The Web Origin Concept

Internet Engineering Task Force (IETF)                          A. Barth
Request for Comments: 6454                                  Google, Inc.
Category: Standards Track                                  December 2011
ISSN: 2070-1721


                         The Web Origin Concept

Abstract

   This document defines the concept of an "origin", which is often used
   as the scope of authority or privilege by user agents.  Typically,
   user agents isolate content retrieved from different origins to
   prevent malicious web site operators from interfering with the
   operation of benign web sites.  In addition to outlining the
   principles that underlie the concept of origin, this document details
   how to determine the origin of a URI and how to serialize an origin
   into a string.  It also defines an HTTP header field, named "Origin",
   that indicates which origins are associated with an HTTP request.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6454.

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.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions  . . . . . . . . . . . . . . . . . . . . . . . . .  3
     2.1.  Conformance Criteria . . . . . . . . . . . . . . . . . . .  3
     2.2.  Syntax Notation  . . . . . . . . . . . . . . . . . . . . .  4
     2.3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Principles of the Same-Origin Policy . . . . . . . . . . . . .  4
     3.1.  Trust  . . . . . . . . . . . . . . . . . . . . . . . . . .  5
       3.1.1.  Pitfalls . . . . . . . . . . . . . . . . . . . . . . .  5
     3.2.  Origin . . . . . . . . . . . . . . . . . . . . . . . . . .  6
       3.2.1.  Examples . . . . . . . . . . . . . . . . . . . . . . .  7
     3.3.  Authority  . . . . . . . . . . . . . . . . . . . . . . . .  7
       3.3.1.  Pitfalls . . . . . . . . . . . . . . . . . . . . . . .  8
     3.4.  Policy . . . . . . . . . . . . . . . . . . . . . . . . . .  8
       3.4.1.  Object Access  . . . . . . . . . . . . . . . . . . . .  8
       3.4.2.  Network Access . . . . . . . . . . . . . . . . . . . .  9
       3.4.3.  Pitfalls . . . . . . . . . . . . . . . . . . . . . . .  9
     3.5.  Conclusion . . . . . . . . . . . . . . . . . . . . . . . . 10
   4.  Origin of a URI  . . . . . . . . . . . . . . . . . . . . . . . 10
   5.  Comparing Origins  . . . . . . . . . . . . . . . . . . . . . . 11
   6.  Serializing Origins  . . . . . . . . . . . . . . . . . . . . . 11
     6.1.  Unicode Serialization of an Origin . . . . . . . . . . . . 12
     6.2.  ASCII Serialization of an Origin . . . . . . . . . . . . . 12
   7.  The HTTP Origin Header Field . . . . . . . . . . . . . . . . . 13
     7.1.  Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     7.2.  Semantics  . . . . . . . . . . . . . . . . . . . . . . . . 13
     7.3.  User Agent Requirements  . . . . . . . . . . . . . . . . . 14
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
     8.1.  Reliance on DNS  . . . . . . . . . . . . . . . . . . . . . 15
     8.2.  Divergent Units of Isolation . . . . . . . . . . . . . . . 15
     8.3.  Ambient Authority  . . . . . . . . . . . . . . . . . . . . 16
     8.4.  IDNA Dependency and Migration  . . . . . . . . . . . . . . 16
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 17
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 17
     10.2. Informative References . . . . . . . . . . . . . . . . . . 18
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 20

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 describes the principles behind the so-called same-
   origin policy as well as the "nuts and bolts" of comparing and
   serializing origins.  This document does not describe all the facets
   of the same-origin policy, the details of which are left to other
   specifications, such as HTML [HTML] and WebSockets [RFC6455], because
   the details are often application-specific.

2.  Conventions

2.1.  Conformance Criteria

   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 [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 rule is used where zero or more linear whitespace octets
   might appear.  OWS SHOULD either not be produced or be produced as a
   single SP.  Multiple OWS octets that occur within field-content
   SHOULD either be replaced with a single SP or transformed to all SP
   octets (each octet other than SP replaced with SP) before
   interpreting the field value or forwarding the message downstream.

   OWS            = *( SP / HTAB / obs-fold )
                  ; "optional" whitespace
   obs-fold       = CRLF ( SP / HTAB )
                  ; obsolete line folding

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).

