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websec                                                          A. Barth
Internet-Draft                                              Google, Inc.
Intended status: Standards Track                       November 26, 2010
Expires: May 30, 2011


                         The Web Origin Concept
                         draft-abarth-origin-09

Abstract

   This document defines the concept of an "origin", which represents a
   web principal.  Typically, user agents isolate content retrieved from
   different origins to prevent a malicious web site operator from
   interfering with the operation of benign web sites.  In particular,
   this document defines how to compute an origin from a URI, how to
   serialize an origin to a string, and an HTTP header, named "Origin",
   for indicating which origin caused the user agent to issue a
   particular HTTP request.

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 May 30, 2011.

Copyright Notice

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




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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (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 BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions  . . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  Conformance Criteria . . . . . . . . . . . . . . . . . . .  4
     2.2.  Syntax Notation  . . . . . . . . . . . . . . . . . . . . .  4
     2.3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Origin . . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   4.  Comparing Origins  . . . . . . . . . . . . . . . . . . . . . .  8
   5.  Serializing Origins  . . . . . . . . . . . . . . . . . . . . .  9
     5.1.  Unicode Serialization of an Origin . . . . . . . . . . . .  9
     5.2.  ASCII Serialization of an Origin . . . . . . . . . . . . .  9
   6.  The HTTP Origin header . . . . . . . . . . . . . . . . . . . . 11
     6.1.  Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     6.2.  Semantics  . . . . . . . . . . . . . . . . . . . . . . . . 11
     6.3.  User Agent Requirements  . . . . . . . . . . . . . . . . . 11
   7.  Privacy Considerations . . . . . . . . . . . . . . . . . . . . 13
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
   10. Implementation Considerations  . . . . . . . . . . . . . . . . 16
     10.1. IDNA dependency and migration  . . . . . . . . . . . . . . 16
   11. Normative References . . . . . . . . . . . . . . . . . . . . . 17
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 18
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 19
















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1.  Introduction

   User agents interact with content created by a large number of
   authors.  Although many of those authors are well-meaning, some
   authors might be malicious.  To the extent that user agents undertake
   actions based on content they process, user agent implementors might
   wish to restrict the ability of malicious authors to disrupt the
   confidentiality or integrity of other content or servers.

   As an example, consider an HTTP user agent that renders HTML content
   retrieved from various servers.  If the user agent executes scripts
   contained in those documents, the user agent implementor might wish
   to prevent scripts retrieved from a malicious server from reading
   documents stored on an honest server, which might, for example, be
   behind a firewall.

   Traditionally, user agents have divided content according to its
   "origin".  More specifically, user agents allow content retrieved
   from one origin to interact freely with other content retrieved from
   that origin, but user agents restrict how that content can interact
   with content from another origin.

   This document does not describe the restrictions user agents ought to
   impose on cross-origin interaction.  Instead, this document defines
   the origin concept itself in such a way that other specifications,
   such for HTTP [cite] or for HTML [cite], can refer to this document
   for a precise, common definition of the web origin concept.

   Specifically, a user agent can compute the origin of a piece of
   content based on the URI from which the user agent retrieved the
   content.  Given two origins computed in this way, the user agent can
   compare the origins to determine if they are "the same", which is
   useful for performing some security checks.  Finally, given an
   origin, the user agent can serialize that origin into either an ASCII
   or a Unicode representation.

   This document also defines one use of the ASCII serialization: the
   HTTP Origin header.  An Origin header attached to an HTTP request
   contains the ASCII serializations of the origins that caused the user
   agent to issue the HTTP request.  The Origin header has a number of
   uses, including for cross-origin resource sharing [cite].










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2.  Conventions

2.1.  Conformance Criteria

   The keywords "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT",
   "RECOMMENDED", "MAY", and "OPTIONAL" in this 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.

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], Section
   1.3).




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   A globally unique identifier is a value which is different from all
   other previously existing values.  For example, a sufficiently long
   random string is likely to be a globally unique identifier.

