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.
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RFC 6454 The Web Origin Concept December 2011
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
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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 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.
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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,
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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:
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:
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.
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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.
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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.
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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.
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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].
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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.
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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.
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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.
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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 ]
;
RFC, FYI, BCP