Internet DRAFT - draft-ietf-websec-x-frame-options
draft-ietf-websec-x-frame-options
WEBSEC D. Ross
Internet-Draft Microsoft
Intended status: Informational T. Gondrom
Expires: February 28, 2014 Thames Stanley
August 27, 2013
HTTP Header Field X-Frame-Options
draft-ietf-websec-x-frame-options-12
Abstract
To improve the protection of web applications against Clickjacking,
this definition describes the X-Frame-Options HTTP response header
field that declares a policy communicated from the server to the
client browser on whether the browser may display the transmitted
content in frames that are part of other web pages. This
informational document serves to document the existing use and
specification of this X-Frame-Options HTTP response header field.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on February 28, 2014.
Copyright Notice
Copyright (c) 2013 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
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to this document. Code Components extracted from this document must
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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 . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. X-Frame-Options Header . . . . . . . . . . . . . . . . . . . 3
2.1. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2. Augmented Backus-Naur Form (ABNF) . . . . . . . . . . . 5
2.2.1. Examples of X-Frame-Options . . . . . . . . . . . . . 5
2.3. Design Issues . . . . . . . . . . . . . . . . . . . . . . 5
2.3.1. Enable HTML content from other domains . . . . . . . 6
2.3.2. Browser Behaviour and Processing . . . . . . . . . . 6
2.3.2.1. Violation of X-Frame-Options . . . . . . . . . . 6
2.3.2.2. Variation in current browser behaviour . . . . . 6
2.3.2.3. Usage design pattern and example scenario for the
ALLOW-FROM parameter . . . . . . . . . . . . . . 8
2.3.2.4. No caching of the X-Frame-Options header . . . . 8
3. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
4.1. Registration Template . . . . . . . . . . . . . . . . . . 9
5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
5.1. Privacy Considerations . . . . . . . . . . . . . . . . . 10
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
6.1. Normative References . . . . . . . . . . . . . . . . . . 10
6.2. Informative References . . . . . . . . . . . . . . . . . 11
Appendix A. Browsers that support X-Frame-Options . . . . . . . 12
Appendix B. Description of a Clickjacking attack . . . . . . . . 12
B.1. Shop . . . . . . . . . . . . . . . . . . . . . . . . . . 12
B.2. Online Shop Confirm Purchase Page . . . . . . . . . . . . 12
B.3. Flash Configuration . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
In 2009 and 2010 many browser vendors ([Microsoft-X-Frame-Options],
[CLICK-DEFENSE-BLOG], [Mozilla-X-Frame-Options]) introduced the use
of a non-standard HTTP [RFC2616] header field "X-Frame-Options" to
protect against Clickjacking [Clickjacking]. HTML-based web
applications can embed or "frame" other web pages. Clickjacking is a
type of attack that occurs when an attacker uses multiple transparent
or opaque layers in the user interface to trick a user into clicking
on a button or link on another page from server B when they were
intending to click on the same place of the overlaying page from
server A. Thus, the attacker is "hijacking" clicks meant for their
page A and routing them to another page B. The attacker is tricking
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the user (who sees the overlaying user interface content from page A)
into clicking specific locations on the underlying page from server
B, triggering some actions on server B and potentially using an
existing session context in that step. This is an attack on both the
user and on server B. And server A may or may not be the attacker.
This specification provides informational documentation about the
current use and definition of the X-Frame-Options HTTP header field.
As described in Section 2.3.2.2 not all browsers implement X-Frame-
Options exactly in the sames way, which can lead to unintended
results. And given that the "X-" construction is deprecated
[RFC6648], the X-Frame-Options header field will in the future be
replaced by the Frame-Options directive in the Content Security
Policy Version 1.1 [CSP-1-1].
Existing anti-ClickJacking measures, e.g. Frame-breaking Javascript,
have weaknesses so that their protection can be circumvented as a
study [FRAME-BUSTING] demonstrated.
Short of configuring the browser to disable frames and script
entirely, which massively impairs browser utility, browser users are
vulnerable to this type of attack.
