websec                                                          A. Barth
Internet-Draft                                              Google, Inc.
Intended status: Standards Track                       December 29, 2010                           June 21, 2011
Expires: July 2, December 23, 2011

                         The Web Origin Concept


   This document defines the concept of an "origin", which represents a
   web principal. is often used
   as the scope of authority or privilege by user agents.  Typically,
   user agents isolate content retrieved from different origins to
   prevent a malicious web site operator operators from interfering with the
   operation of benign web sites.  In particular, addition to outlining the
   principles that underly the origin concept, this document defines how
   to compute an determine the origin from of a URI, how to serialize an origin to into a
   string, and an HTTP header, named "Origin",
   for indicating that indicates which origin caused the user agent to issue a
   origins are associated with an HTTP request.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3  4
   2.  Conventions  . . . . . . . . . . . . . . . . . . . . . . . . .  4  5
     2.1.  Conformance Criteria . . . . . . . . . . . . . . . . . . .  4  5
     2.2.  Syntax Notation  . . . . . . . . . . . . . . . . . . . . .  4  5
     2.3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4  5
   3.  Origin  Principles of the Same-Origin Policy . . . . . . . . . . . . .  7
     3.1.  Trust  . . . . . . . . . . . . . . . . . .  6
   4.  Comparing Origins . . . . . . . .  7
       3.1.1.  Pitfalls . . . . . . . . . . . . . . . . . . . . . . .  8
   5.  Serializing Origins
     3.2.  Origin . . . . . . . . . . . . . . . . . . . . .  9
     5.1.  Unicode Serialization of an Origin . . . . .  8
       3.2.1.  Examples . . . . . . . . . . . . . . . . . . . . . . .  9
     5.2.  ASCII Serialization of an Origin
     3.3.  Authority  . . . . . . . . . . . . . . . . . . . . . . . .  9
   6.  The HTTP Origin header
       3.3.1.  Pitfalls . . . . . . . . . . . . . . . . . . . . 11
     6.1.  Syntax . . . 10
     3.4.  Policy . . . . . . . . . . . . . . . . . . . . . . . . 11
     6.2.  Semantics . . 10
       3.4.1.  Object Access  . . . . . . . . . . . . . . . . . . . . 10
       3.4.2.  Network Access . . . . . . . . . . . . . . . . . . . . 11
     6.3.  User Agent Requirements
       3.4.3.  Pitfalls . . . . . . . . . . . . . . . . . . . . . . . 11
   7.  Privacy Considerations
     3.5.  Conclusion . . . . . . . . . . . . . . . . . . . . . . . . 12
   4.  Origin of a URI  . . . . . . . . . . . . . . . . . . . . . . . 13
   8.  Security Considerations
   5.  Comparing Origins  . . . . . . . . . . . . . . . . . . . . . . 14
   9.  IANA Considerations
   6.  Serializing Origins  . . . . . . . . . . . . . . . . . . . . . 15
   10. Implementation Considerations
     6.1.  Unicode Serialization of an Origin . . . . . . . . . . . . 15
     6.2.  ASCII Serialization of an Origin . . . . 16
     10.1. IDNA dependency and migration . . . . . . . . . 15
   7.  The HTTP Origin header . . . . . 16
   11. Normative References . . . . . . . . . . . . . . . 17
     7.1.  Syntax . . . . . . 17
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . . 18
   Author's Address . . 17
     7.2.  Semantics  . . . . . . . . . . . . . . . . . . . . . . . . 19

1.  Introduction 17
     7.3.  User agents interact with content created by a Agent Requirements  . . . . . . . . . . . . . . . . . 17
   8.  Privacy Considerations . . . . . . . . . . . . . . . . . . . . 19
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 20
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 21
   11. Implementation Considerations  . . . . . . . . . . . . . . . . 22
     11.1. IDNA dependency and migration  . . . . . . . . . . . . . . 22
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 23
     12.2. Informative References . . . . . . . . . . . . . . . . . . 23
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 24
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 25

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 describes 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 principles behind the web so-called same-
   origin concept.

