websec A. Barth
Internet-Draft Google, Inc.
Intended status: Standards Track December 29, 2010
Expires: July 2, 2011
The Web Origin Concept
draft-ietf-websec-origin-00
Abstract
This document defines the concept of an "origin", which represents a
web principal. Typically, user agents isolate content retrieved from
different origins to prevent a malicious web site operator from
interfering with the operation of benign web sites. In particular,
this document defines how to compute an origin from a URI, how to
serialize an origin to a string, and an HTTP header, named "Origin",
for indicating which origin caused the user agent to issue a
particular HTTP request.
Status of this Memo
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Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Conformance Criteria . . . . . . . . . . . . . . . . . . . 4
2.2. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 4
2.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
3. Origin . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Comparing Origins . . . . . . . . . . . . . . . . . . . . . . 8
5. Serializing Origins . . . . . . . . . . . . . . . . . . . . . 9
5.1. Unicode Serialization of an Origin . . . . . . . . . . . . 9
5.2. ASCII Serialization of an Origin . . . . . . . . . . . . . 9
6. The HTTP Origin header . . . . . . . . . . . . . . . . . . . . 11
6.1. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . 11
6.2. Semantics . . . . . . . . . . . . . . . . . . . . . . . . 11
6.3. User Agent Requirements . . . . . . . . . . . . . . . . . 11
7. Privacy Considerations . . . . . . . . . . . . . . . . . . . . 13
8. Security Considerations . . . . . . . . . . . . . . . . . . . 14
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
10. Implementation Considerations . . . . . . . . . . . . . . . . 16
10.1. IDNA dependency and migration . . . . . . . . . . . . . . 16
11. Normative References . . . . . . . . . . . . . . . . . . . . . 17
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 18
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 19
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1. Introduction
User agents interact with content created by a large number of
authors. Although many of those authors are well-meaning, some
authors might be malicious. To the extent that user agents undertake
actions based on content they process, user agent implementors might
wish to restrict the ability of malicious authors to disrupt the
confidentiality or integrity of other content or servers.
As an example, consider an HTTP user agent that renders HTML content
retrieved from various servers. If the user agent executes scripts
contained in those documents, the user agent implementor might wish
to prevent scripts retrieved from a malicious server from reading
documents stored on an honest server, which might, for example, be
behind a firewall.
Traditionally, user agents have divided content according to its
"origin". More specifically, user agents allow content retrieved
from one origin to interact freely with other content retrieved from
that origin, but user agents restrict how that content can interact
with content from another origin.
This document does not describe the restrictions user agents ought to
impose on cross-origin interaction. Instead, this document defines
the origin concept itself in such a way that other specifications,
such for HTTP [cite] or for HTML [cite], can refer to this document
for a precise, common definition of the web origin concept.
Specifically, a user agent can compute the origin of a piece of
content based on the URI from which the user agent retrieved the
content. Given two origins computed in this way, the user agent can
compare the origins to determine if they are "the same", which is
useful for performing some security checks. Finally, given an
origin, the user agent can serialize that origin into either an ASCII
or a Unicode representation.
This document also defines one use of the ASCII serialization: the
HTTP Origin header. An Origin header attached to an HTTP request
contains the ASCII serializations of the origins that caused the user
agent to issue the HTTP request. The Origin header has a number of
uses, including for cross-origin resource sharing [cite].
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2. Conventions
2.1. Conformance Criteria
The keywords "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT",
"RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be
interpreted as described in [RFC2119].
Requirements phrased in the imperative as part of algorithms (such as
"strip any leading space characters" or "return false and abort these
steps") are to be interpreted with the meaning of the key word
("MUST", "SHOULD", "MAY", etc) used in introducing the algorithm.
Conformance requirements phrased as algorithms or specific steps can
be implemented in any manner, so long as the end result is
equivalent. In particular, the algorithms defined in this
specification are intended to be easy to understand and are not
intended to be performant.
2.2. Syntax Notation
This specification uses the Augmented Backus-Naur Form (ABNF)
notation of [RFC5234].
The following core rules are included by reference, as defined in
[RFC5234], Appendix B.1: ALPHA (letters), CR (carriage return), CRLF
(CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote),
HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
sequence of data), SP (space), HTAB (horizontal tab), CHAR (any US-
ASCII character), VCHAR (any visible US-ASCII character), and WSP
(whitespace).
