subsequent request by the user agent to the (same) server for URLs
   of the form /acme/ammo/... would include the following request header
   field:

      Cookie: $Version="2";
              Part_Number="Riding_Rocket_0023";
                          $Domain="www.example.com"; $Path="/acme/ammo";
              Part_Number="Rocket_Launcher_0001";
                          $Domain="www.example.com"; $Path="/acme/"

   Note that the NAME=VALUE pair for the cookie with the more specific
   Path attribute, /acme/ammo, comes before the one with the less
   specific Path attribute, /acme.  Further note that the same cookie
   name appears more than once.

   A subsequent request by the user agent to the (same) server for a URL
   of the form /acme/parts/ would include the following request header
   field:

      Cookie: $Version="2"; Part_Number="Rocket_Launcher_0001";
                            $Domain="www.example.com"; $Path="/acme"

   Here, the second cookie's Path attribute /acme/ammo is not a prefix
   of the request URL, /acme/parts/, so the cookie does not get
   forwarded to the server.


5.  Implementation Considerations

   Here we provide guidance on likely or desirable details for an origin
   server that implements state management.




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5.1.  Set-Cookie2 Content

   An origin server's content should probably be divided into disjoint
   application areas, some of which require the use of state
   information.  The application areas can be distinguished by their
   request URLs.  The Set-Cookie2 header field can incorporate
   information about the application areas by setting the Path attribute
   for each one.

   The session information can obviously be clear or encoded text that
   describes state.  However, if it grows too large, it can become
   unwieldy.  Therefore, an implementor might choose for the session
   information to be a key to a server-side resource.  Of course, using
   a database creates some problems that this state management
   specification was meant to avoid, namely:

   1.  keeping real state on the server side;

   2.  how and when to garbage-collect the database entry, in case the
       user agent terminates the session by, for example, exiting.

5.2.  Stateless Pages

   Caching benefits the scalability of WWW.  Therefore it is important
   to reduce the number of documents that have state embedded in them
   inherently.  For example, if a shopping-basket-style application
   always displays a user's current basket contents on each page, those
   pages cannot be cached, because each user's basket's contents would
   be different.  On the other hand, if each page contains just a link
   that allows the user to "Look at My Shopping Basket", the page can be
   cached.

5.3.  Implementation Limits

   Practical user agent implementations have limits on the number and
   size of cookies that they can store.  In general, user agents' cookie
   support should have no fixed limits.  They should strive to store as
   many frequently-used cookies as possible.  Furthermore, general-use
   user agents SHOULD provide each of the following minimum capabilities
   individually, although not necessarily simultaneously:

   o  at least 300 cookies

   o  at least 4096 bytes per cookie (as measured by the characters that
      comprise the cookie non-terminal in the syntax description of the
      Set-Cookie2 header field, and as received in the Set-Cookie2
      header field)




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   o  at least 20 cookies per unique host or domain name

   User agents created for specific purposes or for limited-capacity
   devices SHOULD provide at least 20 cookies of 4096 bytes, to ensure
   that the user can interact with a session-based origin server.

   The information in a Set-Cookie2 response header field MUST be
   retained in its entirety.  If for some reason there is inadequate
   space to store the cookie, it MUST be discarded, not truncated.

   Applications should use as few and as small cookies as possible, and
   they should cope gracefully with the loss of a cookie.

5.3.1.  Denial of Service Attacks

   User agents MAY choose to set an upper bound on the number of cookies
   to be stored from a given host or domain name or on the size of the
   cookie information.  Otherwise a malicious server could attempt to
   flood a user agent with many cookies, or large cookies, on successive
   responses, which would force out cookies the user agent had received
   from other servers.  However, the minima specified above SHOULD still
   be supported.

5.4.  Backwards Compatibility

   Servers that send cookies according to this specification and that
   wish to send cookies with the same properties to a client following
   the RFC2965 specification MAY send Domain and Path attributes in the
   same header field as the version 2 arguments.  Clients following this
   specification MUST ignore these attributes.


6.  Privacy

   Informed consent should guide the design of systems that use cookies.
   A user should be able to find out how a web site plans to use
   information in a cookie and should be able to choose whether or not
   those policies are acceptable.  Both the user agent and the origin
   server must assist informed consent.

6.1.  User Agent Control

   An origin server could create a Set-Cookie2 header field to track the
   path of a user through the server.  Users may object to this behavior
   as an intrusive accumulation of information, even if their identity
   is not evident.  (Identity might become evident, for example, if a
   user subsequently fills out a form that contains identifying
   information.)  This state management specification therefore requires



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   that a user agent give the user control over such a possible
   intrusion, although the interface through which the user is given
   this control is left unspecified.  However, the control mechanisms
   provided SHALL at least allow the user

   o  to completely disable the sending and saving of cookies.

   o  to determine whether a stateful session is in progress.

   o  to control the saving of a cookie on the basis of the cookie's
      Domain attribute.