   Specifically, a user agent can compute policy as well as the origin of a piece "nuts and bolts" 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. comparing and
   serializing origins.  This document also defines one use of the ASCII serialization: the
   HTTP Origin header.  An Origin header attached to an HTTP request
   contains does not describe all the ASCII serializations facets
   of the origins that caused same-origin policy, the user
   agent details of which are left to issue other
   specifications, such as HTML [HTML] and WebSockets [WEBSOCKETS],
   because the HTTP request.  The Origin header has a number of
   uses, including for cross-origin resource sharing [cite]. details are often application-specific.

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

   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

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

   A globally unique identifier is a value which is different from all
   other previously existing values. 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 of [RFC5890]) and/or A-labels according to the
       appropriate IDNA specification [RFC5891] or [RFC3490] (see
       Section 11.1 of this specification)

   2.  Convert the labels to lower case.

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

3.  Principles of the Same-Origin Policy

   Many user agents undertake actions on behalf of remote parties.  For
   example, HTTP user agents follow redirects, which are instructions
   from remote servers and HTML user agents expose rich DOM interfaces
   to scripts retrieved from remote servers.

   Without any security model, user agents might undertake actions
   detrimental to the user or to other parties.  Over time, many web-
   related technologies have converged towards a common security model,
   known colloquially as the "same-origin policy".  Although this
   security model evolved largely organically, the same-origin policy
   can be understood in terms of a handful of key concepts.  This
   section presents those concepts and provides advice about how to use
   these concepts securely.

3.1.  Trust

   The same-origin policy specifies trust by URI.  For example, HTML
   documents designate which script to run with a URI:

   <script src="https://example.com/library.js"></script>

   When a user agent process this element, the user agent will fetch the
   script at the designated URI and execute the script with the
   privileges of the document.  In this way, the document grants all the
   privileges it has to the resource designated by the URI.  In essence,
   the document declares that it trusts the integrity of information
   retrieved from that URI.

   In addition to importing libraries from URIs, user agents also send
   information to remote parties designated by URI.  For example,
   consider the HTML form element:

   <form method="POST" action="https://example.com/login">
    ... <input type="password"> ...

   When the user enters his or her password and submits the form, the
   user agent sends the password to the network endpoint designated by
   the URI.  In this way, the document exports its secret data to that
   URI, in essence declaring that it trusts the confidentiality of
   information sent to that URI.

3.1.1.  Pitfalls

   When designing new protocols that use the same-origin policy, make
   sure that important trust distinctions are visible in URIs.  For
   example, if both TLS and non-TLS protected resources used the "http"
   URI scheme (as in [RFC2817]), a document would be unable to specify
   that it wished to retrieve a script only over TLS.  By using the
   "https" URI scheme, documents are able to indicate that they wish to
   interact with resources that are protected from active network

3.2.  Origin

   In principle, user agents could treat every URI as a separate
   protection domain and require explicit content for content retrieved
   from one URI to interact with another URI.  Unfortunately, this
   design is cumbersome for developers because web applications often
   consist of a number of resources acting in concert.

   Instead, user agents group URIs together into protection domains
   called "origins".  Roughly speaking, two URIs are part of the same
   origin (i.e., represent the same principal) if they have the same
   scheme, host, and port.  (See Section 4 for full details.)

   Q: Why not just use the host?

   A: Including the scheme in the origin tuple is essential for
   security.  If user agents did not include the scheme, there would be
   no isolation between http://example.com and https://example.com
   because the two have the same host.  However, without this isolation,
   an active network attacker could corrupt content retrieved from
   http://example.com and have that content instruct the user agent to
   compromise the confidentiality and integrity of content retrieved
   from https://example.com, bypassing the protections afforded by TLS

   Q: Why use the fully qualified host name instead of just the "top-
   level" domain?

