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Working Group                                                   A. Barth
Internet-Draft                                             U.C. Berkeley
Expires: December 26, 2009                                    C. Jackson
                                                     Stanford University
                                                              I. Hickson
                                                            Google, Inc.
                                                           June 24, 2009


                        The HTTP Sec-From Header
                         draft-abarth-origin-01

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
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   This Internet-Draft will expire on December 26, 2009.

Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
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Abstract

   This document defines the HTTP Sec-From header.  The Sec-From header
   is added by the user agent to describe the security contexts that
   caused the user agent to initiate an HTTP request.  HTTP servers can
   use the Sec-From header to mitigate against Cross-Site Request
   Forgery (CSRF) vulnerabilities.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Origin . . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Comparing Origins  . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Serializing Origins  . . . . . . . . . . . . . . . . . . . . .  6
     4.1.  Unicode Serialization of an Origin . . . . . . . . . . . .  6
     4.2.  ASCII Serialization of an Origin . . . . . . . . . . . . .  6
   5.  User Agent Behavior  . . . . . . . . . . . . . . . . . . . . .  8
   6.  HTTP Server Behavior . . . . . . . . . . . . . . . . . . . . .  9
   7.  Privacy Considerations . . . . . . . . . . . . . . . . . . . . 10
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 12
   10. TODO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14



























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1.  Introduction

   This document describes the HTTP Sec-From header.  The Sec-From
   header identifies the security contexts that caused the user agent to
   initiate an HTTP request.  HTTP servers can mitigate cross-site
   request forgery vulnerabilies by accepting requests only if the Sec-
   From header contains only white-listed origins.

   TODO: Discuss other CSRF defenses.










































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2.  Origin

   The following algoritm MUST be used to compute the origin of a URI.

   1.   Let /uri/ be the URI for which the origin is being determined.

   2.   Parse /uri/.

   3.   If /uri/ does not use a server-based naming authority, or if
        parsing /uri/ failed, or if /uri/ is not an absolute URI, then
        return an implementation-defined value.

   4.   Let /scheme/ be the scheme component of /uri/, converted to
        lowercase.

   5.   If the implementation doesn't support the protocol given by
        /scheme/, then return an implementaion-defined value.

   6.   If /scheme/ is "file", then the implementation MAY return a
        implementation-defined value.

   7.   Let /host/ be the host component of /uri/.

   8.   Apply the IDNA ToASCII algorithm to /host/, with both the
        AllowUnassigned and UseSTD3ASCIIRules flags set.  Let /host/ be
        the result of the ToASCII algorithm.

   9.   If ToASCII fails to convert one of the components of the string
        (e.g. because it is too long or because it contains invalid
        characters), then return an implementation-defined value.

   10.  Let /host/ be the result of converting /host/ to lowercase.

   11.  If there is no port component of /uri/, then let /port/ be the
        default port for the protocol given by /scheme/.  Otherwise, let
        /port/ be the port component of /uri/.

   12.  Return the tuple (/scheme/, /host/, /port/).

   Implementations MAY define other types of origins in addition to the
   scheme/host/port tuple type defined above.  (For example, user agents
   could implement globally unique origins or certificate-based
   origins.)








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3.  Comparing Origins

   Implementations MUST use the following algorithm to test whether two
   origins are the "same origin".

   1.  Let /A/ be the first origin being compared, and let /B/ be the
       second origin being compared.

   2.  If either /A/ or /B/ is not a scheme/host/port tuple, return an
       implementation-defined value.

   3.  If /A/ and /B/ have scheme components that are not identical,
       return false.

   4.  If /A/ and /B/ have host components that are not identical,
       return false.

   5.  If /A/ and /B/ have port components that are not identical,
       return false.

   6.  Return true.






























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4.  Serializing Origins

4.1.  Unicode Serialization of an Origin

   Implementations MUST using the following algorithm to compute the
   Unicode serialization of an origin:

   1.  If the origin in question is not a scheme/host/port tuple, then
       return the string

          null

       (i.e., the character sequence U+006E, U+0075, U+006C, U+006C) and
       abort these steps.

   2.  Otherwise, let /result/ be the scheme part of the origin tuple.

   3.  Append the string "://" to /result/.

   4.  Apply the IDNA ToUnicode algorithm to each component of the host
       part of the origin tuple, and append the results of each
       component, in the same order, separated by U+002E FULL STOP
       characters (".") to /result/.

   5.  If the port part of the origin tuple gives a port that is
       different from the default port for the protocol given by the
       scheme part of the origin tuple, then append a U+003A COLON
       character (":") and the given port, in base ten, to /result/.

   6.  Return /result/.

   TODO: Check that we handle IPv6 literals correctly.

4.2.  ASCII Serialization of an Origin

   Implementations MUST using the following algorithm to compute the
   ASCII serialization of an origin:

   1.  If the origin in question is not a scheme/host/port tuple, then
       return the string

          null

       (i.e., the character sequence U+006E, U+0075, U+006C, U+006C) and
       abort these steps.

   2.  Otherwise, let /result/ be the scheme part of the origin tuple.




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   3.  Append the string "://" to /result/.

   4.  Apply the IDNA ToASCII algorithm the host part of the origin
       tuple, with both the AllowUnassigned and UseSTD3ASCIIRules flags
       set, and append the result to /result/.