"X-Frame-Options" allows a web page from host B to declare that its
content (for example a button, links, text, etc.) must not be
displayed in a frame (<frame> or <iframe>) of another page (e.g. from
host A). This is done by a policy declared in the HTTP header and
enforced by browser implementations as documented here.
1.1. Requirements Language
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 RFC 2119 [RFC2119].
2. X-Frame-Options Header
The X-Frame-Options HTTP response header field indicates a policy on
whether the browser should render the transmitted resource within a
<frame> or <iframe>. Servers can declare this policy in the header
of their HTTP responses to prevent clickjacking attacks, and by this
ensuring that their content is not embedded into other pages or
frames.
2.1. Syntax
The header field name is:
X-Frame-Options
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There are three different values for the header field. These values
are mutually exclusive, that is exactly one of the three values MUST
be set.
DENY
A browser receiving content with this header MUST NOT display
this content in any frame.
SAMEORIGIN
A browser receiving content with this header field MUST NOT
display this content in any frame from a page of different
origin than the content itself.
If a browser or plugin can not reliably determine whether the
origin of the content and the frame have the same origin, this
MUST be treated as "DENY".
Please note that current implementations vary on the
interpretation of this criteria: In some it only allows a page
to be framed if the origin of the top-level browsing-context is
identical to the origin of the content using the X-FRAME-
OPTIONS directive; in others it may consider the origin of the
framing page instead. See also section 2.3.2.2 for more
details on the nesting of frames and variations in the handling
of this header field by different browsers. And refer to
section 5 paragraph 2 for the resulting potential security
problems.
ALLOW-FROM (followed by a serialized-origin [RFC6454])
A browser receiving content with this header MUST NOT display
this content in a frame from any page with a top-level browsing
context of different origin than the specified origin. While
this can expose the page to risks by the trusted origin, in
some cases it may be necessary to allow the framing by content
from other domains.
The meaning of the term "serialized-origin" is given in [RFC6454].
If the ALLOW-FROM value is used, it MUST be followed by a valid
origin [RFC6454] (as a subset of URI [RFC3986])
Any data beyond the domain address (i.e. any data after the "/"
separator) is to be ignored. And the algorithm to compare origins
from [RFC6454] SHOULD be used to verify that a referring page is of
the same origin as the content (in the case of SAMEORIGIN) or that
the referring page's origin is identical with the ALLOW-FROM
serialized-origin (in the case of ALLOW-FROM). Though in conflict
with [RFC6454], current implementations do not consider the port as a
defining component of the origin. I.e. existing implementations
differ with [RFC6454] in that origins with the same protocol but
different port values are considered equivalent.
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Wildcards or lists to declare multiple domains in one ALLOW-FROM
statement are not permitted (see Section 2.3.2.3).
2.2. Augmented Backus-Naur Form (ABNF)
The RFC 5234 [RFC5234] ABNF of the X-Frame-Options header field value
is the following.
X-Frame-Options = "DENY"
/ "SAMEORIGIN"
/ ( "ALLOW-FROM" RWS SERIALIZED-ORIGIN )
RWS = 1*( SP / HTAB )
; required whitespace
With serialized-origin as defined in [RFC6454] and the definition of
RWS (required whitespace) is the same as in [HTTPbis-P1].
RWS is used when at least one linear whitespace octet is required to
separate field tokens. RWS SHOULD be generated as a single space
(SP). Multiple RWS octets that occur within field-content SHOULD
either be replaced with a single SP or transformed to all SP octets
before interpreting the field value or forwarding the message
downstream.
And SP (space) and HTAB (horizontal tab) are as defined in RFC 5234
[RFC5234], Appendix B.1.
The values are specified as ABNF strings, and therefore are case-
insensitive.
2.2.1. Examples of X-Frame-Options
X-FRAME-OPTIONS: DENY
X-FRAME-OPTIONS: SAMEORIGIN
X-FRAME-OPTIONS: ALLOW-FROM https://example.com/
2.3. Design Issues
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2.3.1. Enable HTML content from other domains
There are a number of main direct vectors that enable HTML content
from other domains and browser implementations of X-Frame-Options
cover all of them:
o IFRAME tag
o Frame tag
o The Object tag (requires a redirect)
o Applet tag
o Embed tag
Besides these, other ways to host HTML content can be possible. For
example some plugins may host HTML views directly. If these plugins
appear essentially as frames (as opposed to top-level windows), the
plugins must conform to the X-FRAME-OPTIONS policy as specified in
this document as well.