   A globally unique identifier is a value that is different from all
   other previously existing values.  For example, a sufficiently long
   random string is likely to be a globally unique identifier.  If the
   origin value never leaves the user agent, a monotonically increasing
   counter local to the user agent can also serve as a globally unique
   identifier.

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 Document Object
   Model (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 processes 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"> ...
   </form>

   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 Transport Layer Security (TLS) and non-TLS protected
   resources use the "http" URI scheme (as in [RFC2817]), a document
   would be unable to specify that it wishes 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 attackers.

3.2.  Origin

   In principle, user agents could treat every URI as a separate
   protection domain and require explicit consent 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
   [RFC5246].

   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
   evolved.

3.2.1.  Examples

   All of the following resources have the same origin:

   http://example.com/
   http://example.com:80/
   http://example.com/path/file

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

   Each of the following resources has a different origin from the
   others.

   http://example.com/
   http://example.com:8080/
   http://www.example.com/
   https://example.com:80/
   https://example.com/
   http://example.org/
   http://ietf.org/

   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 [RFC3986] 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 agents 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 it 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
   factors.

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 retrieved 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 the 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
   example, 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
   attackers.

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 to 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 [RFC6455].

3.4.3.  Pitfalls

   Whenever user agents allow one origin to interact with resources from
   another origin, they invite security issues.  For example, the
   ability to display images from another origin leaks their height and
   width.  Similarly, the 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 hyperlinks, a core
   feature of the web.

   When adding new functionality to the web platform, it can be tempting
   to grant a privilege to one resource but to withhold that privilege
   from another resource in the same origin.  However, withholding
   privileges in this way is ineffective because the resource without
   the privilege can usually obtain the privilege anyway because user
   agents do not isolate resources within an origin.  Instead,
   privileges should be granted or withheld from origins as a whole
   (rather than discriminating between individual resources within an
   origin) [BOFGO].

3.5.  Conclusion

   The same-origin policy uses URIs to designate trust relationships.
   URIs are grouped together into origins, which represent protection
   domains.  Some resources in an origin (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

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

   1.  If the URI does not use a hierarchical element as a naming
       authority (see [RFC3986], Section 3.2) or if the URI is not an
       absolute URI, then generate a fresh globally unique identifier
       and return that value.

          NOTE: Running this algorithm multiple times for the same URI
          can produce different values each time.  Typically, user
          agents compute the origin of, for example, an HTML document
          once and use that origin for subsequent security checks rather
          than recomputing the origin for each security check.

   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 generate a fresh globally unique identifier and
       return that value.

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

          NOTE: Historically, user agents have granted content from the
          file scheme a tremendous amount of 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 agents
          use globally unique identifiers for each file URI, which is
          the most secure option.

   5.  Let uri-host be the host component of the URI, converted to lower
       case (using the i;ascii-casemap collation defined in [RFC4790]).

          NOTE: This document assumes that the user agent performs
          Internationalizing Domain Names in Applications (IDNA)
          processing and validation when constructing the URI.  In
          particular, this document assumes the uri-host will contain
          only LDH labels because the user agent will have already
          converted any non-ASCII labels to their corresponding A-labels
          (see [RFC5890]).  For this reason, origin-based security
          policies are sensitive to the IDNA algorithm employed by the
          user agent.  See Section 8.4 for further discussion.

   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).

5.  Comparing Origins

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

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

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

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

      NOTE: A URI is not necessarily same-origin with itself.  For
      example, a data URI [RFC2397] 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
   [Unicode6] string and to an ASCII [RFC20] 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
       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 each component of the host part of the origin triple
       (converted as follows) to the result, separated by U+002E FULL
       STOP code points ("."):

       1.  If the component is an A-label, use the corresponding U-label
           instead (see [RFC5890] and [RFC5891]).

       2.  Otherwise, use the component verbatim.

   5.  If the port part of the origin triple is different from 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.

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
       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 host part of the origin triple to result.

   5.  If the port part of the origin triple is different from 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.

7.  The HTTP Origin Header Field

   This section defines the HTTP Origin header field.