   5.  If ToASCII fails to convert one of the components of the string,
       e.g. because it is too long or because it contains invalid
       characters, then return the literal string "null" and abort these
       steps.

   6.  If the port part of the origin tuple gives a port that is
       different from the default port for the protocol given by the
       scheme part of the origin tuple, then append a U+003A COLON
       character (":") and the given port, in base ten, to /result/.

   7.  Return /result/.


































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5.  User Agent Behavior

   Whenever a user agent issues an HTTP request, the user agent MUST
   include exactly one HTTP header named "Sec-From".

   The value of the Sec-From header MUST either be

   1.  the string "null" (i.e., the character sequence U+006E, U+0075,
       U+006C, U+006C) or

   2.  a sequence of non-"null" ASCII serializations of origins
       separated by at least one LWS character.

   Whenever a user agent would send a Sec-From header containing two
   consecutive, idential origin serializations, the user agent MAY
   remove one such origin serialization from the header.

   Whenever a user agent issues an HTTP request from a "privacy-
   sensitive" context, the user agent MUST send the value "null" in the
   Sec-From header.

   Whenever a user agent issues an HTTP request to URI /A/ as a result
   of an HTTP redirect from URI /B/, the user agent MUST either:

   1.  set the value of the Sec-From header in the HTTP request to /A/
       to "null" (i.e., the character sequence U+006E, U+0075, U+006C,
       U+006C),

   2.  set the value of the Sec-From header in the /A/ request to the
       value of the Sec-From header in the /B/ request extended with a
       space and the ASCII serialization of the origin of /B/, unless
       this would result in the header containing the origin
       serialization "null".

   Whenever a user agent issues an HTTP request that (1) is *not* the
   result of an HTTP redirect and (2) is *not* initiated from a
   "privacy-sensitive" context, the user agent SHOULD set the value of
   the Sec-From header to the ASCII serialization of the origin that
   initiated the HTTP request.

   Note: This behavior differs from that of the HTTP Referer header,
   which user agents often suppress when an origin with an "https"
   scheme issues a request for a URI with an "http" scheme.








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6.  HTTP Server Behavior

   HTTP Servers MAY use the Sec-From header to "defend themselves
   against CSRF attacks."  Such servers are known as "participating
   servers" in this section.

   Let the /origin white list/ of a participating server be a set of
   strings selected by the operator of that server.

   The string "null" MUST NOT be a member of the /origin white list/ for
   any participating server.

      Example: The origin white list for the example.com Web server
      could be the strings "http://example.com", "https://example.com",
      "http://www.example.com", and "https://www.example.com".

   A participating server MUST use the following algorithm when
   determining whether to modify state in response to an HTTP request:

   1.  If the request method is safe (as defined by RFC 2616, Section 
       9.1.1, e.g. either "GET" nor "HEAD"), return "MUST NOT modify
       state" and abort these steps.

   2.  If the request does not contain a header named "Sec-From", return
       "MAY modify state" abort these stepts.

   3.  For each request header named "Sec-From", let the /initiating
       origin list/ be the list of origins represented in the header:

       1.  If there exists a origin in the /initiating origin list/ is
           not a member of the /origin white list/ for this server,
           return "MUST NOT modify state" and abort these steps.

   4.  Return "MAY modify state".

      Example: A Web server could modify state in response to POST
      requests that lack an Sec-From header (because these requests are
      sent by non-supporting user agents) and could modify state in
      response to POST requests that have an Sec-From header of
      "http://example.com", "https://example.com",
      "http://www.example.com", or "https://www.example.com".










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7.  Privacy Considerations

   This section is not normative.

   The Sec-From header improves on the Referer header by respecting the
   user's privacy: The Sec-From header includes only the information
   required to identify the principal that initiated the request
   (typically the scheme, host, and port of initiating origin).  In
   particular, the Sec-From header does not contain the path or query
   portions of the URI included in the Referer header that invade
   privacy without providing additional security.

   The Sec-From header also improves on the Referer header by NOT
   leaking intranet host names to external Web sites when a user follows
   a hyperlink from an intranet host to an external site because
   hyperlinks generate privacy-sensitive requests.



































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8.  Security Considerations

   This section is not normative.

   Because a supporting user agent will always include the Sec-From
   header when making HTTP requests, HTTP servers can detect that a
   request was initiated by a supporting user agent by observing the
   presence of the header.  This design prevents an attacker from making
   a supporting user agent appear to be a non-supporting user agent.
   Unlike the Referer header, which is absent when suppressed by the
   user agent, the Sec-From header takes on the value "null" when
   suppressed by the user agent.

   In some legacy user agents, The Sec-From header can be spoofed for
   same-site XMLHttpRequests.  Sites that rely only on network
   connectivity for authentication should use a DNS rebinding defense,
   such as validating the HTTP Host header, in addition to CSRF
   protection.

































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9.  IANA Considerations

   TODO: The "Sec-From" header should be registered.
















































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10.  TODO

   Think about how this interacts with proxies.

   Think about how this interacts with caches.














































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Authors' Addresses

   Adam Barth
   University of California, Berkeley

   Email: abarth@eecs.berkeley.edu
   URI:   http://www.adambarth.com/


   Collin Jackson
   Stanford University

   Email: collinj@cs.stanford.edu
   URI:   http://www.collinjackson.com/


   Ian Hickson
   Google, Inc.

   Email: ian@hixie.ch
   URI:   http://ln.hixie.ch/






























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