   A idna-canonicalization host name is the string generated by the
   following algorithm:

   1.  Convert the host name to a sequence of NR-LDH labels (see Section
       2.3.2.2 of [RFC5890]) and/or A-labels according to the
       appropriate IDNA specification [RFC5891] or [RFC3490] (see
       Section 10.1 of this specification)

   2.  Convert the labels to lower case.

   3.  Concatenate the labels, separating each label from the next with
       a %x2E (".") character.



































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3.  Origin

   An origin represents a web principal.  Typically, user agents
   determine the origin of a piece of content from the URI from which
   they retrieved the content.  In this section, we define how to
   compute an origin from a URI.

   The origin of a URI is the value computed by the following algorithm:

   1.  If the URI does not use a server-based naming authority, or if
       the URI is not an absolute URI, then return a globally unique
       identifier.

   2.  Let uri-scheme be the scheme component of the URI, converted to
       lowercase.

   3.  If the implementation doesn't support the protocol given by uri-
       scheme, then return a globally unique identifier.

   4.  If uri-scheme is "file", the implementation MAY return an
       implementation-defined value.

       1.  NOTE: Historically, user agents have granted content from the
           file scheme a tremendous number of privileges.  However,
           granting all local files such wide privileges can lead to
           privilege escalation attacks.  Some user agents have had
           success granting local files directory-based privileges, but
           this approach has not been widely adopted.  Other user agent
           use a globally unique identifier each file URI, which is the
           most secure option.

   5.  Let uri-host be the idna-canonicalization of the host component
       of the URI.

   6.  If there is no port component of the URI:

       1.  Let uri-port be the default port for the protocol given by
           uri-scheme.

       Otherwise:

       2.  Let uri-port be the port component of the URI.

   7.  Return the triple (uri-scheme, uri-host, uri-port).

   Implementations MAY define other types of origins in addition to the
   scheme/host/port triple type defined above.  For example, an
   implementation might define an origin based on a public key or an



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   implementation might append addition "sandbox" bits to a scheme/host/
   port triple.

















































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4.  Comparing Origins

   To origins are "the same" if, and only if, they are identical.  In
   particular:

   o  If the two origins are scheme/host/port triple, the two origins
      are the same if, and only if, they have identical schemes, hosts,
      and ports.

   o  An origin that is globally unique identifier cannot be the same as
      an origin that is a scheme/host/port triple.

   o  Two origins that are globally unique identifiers cannot be the
      same if they were created at different times, even if they were
      created for the same URI.

   Two URIs are the same-origin if their origins are the same.

      NOTE: A URI is not necessarily same-origin with itself.  For
      example, a data URI is not same-origin with itself because data
      URIs do not use a server-based naming authority and therefore have
      globally unique identifiers as origins.





























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5.  Serializing Origins

   This section defines how to serialize an origin to a unicode string
   and to an ASCII string.

5.1.  Unicode Serialization of an Origin

   The unicode-serialization of an origin is the value returned by the
   following algorithm:

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

          null

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

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

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

   4.  Append the [TODO: IDNA ToUnicode] algorithm to each component of
       the host part of the origin triple, and append the results of
       each component, in the same order, separated by U+002E FULL STOP
       code points (".") to result.

   5.  If the port part of the origin triple is different than the
       default port for the protocol given by the scheme part of the
       origin triple:

       1.  Append a U+003A COLON code point (":") and the given port, in
           base ten, to result.

   6.  Return result.

   [TODO: Check that we handle IPv6 literals correctly.]

5.2.  ASCII Serialization of an Origin

   The ascii-serialization of an origin is the value returned by the
   following algorithm:

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

          null




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

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

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

   4.  Append the host port of the origin triple to result.

   5.  If the port part of the origin triple is different than the
       default port for the protocol given by the scheme part of the
       origin triple:

       1.  Append a U+003A COLON code points (":") and the given port,
           in base ten, to result.

   6.  Return result.


































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6.  The HTTP Origin header

   This section defines the HTTP Origin header.