7.1.  Syntax

   The Origin header field has the following syntax:

   origin              = "Origin:" OWS origin-list-or-null OWS
   origin-list-or-null = %x6E %x75 %x6C %x6C / origin-list
   origin-list         = serialized-origin *( SP serialized-origin )
   serialized-origin   = scheme "://" host [ ":" port ]
                       ; <scheme>, <host>, <port> from RFC 3986

7.2.  Semantics

   When included in an HTTP request, the Origin header field indicates
   the origin(s) that "caused" the user agent to issue the request, as
   defined by the API that triggered 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 MAY use the Origin header field to
   inform the server of the security context in which the script was
   executing when it caused the user agent to issue the request.

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

7.3.  User Agent Requirements

   The user agent MAY include an Origin header field 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 field.

      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 header fields.

   When generating an Origin header field, 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.

8.  Security Considerations

   The same-origin policy is one of the cornerstones of security for
   many user agents, including web browsers.  Historically, some user
   agents tried other security models, including taint tracking and
   exfiltration prevention, but those models proved difficult to
   implement at the time (although there has been recent interest in
   reviving some of these ideas).

   Evaluating the security of the same-origin policy is difficult
   because the origin concept itself plays such a central role in the
   security landscape.  The notional origin itself is just a unit of
   isolation, imperfect as are most one-size-fits-all notions.  That
   said, there are some systemic weaknesses, discussed below.

8.1.  Reliance on DNS

   In practice, the same-origin policy relies upon the Domain Name
   System (DNS) for security because many commonly used URI schemes,
   such as http, use DNS-based naming authorities.  If the DNS is
   partially or fully compromised, the same-origin policy might fail to
   provide the security properties required by applications.

   Some URI schemes, such as https, are more resistant to DNS compromise
   because user agents employ other mechanisms, such as certificates, to
   verify the source of content retrieved from these URIs.  Other URI
   schemes, such as the chrome-extension URI scheme (see Section 4.3 of
   [CRX]), use a public-key-based naming authority and are fully secure
   against DNS compromise.

   The web origin concept isolates content retrieved from different URI
   schemes; this is essential to containing the effects of DNS
   compromise.

8.2.  Divergent Units of Isolation

   Over time, a number of technologies have converged on the web origin
   concept as a convenient unit of isolation.  However, many
   technologies in use today, such as cookies [RFC6265], pre-date the
   modern web origin concept.  These technologies often have different
   isolation units, leading to vulnerabilities.

   One alternative is to use only the "registry-controlled" domain
   rather than the fully qualified domain name as the unit of isolation
   (e.g., "example.com" instead of "www.example.com").  This practice is
   problematic for a number of reasons and is NOT RECOMMENDED:

   1.  The notion of a "registry-controlled" domain is a function of
       human practice surrounding the DNS rather than a property of the
       DNS itself.  For example, many municipalities in Japan run public
       registries quite deep in the DNS hierarchy.  There are widely
       used "public suffix lists", but these lists are difficult to keep
       up to date and vary between implementations.

   2.  This practice is incompatible with URI schemes that do not use a
       DNS-based naming authority.  For example, if a given URI scheme
       uses public keys as naming authorities, the notion of a
       "registry-controlled" public key is somewhat incoherent.  Worse,
       some URI schemes, such as nntp, use dotted delegation in the
       opposite direction from DNS (e.g., alt.usenet.kooks), and others
       use the DNS but present the labels in the reverse of the usual
       order (e.g., com.example.www).

   At best, using "registry-controlled" domains is URI-scheme- and
   implementation-specific.  At worst, differences between URI schemes
   and implementations can lead to vulnerabilities.

8.3.  Ambient Authority

   When using the same-origin policy, user agents grant authority to
   content based on its URI rather than based on which objects the
   content can designate.  This disentangling of designation from
   authority is an example of ambient authority and can lead to
   vulnerabilities.

   Consider, for example, cross-site scripting in HTML documents.  If an
   attacker can inject script content into an HTML document, those
   scripts will run with the authority of the document's origin, perhaps
   allowing the script access to sensitive information, such as the
   user's medical records.  If, however, the script's authority were
   limited to those objects that the script could designate, the
   attacker would not gain any advantage by injecting the script into an
   HTML document hosted by a third party.

