websec A. Barth
Internet-Draft Google, Inc.
Intended status: Standards Track June 21, 2011
Expires: December 23, 2011
The Web Origin Concept
draft-ietf-websec-origin-01
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 underly the origin concept, this document defines how
to determine the origin of a URI, how to serialize an origin into a
string, and an HTTP header, named "Origin", that indicates which
origins are associated with an HTTP request.
Status of this Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on December 23, 2011.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Conformance Criteria . . . . . . . . . . . . . . . . . . . 5
2.2. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 5
2.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
3. Principles of the Same-Origin Policy . . . . . . . . . . . . . 7
3.1. Trust . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1.1. Pitfalls . . . . . . . . . . . . . . . . . . . . . . . 8
3.2. Origin . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.2.1. Examples . . . . . . . . . . . . . . . . . . . . . . . 9
3.3. Authority . . . . . . . . . . . . . . . . . . . . . . . . 9
3.3.1. Pitfalls . . . . . . . . . . . . . . . . . . . . . . . 10
3.4. Policy . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.4.1. Object Access . . . . . . . . . . . . . . . . . . . . 10
3.4.2. Network Access . . . . . . . . . . . . . . . . . . . . 11
3.4.3. Pitfalls . . . . . . . . . . . . . . . . . . . . . . . 11
3.5. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . 12
4. Origin of a URI . . . . . . . . . . . . . . . . . . . . . . . 13
5. Comparing Origins . . . . . . . . . . . . . . . . . . . . . . 14
6. Serializing Origins . . . . . . . . . . . . . . . . . . . . . 15
6.1. Unicode Serialization of an Origin . . . . . . . . . . . . 15
6.2. ASCII Serialization of an Origin . . . . . . . . . . . . . 15
7. The HTTP Origin header . . . . . . . . . . . . . . . . . . . . 17
7.1. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . 17
7.2. Semantics . . . . . . . . . . . . . . . . . . . . . . . . 17
7.3. User 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
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Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 25
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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 [WEBSOCKETS],
because the details are often application-specific.
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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 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.
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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"> ...
</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.
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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
attackers.
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
[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
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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
http://example.com/
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 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
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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
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 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
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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
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 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, 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
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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 designates 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.
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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 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 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 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
implementation might append addition "sandbox" bits to a scheme/host/
port triple.
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5. 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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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
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 IDNA 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.
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 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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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" 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 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.
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.
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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.
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8. Privacy Considerations
[TODO: Privacy considerations.]
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9. Security Considerations
[TODO: Security considerations.]
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10. IANA Considerations
[TODO: Register the Origin header.]
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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].
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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 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.
[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".
[WEBSOCKETS]
"FIXME: WebSockets".
[CORS] "FIXME: Cross-Origin Resource Sharing".
[CSRF] "FIXME: Robust Defenses to Cross-Site Request Forgery".
[BOFGO] "FIXME: Beware of Finer-Grained Origins".
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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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