2.3.2. Browser Behaviour and Processing
To allow secure implementations, browsers must behave in a consistent
and reliable way.
If an X-Frame-Options HTTP header field prohibits framing, the user-
agent of the browser MAY immediately abort downloading or parsing of
the document.
2.3.2.1. Violation of X-Frame-Options
When a browser discovers that loaded content with the X-FRAME-OPTIONS
header field would be displayed in a frame against the specified
orders of the header, the browser SHOULD redirect as soon as possible
to a "No-Frame" page. For example this can be a noframe.html page
that also states the full URL and hostname of the protected page.
The NoFrame page could provide the user with an option to open the
target URL in a new window.
Implementations of this vary, some browsers will show a message that
allows the user to safely open the target page in a new window.
Other implementations will simply render an empty frame.
2.3.2.2. Variation in current browser behaviour
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There are currently variations in the implementation of the X-FRAME-
OPTIONS header. For example not all browsers support the "ALLOW-
FROM" option. "ALLOW-FROM" was initially an Internet Explorer
extension and at the time of writing has not been uniformly
implemented by other user agents.
Furthermore the criteria for the SAMEORIGIN (and ALLOW-FROM)
directive may not be evaluated unanimously either: The known
implementations in Appendix A evaluate the SAMEORIGIN directive based
on the origin of the framed page and the top-level browsing-context,
while other implementations might evaluate based on the framed page
and the framing page, or the whole chain of nested frames inbetween.
To illustrate the difference between the comparison with "framing
page" and the "top-level browsing-context" consider the following
scenario: Web pages may embed frames with other pages which in turn
embed frames with other pages as well and so on. In theory this can
result in an infinite nesting of framed pages. For example web page
A may contain in a frame web page B, and web page B contains in a
frame web page C.
Web page A
<html>
....
<frame src="https://URI_of_web_page_B" />
</html>
Web Page B
<html>
....
<frame src="https://URI_of_web_page_C" />
</html>
And so forth...
In this example, for the nested frames with the inner framed web page
C, the most outer web page A would be the "top-level browsing-
context" and web page B would be the "framing page"
These potential variations in the evaluation of the header by
different implementations impair the useage and reliability of this
http header and have security implications as described in section 5.
A revised version of x-frame-options in the form of a frame-options
directive in the CSP 1.1[CSP-1-1] will unify the behaviour and it is
expected that newer implementations will use it rather than the
mechanisms documented here.
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2.3.2.3. Usage design pattern and example scenario for the ALLOW-FROM
parameter
As the "ALLOW-FROM" field only supports one serialized-origin, in
cases when the server wishes to allow more than one resource to frame
its content, the following design pattern can fulfil that need:
1. A page that wants to render the requested content in a frame
supplies its own origin information to the server providing the
to-be-framed content via a querystring parameter.
2. The Server verifies the hostname meets its criteria so that the
page can be allowed to be framed by the target resource. This
may for example happen via a look-up of a white-list of trusted
domain names that are allowed to frame the page. For example,
for a Facebook "Like" button, the server can check to see that
the supplied hostname matches the hostname(s) expected for that
"Like" button.
3. The server returns the hostname in X-FRAME-OPTIONS: ALLOW-FROM if
the proper criteria was met in step #2.
4. The browser enforces the X-FRAME-OPTIONS: ALLOW-FROM header.
2.3.2.4. No caching of the X-Frame-Options header
It is not recommended to cache the X-Frame-Options header for a
resource. Caching the X-Frame-Options response could result in
problems because:
1. The browser has to check for every http-request of the resource
whether the X-Frame-Options header has been set and then act
accordingly, as a resource itself might be created dynamically
and the header could change with it, too.
2. And also, as outlined in section 2.3.2.3., servers may generate X
-Frame-Options header responses depending on the request.