   A: Although the DNS has hierarchical delegation, the trust
   relationships between host names vary by deployment.  For example, at
   many educational institutions, students can host content at
   https://example.edu/~student/, but that does not mean a document
   authored by a student should be part of the same origin (i.e.,
   inhabit the same protection domain) as a web application for managing
   grades hosted at https://grades.example.edu/.

   The example.edu deployment illustrates that grouping resources by
   origin does not always align perfectly with every deployment
   scenario.  In this deployment every student's web site inhabits the
   same origin, which might not be desirable.  In some sense, the origin
   granularity is a historical artifact of how the security model

3.2.1.  Examples

   All of the following resources have the same origin:


   Each of the URIs has the same scheme, host, and port components.

   Each of the following resources has a different origin from the


   In each case, at least one of the scheme, host, and port component
   will differ from the others in the list.

3.3.  Authority

   Although user agents group URIs into origins, not every resource in
   an origin carries the same authority (in the security sense of the
   word "authority", not in the RFC 3986 sense).  For example, an image
   is passive content and, therefore, carries no authority, meaning the
   image has no access to the objects and resources available to its
   origin.  By contrast, an HTML document carries the full authority of
   its origin and scripts within (or imported into) the document can
   access every resource in its origin.

   User agent determine how much authority to grant a resource by
   examining its media type.  For example, resources with a media type
   of image/png are treated as images and resources with a media type of
   text/html are treated as HTML documents.

   When hosting untrusted content (such as user-generated content), web
   applications can limit that content's authority by restricting its
   media type.  For example, serving user-generated content as image/png
   is less risky than serving user-generated content as text/html.  Of
   course many web applications incorporate untrusted content in their
   HTML documents.  If not done carefully, these applications risk
   leaking their origin's authority to the untrusted content, a
   vulnerability commonly known as cross-site scripting.

3.3.1.  Pitfalls

   When designing new pieces of the web platform, be careful not to
   grant authority to resources irrespective of media type.  Many web
   applications serve untrusted content with restricted media types.  A
   new web platform feature that grants authority to these pieces of
   content risks introducing vulnerabilities into existing applications.
   Instead, prefer to grant authority to media types that already
   possess the origin's full authority or to new media types designed
   specifically to carry the new authority.

   In order to remain compatible with servers that supply incorrect
   media types, some user agents employ "content sniffing" and treat
   content as if had a different media type than the media type supplied
   by the server.  If not done carefully, content sniffing can lead to
   security vulnerabilities because user agents might grant low-
   authority media types, such as images, the privileges of high-
   authority media types, such as HTML documents [SNIFF].

3.4.  Policy

   Generally speaking, user agents isolate different origins and permit
   controlled communication between origins.  The details of how user
   agents provide isolation and communication vary depending on several

3.4.1.  Object Access

   Most objects (also known as application programming interfaces or
   APIs) exposed by the user agent are available only to the same
   origin.  Specifically, content retrieve from one URI can access
   objects associated with content retrieved from another URI if, and
   only if, the two URIs belong to the same origin, e.g., have same
   scheme, host, and port.

   There are some exceptions to this general rule.  For example, some
   parts of HTML's Location interface are available across origins
   (e.g., to allow for navigating other browsing contexts).  As another
   sample, HTML's postMessage interface is visible across origins
   explicitly to facilitate cross-origin communication.  Exposing
   objects to foreign origins is dangerous and should be done only with
   great care because doing so exposes these objects to potential

3.4.2.  Network Access

   Access to network resources varies depending on whether the resources
   are in the same origin as the content attempting to access them.

   Generally, reading information from another origin is forbidden.
   However, an origin is permitted use some kinds of resources retrieved
   from other origins.  For example, an origin is permitted to execute
   script, render images, and apply style sheets from any origin.
   Likewise, an origin can display content from another origin, such as
   an HTML document in an HTML frame.  Network resources can also opt
   into letting other origins read their information, for example using
   Cross-Origin Resource Sharing [CORS].  In these cases, access is
   typically granted on a per-origin basis.