The OWS (optional whitespace) rule is used where zero or more linear
whitespace characters MAY appear:
OWS = *( [ obs-fold ] WSP )
; "optional" whitespace
obs-fold = CRLF
OWS SHOULD either not be produced or be produced as a single SP
character.
2.3. Terminology
The terms user agent, client, server, proxy, and origin server have
the same meaning as in the HTTP/1.1 specification ([RFC2616], Section
1.3).
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A globally unique identifier is a value which is different from all
other previously existing values. For example, a sufficiently long
random string is likely to be a globally unique identifier.
A idna-canonicalization host name is the string generated by the
following algorithm:
1. Convert the host name to a sequence of NR-LDH labels (see Section
2.3.2.2 of [RFC5890]) and/or A-labels according to the
appropriate IDNA specification [RFC5891] or [RFC3490] (see
Section 10.1 of this specification)
2. Convert the labels to lower case.
3. Concatenate the labels, separating each label from the next with
a %x2E (".") character.
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3. Origin
An origin represents a web principal. Typically, user agents
determine the origin of a piece of content from the URI from which
they retrieved the content. In this section, we define how to
compute an origin from a URI.
The origin of a URI is the value computed by the following algorithm:
1. If the URI does not use a server-based naming authority, or if
the URI is not an absolute URI, then return a globally unique
identifier.
2. Let uri-scheme be the scheme component of the URI, converted to
lowercase.
3. If the implementation doesn't support the protocol given by uri-
scheme, then return a globally unique identifier.
4. If uri-scheme is "file", the implementation MAY return an
implementation-defined value.
1. NOTE: Historically, user agents have granted content from the
file scheme a tremendous number of privileges. However,
granting all local files such wide privileges can lead to
privilege escalation attacks. Some user agents have had
success granting local files directory-based privileges, but
this approach has not been widely adopted. Other user agent
use a globally unique identifier each file URI, which is the
most secure option.
5. Let uri-host be the idna-canonicalization of the host component
of the URI.
6. If there is no port component of the URI:
1. Let uri-port be the default port for the protocol given by
uri-scheme.
Otherwise:
2. Let uri-port be the port component of the URI.
7. Return the triple (uri-scheme, uri-host, uri-port).
Implementations MAY define other types of origins in addition to the
scheme/host/port triple type defined above. For example, an
implementation might define an origin based on a public key or an
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implementation might append addition "sandbox" bits to a scheme/host/
port triple.
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4. Comparing Origins
To origins are "the same" if, and only if, they are identical. In
particular:
o If the two origins are scheme/host/port triple, the two origins
are the same if, and only if, they have identical schemes, hosts,
and ports.
o An origin that is globally unique identifier cannot be the same as
an origin that is a scheme/host/port triple.
o Two origins that are globally unique identifiers cannot be the
same if they were created at different times, even if they were
created for the same URI.
Two URIs are the same-origin if their origins are the same.
NOTE: A URI is not necessarily same-origin with itself. For
example, a data URI is not same-origin with itself because data
URIs do not use a server-based naming authority and therefore have
globally unique identifiers as origins.
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5. Serializing Origins
This section defines how to serialize an origin to a unicode string
and to an ASCII string.
5.1. Unicode Serialization of an Origin
The unicode-serialization of an origin is the value returned by the
following algorithm:
1. If the origin is not a scheme/host/port triple, then return the
string
null
(i.e., the code point sequence U+006E, U+0075, U+006C, U+006C)
and abort these steps.
2. Otherwise, let result be the scheme part of the origin triple.
3. Append the string "://" to result.
4. Append the [TODO: IDNA ToUnicode] algorithm to each component of
the host part of the origin triple, and append the results of
each component, in the same order, separated by U+002E FULL STOP
code points (".") to result.
5. If the port part of the origin triple is different than the
default port for the protocol given by the scheme part of the
origin triple:
1. Append a U+003A COLON code point (":") and the given port, in
base ten, to result.
6. Return result.
[TODO: Check that we handle IPv6 literals correctly.]
5.2. ASCII Serialization of an Origin
The ascii-serialization of an origin is the value returned by the
following algorithm:
1. If the origin is not a scheme/host/port triple, then return the
string
null
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(i.e., the code point sequence U+006E, U+0075, U+006C, U+006C)
and abort these steps.