8.4.  IDNA Dependency and Migration

   The security properties of the same-origin policy can depend
   crucially on details of the IDNA algorithm employed by the user
   agent.  In particular, a user agent might map some international
   domain names (for example, those involving the U+00DF character) to
   different ASCII representations depending on whether the user agent
   uses IDNA2003 [RFC3490] or IDNA2008 [RFC5890].

   Migrating from one IDNA algorithm to another might redraw a number of
   security boundaries, potentially erecting new security boundaries or,
   worse, tearing down security boundaries between two mutually
   distrusting entities.  Changing security boundaries is risky because
   combining two mutually distrusting entities into the same origin
   might allow one to attack the other.

9.  IANA Considerations

   The permanent message header field registry (see [RFC3864]) has been
   updated with the following registration:

   Header field name: Origin

   Applicable protocol: http

   Status: standard

   Author/Change controller: IETF

   Specification document: this specification (Section 7)

10.  References

10.1.  Normative References

   [RFC20]     Cerf, V., "ASCII format for network interchange", RFC 20,
               October 1969.

   [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.

   [RFC3864]   Klyne, G., Nottingham, M., and J. Mogul, "Registration
               Procedures for Message Header Fields", BCP 90, RFC 3864,
               September 2004.

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

   [RFC4790]   Newman, C., Duerst, M., and A. Gulbrandsen, "Internet
               Application Protocol Collation Registry", RFC 4790,
               March 2007.

   [RFC5234]   Crocker, D., Ed. and P. Overell, "Augmented BNF for
               Syntax Specifications: ABNF", STD 68, RFC 5234,
               January 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.

   [Unicode6]  The Unicode Consortium, "The Unicode Standard, Version
               6.0.0", 2011,
               <http://www.unicode.org/versions/Unicode6.0.0/>.

10.2.  Informative References

   [BOFGO]     Jackson, C. and A. Barth, "Beware of Finer-Grained
               Origins", 2008,
               <http://w2spconf.com/2008/papers/s2p1.pdf>.

   [CORS]      van Kesteren, A., "Cross-Origin Resource Sharing", W3C
               Working Draft WD-cors-20100727, July 2010,
               <http://www.w3.org/TR/2010/WD-cors-20100727/>.

               Latest version available at <http://www.w3.org/TR/cors/>.

   [CRX]       Barth, A., Felt, A., Saxena, P., and A. Boodman,
               "Protecting Browsers from Extension Vulnerabilities",
               2010, <http://www.isoc.org/isoc/conferences/ndss/10/pdf/
               04.pdf>.

   [CSRF]      Barth, A., Jackson, C., and J. Mitchell, "Robust Defenses
               for Cross-Site Request Forgery", 2008,
               <http://portal.acm.org/citation.cfm?id=1455770.1455782>.

   [HTML]      Hickson, I., "HTML5", W3C Working Draft WD-html5-
               20110525, May 2011,
               <http://www.w3.org/TR/2011/WD-html5-20110525/>.

               Latest version available at
               <http://www.w3.org/TR/html5/>.

   [RFC2397]   Masinter, L., "The "data" URL scheme", RFC 2397,
               August 1998.

   [RFC2817]   Khare, R. and S. Lawrence, "Upgrading to TLS Within
               HTTP/1.1", RFC 2817, May 2000.

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

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

   [RFC6265]   Barth, A., "HTTP State Management Mechanism", RFC 6265,
               April 2011.

   [RFC6455]   Fette, I. and A. Melnikov, "The WebSocket Protocol",
               RFC 6455, December 2011.

   [SNIFF]     Barth, A. and I. Hickson, "Media Type Sniffing", Work
               in Progress, May 2011.

Appendix A.  Acknowledgements

   We would like to thank Lucas Adamski, Stephen Farrell, Miguel A.
   Garcia, Tobias Gondrom, Ian Hickson, Anne van Kesteren, Jeff Hodges,
   Collin Jackson, Larry Masinter, Alexey Melnikov, Mark Nottingham,
   Julian Reschke, Peter Saint-Andre, Jonas Sicking, Sid Stamm, Daniel
   Veditz, and Chris Weber for their valuable feedback on this document.

Author's Address

   Adam Barth
   Google, Inc.

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