   Sending information to another origin is permitted.  However, sending
   information over the network in arbitrary formats is dangerous.  For
   this reason, user agents restrict documents to sending information
   using particular protocols, such as in an HTTP request without custom
   headers.  Expanding the set of allowed protocols, for example by
   adding support for WebSockets, must be done carefully to avoid
   introducing vulnerabilities [WEBSOCKETS].

3.4.3.  Pitfalls

   Whenever user agents allow one origin to interact with resources from
   another origin, they invite security issues.  For example, a sufficiently long
   random string is likely the
   ability to be a globally unique identifier.

   A idna-canonicalization host name is display images from another origin leaks their height and
   width.  Similarly, the string generated by ability to send network requests to another
   origin gives rise to cross-site request forgery vulnerabilities
   [CSRF].  However, user agent implementors often balance these risks
   against the benefits of allowing the cross-origin interaction.  For
   example, an HTML user agent that blocked cross-origin network
   requests would prevent its users from following algorithm:

   1.  Convert hyperlinks, a core
   feature of the host name web.

   When adding new functionality to the web platform, it can be tempting
   to grant a sequence of NR-LDH labels (see Section of [RFC5890]) and/or A-labels according privilege to one resource but to withhold that privilege
   from another resource in the
       appropriate IDNA specification [RFC5891] or [RFC3490] (see
       Section 10.1 of same origin.  However, withholding
   privileges in this specification)

   2.  Convert way is ineffective because the labels to lower case.

   3.  Concatenate resource without
   the labels, separating each label from privilege can usually obtain the next with
       a %x2E (".") character.

3.  Origin

   An origin represents a web principal.  Typically, privilege anyway because user
   determine the origin of a piece of content from the URI do not isolate resources within an origin.  Instead,
   privileges should be granted or withheld from which
   they retrieved the content.  In this section, we define how origins as a whole
   (rather than discriminating between individual resources within an
   origin) [BOFGO].

3.5.  Conclusion

   The same-origin policy uses URIs to
   compute designates trust relationships.
   URIs are grouped together into origins, which represent protection
   domains.  Some resources in an origin from (e.g., active content) are
   granted the origin's full authority, whereas other resources in the
   origin (e.g., passive content) are not granted the origin's
   authority.  Content that carries its origin's authority is granted
   access to objects and network resources within its own origin.  This
   content is also granted limited access to objects and network
   resources of other origins, but these cross-origin privileges must be
   designed carefully to avoid security vulnerabilities.

4.  Origin of a URI. 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

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

   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 amount of privileges. privilege.  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


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


5.  Comparing Origins

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

   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.


6.  Serializing Origins

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


6.1.  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


       (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] ToUnicode algorithm [RFC5891] 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.]


6.2.  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


       (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 part 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.


7.  The HTTP Origin header

   This section defines the HTTP Origin header.


7.1.  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


7.2.  Semantics

   When included in an HTTP request, the Origin header indicates the
   origin(s) that caused "caused" the user agent to issue the request, as
   defined by the API that triggered the user agent to issue the

   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


7.3.  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
      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,

      *  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.


8.  Privacy Considerations

   [TODO: Privacy considerations.]


9.  Security Considerations

   [TODO: Security considerations.]


10.  IANA Considerations

   [TODO: Register the Origin header.]


11.  Implementation Considerations


11.1.  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].


12.  References

12.1.  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 11.1 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.

12.2.  Informative References

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC2817]  "FIXME: RFC2817".

   [SNIFF]    "FIXME: Media Type Sniffing".

   [HTML]     "FIXME: HTML5".

              "FIXME: WebSockets".

   [CORS]     "FIXME: Cross-Origin Resource Sharing".

   [CSRF]     "FIXME: Robust Defenses to Cross-Site Request Forgery".

   [BOFGO]    "FIXME: Beware of Finer-Grained Origins".

Appendix A.  Acknowledgements
Author's Address

   Adam Barth
   Google, Inc.

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