   Such control could be provided, for example, by mechanisms

   o  to notify the user when the user agent is about to send a cookie
      to the origin server, to offer the option not to begin a session.

   o  to display a visual indication that a stateful session is in
      progress.

   o  to let the user decide which cookies, if any, should be saved when
      the user concludes a window or user agent session.

   o  to let the user examine and delete the contents of a cookie at any
      time.

   A user agent usually begins execution with no remembered state
   information.  It SHOULD be possible to configure a user agent never
   to send a Cookie header field to an origin server, in which case it
   can never sustain state with an origin server.  (The user agent would
   then behave like one that is unaware of how to handle Set-Cookie2
   response header fields.)

   When the user agent terminates execution, it SHOULD let the user
   discard all state information.  Alternatively, the user agent MAY ask
   the user whether state information should be retained; the default
   should be "no".  If the user chooses to retain state information, it
   would be restored the next time the user agent runs.

   NOTE: User agents should probably be cautious about using files to
   store cookies long-term.  If a user runs more than one instance of
   the user agent, the cookies could be commingled or otherwise
   corrupted.

6.2.  Origin Server Role

   An origin server SHOULD promote informed consent by adding CommentURL
   or Comment information to the cookies it sends.  CommentURL is



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   preferred because of the opportunity to provide richer information in
   a multiplicity of languages.

6.3.  Clear Text

   The information transmitted in the Set-Cookie2 and Cookie header
   fields is unprotected.  As a consequence:

   1.  Any sensitive information that is conveyed in them is exposed to
       intruders.

   2.  A malicious intermediary could alter the header fields as they
       travel in either direction, with unpredictable results.

   These facts imply that information of a personal and/or financial
   nature should only be sent over a secure channel.  For less sensitive
   information, or when the content of the header field is a database
   key, an origin server should be vigilant to prevent a bad Cookie
   value from causing failures.

   A user agent in a shared user environment poses a further risk.
   Using a cookie inspection interface, User B could examine the
   contents of cookies that were saved when User A used the machine.


7.  IANA Considerations

   This document makes no request of IANA.

   Note to RFC Editor: this section may be removed on publication as an
   RFC.


8.  Security Considerations

8.1.  Protocol Design

   The restrictions on the value of the Domain state attribute by using
   the SubDomain attribute, and the rules concerning unverifiable
   transactions, are meant to reduce the ways that cookies can "leak" to
   the "wrong" site.  The intent is to restrict cookies to one host, or
   a closely related set of hosts.  We consider it acceptable for hosts
   host1.foo.com and host2.foo.com to share cookies, but not a.com and
   b.com.  Because of the many hierarchies used to organize domain
   names, it is not possible to define a small set of rules that can
   tell the client if a domain name is in fact similar to .com, or not.
   For that reason, this specification introduces the "Subdomain"
   attribute as a replacement of the "Domain" attribute defined by



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   [RFC2965], so that only hosts in the domain below the server
   top.example.com can receive the cookie.

   Similarly, a server can only share cookies with resource in
   subfolders of the default path derived from the request-URI.

8.2.  Cookie Spoofing

   Proper application design can avoid spoofing attacks from related
   domains.  Consider:

   1.  User agent makes request to victim.cracker.edu, gets back cookie
       session_id="1234" and sets the default domain victim.cracker.edu.

   2.  User agent makes request to cracker.edu, gets back cookie
       session-id="1111", with a SubDomain attribute.

   3.  User agent makes request to victim.cracker.edu again, and passes

      Cookie: $Version="1"; session_id="1234";
                       $Domain="victim.cracker.edu"; $Path="/example/" ,
              $Version="1"; session_id="1111";
                       $Domain=".cracker.edu"; $Path="/"
       The server at victim.cracker.edu should detect that the second
       cookie was not one it originated by noticing that the Domain
       attribute is not for itself and ignore it.

8.3.  Unexpected Cookie Sharing

   A user agent SHOULD make every attempt to prevent the sharing of
   session information between hosts that are in different domains.
   Embedded or inlined objects may cause particularly severe privacy
   problems if they can be used to share cookies between disparate
   hosts.  For example, a malicious server could embed cookie
   information for host a.com in a URI for a CGI on host b.com.  User
   agent implementors are strongly encouraged to prevent this sort of
   exchange whenever possible.

8.4.  Cookies for Account Information

   While it is common practice to use them this way, cookies are not
   designed or intended to be used to hold authentication information,
   such as account names and passwords.  Unless such cookies are
   exchanged over an encrypted path, the account information they
   contain is highly vulnerable to perusal and theft.