2. Otherwise, let result be the scheme part of the origin triple.
3. Append the string "://" to result.
4. Append the host port of the origin triple to result.
5. If the port part of the origin triple is different than the
default port for the protocol given by the scheme part of the
origin triple:
1. Append a U+003A COLON code points (":") and the given port,
in base ten, to result.
6. Return result.
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6. The HTTP Origin header
This section defines the HTTP Origin header.
6.1. Syntax
The Origin header has the following syntax:
origin = "Origin:" OWS origin-list-or-null OWS
origin-list-or-null = "null" / origin-list
origin-list = serialized-origin *( SP serialized-origin )
serialized-origin = scheme "://" host [ ":" port ]
; <scheme>, <host>, <port> productions from RFC3986
6.2. Semantics
When included in an HTTP request, the Origin header indicates the
origin(s) that caused the user agent to issue the request.
For example, consider a user agent that executes scripts on behalf of
origins. If one of those scripts causes the user agent to issue an
HTTP request, the user agent might wish to use the Origin header to
inform the server that the request was issued by the script.
In some cases, a number of origins contribute to causing the user
agents to issue an HTTP request. In those cases, the user agent can
list all the origins in the Origin header. For example, if the HTTP
request was initially issued by one origin but then later redirected
by another origin, the user agent might wish to inform the server
that two origins were involved in causing the user agent to issue the
request.
6.3. User Agent Requirements
The user agent MAY include an Origin header in any HTTP request.
The user agent MUST NOT include more than one Origin header field in
any HTTP request.
Whenever a user agent issues an HTTP request from a "privacy-
sensitive" context, the user agent MUST send the value "null" in the
Origin header.
NOTE: This document does not define the notion of a privacy-
sensitive context. Applications that generate HTTP requests can
designate contexts as privacy-sensitive to impose restrictions on
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how user agents generate Origin headers.
When generating an Origin header, the user agent MUST meet the
following requirements:
o Each of the serialized-origin productions in the grammar MUST be
the ascii-serialization of an origin.
o No two consecutive serialized-origin productions in the grammar
can be identical. In particular, if the user agent would generate
two consecutive serialized-origins, the user agent MUST NOT
generate the second one.
If the user agent issued an HTTP request current-request because the
user agent received 3xx Status Code response to another HTTP request
previous-request for URI previous-uri:
o The HTTP request current-request MUST include an Origin header.
o The value of the Origin header MUST be either:
* The string "null" (i.e., the byte sequence %x6E, %x75, %x6C,
%x6C).
* The value of the Origin header in the previous-request. The
user agent MUST NOT choose this option if the ascii-
serialization of previous-uri is not identical to the last
serialized-origin in the Origin header of the previous request.
* The value of the Origin header in previous header extended with
a space and the ascii-serialization of the origin of previous-
uri. The user agent MUST NOT choose this option if the ascii-
serialization of the origin of previous-uri is "null".
The user agent SHOULD include the Origin header in an HTTP request if
the user agent issues the HTTP request on behalf of an origin (e.g.,
not by the user operating a trusted user interface surface). In this
case, the user agent SHOULD set the value of the Origin header to the
ascii-serialization of that origin.
NOTE: This behavior differs from the usual user agent behavior for
the HTTP Referer header, which user agents often suppress when an
origin with an "https" scheme issues a request for a URI with an
"http" scheme.
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7. Privacy Considerations
[TODO: Privacy considerations.]
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8. Security Considerations
[TODO: Security considerations.]
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9. IANA Considerations
[TODO: Register the Origin header.]
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10. Implementation Considerations
10.1. IDNA dependency and migration
IDNA2008 [RFC5890] supersedes IDNA2003 [RFC3490] but is not
backwards-compatible. For this reason, there will be a transition
period (possibly of a number of years). User agents SHOULD implement
IDNA2008 [RFC5890] and MAY implement [Unicode Technical Standard #46
<http://unicode.org/reports/tr46/>] in order to facilitate a smoother
IDNA transition. If a user agent does not implement IDNA2008, the
user agent MUST implement IDNA2003 [RFC3490].
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11. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications (IDNA)",
RFC 3490, March 2003.
See Section 10.1 for an explanation why the normative
reference to an obsoleted specification is needed.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, August 2010.
[RFC5891] Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", RFC 5891, August 2010.
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Appendix A. Acknowledgements
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Author's Address
Adam Barth
Google, Inc.
Email: ietf@adambarth.com
URI: http://www.adambarth.com/
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