6.1.  Syntax

   The Origin header has the following syntax:


 origin              = "Origin:" OWS origin-list-or-null OWS
 origin-list-or-null = "null" / origin-list
 origin-list         = serialized-origin *( SP serialized-origin )
 serialized-origin   = scheme "://" host [ ":" port ]
                     ; <scheme>, <host>, <port> productions from RFC3986


6.2.  Semantics

   When included in an HTTP request, the Origin header indicates the
   origin(s) that caused the user agent to issue the request.

   For example, consider a user agent that executes scripts on behalf of
   origins.  If one of those scripts causes the user agent to issue an
   HTTP request, the user agent might wish to use the Origin header to
   inform the server that the request was issued by the script.

   In some cases, a number of origins contribute to causing the user
   agents to issue an HTTP request.  In those cases, the user agent can
   list all the origins in the Origin header.  For example, if the HTTP
   request was initially issued by one origin but then later redirected
   by another origin, the user agent might wish to inform the server
   that two origins were involved in causing the user agent to issue the
   request.

6.3.  User Agent Requirements

   The user agent MAY include an Origin header in any HTTP request.

   The user agent MUST NOT include more than one Origin header field in
   any HTTP request.

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

      NOTE: This document does not define the notion of a privacy-
      sensitive context.  Applications that generate HTTP requests can
      designate contexts as privacy-sensitive to impose restrictions on



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      how user agents generate Origin headers.

   When generating an Origin header, the user agent MUST meet the
   following requirements:

   o  Each of the serialized-origin productions in the grammar MUST be
      the ascii-serialization of an origin.

   o  No two consecutive serialized-origin productions in the grammar
      can be identical.  In particular, if the user agent would generate
      two consecutive serialized-origins, the user agent MUST NOT
      generate the second one.

   If the user agent issued an HTTP request current-request because the
   user agent received 3xx Status Code response to another HTTP request
   previous-request for URI previous-uri:

   o  The HTTP request current-request MUST include an Origin header.

   o  The value of the Origin header MUST be either:

      *  The string "null" (i.e., the byte sequence %x6E, %x75, %x6C,
         %x6C).

      *  The value of the Origin header in the previous-request.  The
         user agent MUST NOT choose this option if the ascii-
         serialization of previous-uri is not identical to the last
         serialized-origin in the Origin header of the previous request.

      *  The value of the Origin header in previous header extended with
         a space and the ascii-serialization of the origin of previous-
         uri.  The user agent MUST NOT choose this option if the ascii-
         serialization of the origin of previous-uri is "null".

   The user agent SHOULD include the Origin header in an HTTP request if
   the user agent issues the HTTP request on behalf of an origin (e.g.,
   not by the user operating a trusted user interface surface).  In this
   case, the user agent SHOULD set the value of the Origin header to the
   ascii-serialization of that origin.

      NOTE: This behavior differs from the usual user agent behavior for
      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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7.  Privacy Considerations

   [TODO: Privacy considerations.]
















































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8.  Security Considerations

   [TODO: Security considerations.]
















































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9.  IANA Considerations

   [TODO: Register the Origin header.]
















































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10.  Implementation Considerations

10.1.  IDNA dependency and migration

   IDNA2008 [RFC5890] supersedes IDNA2003 [RFC3490] but is not
   backwards-compatible.  For this reason, there will be a transition
   period (possibly of a number of years).  User agents SHOULD implement
   IDNA2008 [RFC5890] and MAY implement [Unicode Technical Standard #46
   <http://unicode.org/reports/tr46/>] in order to facilitate a smoother
   IDNA transition.  If a user agent does not implement IDNA2008, the
   user agent MUST implement IDNA2003 [RFC3490].








































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11.  Normative References

   [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.

              See Section 10.1 for an explanation why the normative
              reference to an obsoleted specification is needed.

   [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.

   [RFC5890]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, August 2010.

   [RFC5891]  Klensin, J., "Internationalized Domain Names in
              Applications (IDNA): Protocol", RFC 5891, August 2010.























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Appendix A.  Acknowledgements


















































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Author's Address

   Adam Barth
   Google, Inc.

   Email: ietf@adambarth.com
   URI:   http://www.adambarth.com/












































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