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9.  Historical

9.1.  Compatibility with Existing Implementations

   Existing cookie implementations, based on the Netscape specification,
   use the Set-Cookie (not Set-Cookie2) header field.  User agents that
   receive in the same response both a Set-Cookie and a Set-Cookie2
   response header field for the same cookie MUST discard the Set-Cookie
   information and use only the Set-Cookie2 information.  Furthermore, a
   user agent MUST assume, if it received a Set-Cookie2 response header
   field, that the sending server complies with this document and will
   understand Cookie request header fields that also follow this
   specification.

   New cookies MUST replace both equivalent old- and new-style cookies.
   That is, if a user agent that follows both this specification and
   Netscape's original specification receives a Set-Cookie2 response
   header field, and the NAME and the Domain and Path state attributes
   match (per the Cookie Management section) a Netscape-style cookie,
   the Netscape-style cookie MUST be discarded, and the user agent MUST
   retain only the cookie adhering to this specification.

   Older user agents that do not understand this specification, but that
   do understand Netscape's original specification, will not recognize
   the Set-Cookie2 response header field and will receive and send
   cookies according to the older specification.

   A user agent that supports both this specification and Netscape-style
   cookies SHOULD still send a Cookie request header field that follows
   the format specified in this document, as the benefit of adding
   domain and path information to each cookie and thus providing even
   older server with the ability to detect incorrectly set cookies
   outweigh the potential problems unknown cookienames may cause.

   The client should also send this header field in requests to servers
   that receive cookies that are not of the version specified by this
   document

      Cookie2: $Version="2"

   The Cookie2 header field advises the server that the user agent
   understands new-style cookies.  If the server understands new-style
   cookies, as well, it SHOULD continue the stateful session by sending
   a Set-Cookie2 response header field, rather than Set-Cookie.  A
   server that does not understand new-style cookies will simply ignore
   the Cookie2 request header field.





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9.2.  Caching and HTTP/1.0

   Some caches, such as those conforming to HTTP/1.0, will inevitably
   cache the Set-Cookie2 and Set-Cookie header fields, because there was
   no mechanism to suppress caching of headers prior to HTTP/1.1.  This
   caching can lead to security problems.  Documents transmitted by an
   origin server along with Set-Cookie2 and Set-Cookie header fields
   usually either will be uncachable, or will be "pre-expired".  As long
   as caches obey instructions not to cache documents (following
   Expires: <a date in the past> or Pragma: no-cache (HTTP/1.0), or
   Cache-control: no-cache (HTTP/1.1)) uncachable documents present no
   problem.  However, pre-expired documents may be stored in caches.
   They require validation (a conditional GET) on each new request, but
   some cache operators loosen the rules for their caches, and sometimes
   serve expired documents without first validating them.  This
   combination of factors can lead to cookies meant for one user later
   being sent to another user.  The Set-Cookie2 and Set-Cookie header
   fields are stored in the cache, and, although the document is stale
   (expired), the cache returns the document in response to later
   requests, including cached header fields.


10.  Acknowledgements

   This document is based on [RFC2965] by David Kristol and Lou Montulli
   and the collective efforts of the HTTP Working Group of the IETF and,
   particularly, the following people, in addition to the authors of RFC
   2965: Roy Fielding, Yaron Goland, Marc Hedlund, Ted Hardie, Koen
   Holtman, Shel Kaphan, Rohit Khare, Foteos Macrides, David W. Morris.

   This document include some changes suggested by RFC 2965 errata
   drafts posted by Arne Thomassen [ERRATA1] and David Kristol
   [ERRATA2].


11.  References

11.1.  Normative References

   [I-D.ietf-httpstate-cookie]
              Barth, A., "HTTP State Management Mechanism",
              draft-ietf-httpstate-cookie-23 (work in progress),
              March 2011.

   [Netscape]
              "Persistent Client State -- HTTP Cookies",
              <http://www.netscape.com/newsref/std/cookie_spec.html>.




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              available at
              <http://www.netscape.com/newsref/std/cookie_spec.html>

   [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.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

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

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

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

11.2.  Non-normative References

   [ERRATA1]  "RFC Errata: HTTP State Management Mechanism (draft)",
              October 2000,
              <http://retawq.sourceforge.net/cookies/
              cookie-errata.html>.

   [ERRATA2]  "[Draft of] Errata to RFC 2965", May 2003,
              <http://kristol.org/cookie/errata.html>.


Appendix A.  Open issues

   o  Should cookies with cookie-values with unquoted whitespace be
      rejected?














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Author's Address

   Yngve N Pettersen
   Opera Software ASA
   Waldemar Thranes gate 98
   N-0175 OSLO,
   Norway

   Email: yngve@opera.com










































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