Example case: Considering that we have only one serialized-origin
in the ALLOW-FROM directive, imagine a user has multiple pages
open in his browser tabs with one of web page 1 from domain A and
the second of web page 2 from domain B, both frame the same page
from domain C with the ALLOW-FROM directive. In that case the
page needs to reply to both requests with different X-Frame-
Options headers, the first pointing to origin A, the second to
origin B.
However, we found that none of the major browsers listed in
Appendix A cache the responses.
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3. Acknowledgements
This document was derived from input from specifications published by
various browser vendors such as Microsoft (Eric Lawrence, David
Ross), Mozilla, Google, Opera and Apple.
4. IANA Considerations
This memo is a request to IANA to include the specified HTTP header
in the registry as outlined in Registration Procedures for Message
Header Fields [RFC3864]
4.1. Registration Template
PERMANENT MESSAGE HEADER FIELD REGISTRATION TEMPLATE:
Header field name: X-Frame-Options
Applicable protocol: http [RFC2616]
Status: informational
Author/Change controller: IETF
Specification document(s): draft-ietf-websec-x-frame-options
Related information:
Figure 1
5. Security Considerations
The introduction of the X-FRAME-OPTIONS http header field does
improve the protection against Clickjacking. However, it is not
self-sufficient on its own to protect against all kinds of these
attack vectors. It must be used in conjunction with other security
measures like secure coding (e.g. input validation, output encoding,
...) and the Content Security Policy [CSP].
It is important to note that current implementations do not check the
origins of the entire ancestor tree of frames of the framing
resources, and this may expose the resource to attack in multiple-
nested scenarios.
The browser implementations evaluate based on the origin of the
framed page and the top-level browsing-context (i.e. most outer
frame):
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If a resource from origin A embeds untrusted content from origin B,
that untrusted content can embed another resource from origin A with
an X-Frame-Options: SAMEORIGIN policy and that check would pass when
the user agent only verifies the top-level browsing context.
Therefore web developers should be aware that embedding content from
other sites can leave their web pages vulnerable to clickjacking even
if the X-Frame-Options header is used.
Furthermore, X-Frame-Options must be sent as an HTTP header field and
is explicitly ignored by user agents when declared with a meta http-
equiv tag.
5.1. Privacy Considerations
There are two kinds of potential data leakage to consider:
1. Using X-FRAME-OPTIONS with the parameter ALLOW-FROM allows a page
to guess or infer information about who is framing it. A web
server may answer requests with the X-FRAME-OPTIONS ALLOW-FROM
header and by thus determine which other page is framing it.
This is inherent by design, but may lead to data leakage or data
protection concerns.
2. The web server using the ALLOW-FROM directive may disclose to
other parties who request the page in the header by which page it
is allowed to be framed. If a web server wishes to reduce this
leakage, it is recommended to generate the ALLOW-FROM header for
each request based on the design pattern as described in section
2.3.2.3.
6. References
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC
3986, January 2005.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454, December
2011.
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6.2. Informative References
[CLICK-DEFENSE-BLOG]
Microsoft, "Clickjacking Defense", 2009, <http://
blogs.msdn.com/b/ie/archive/2009/01/27/ie8-security-part-
vii-clickjacking-defenses.aspx>.
[CSP-1-1] Barth, A. and M. West, "Content Security Policy 1.1", W3C
Working Draft WD-CSP11-20130604, June 2013,
<http://www.w3.org/TR/2013/WD-CSP11-20130604/>.
Latest version available at
[CSP] Sterne, B. and A. Barth, "Content Security Policy 1.0",
W3C Candidate Recommendation CR-CSP-20121115, November
2012, <http://www.w3.org/TR/2012/CR-CSP-20121115/>.
Latest version available at
[CSRF] OWASP (Open Web Application Security Project), "OWASP
Top-10: Cross-Site Request Forgery (CSRF)", 2010, <https:/
/www.owasp.org/index.php/Top_10_2013-A8-Cross-
Site_Request_Forgery_%28CSRF%29>.
[Clickjacking]
OWASP (Open Web Application Security Project),
"Clickjacking", 2010,
<http://www.owasp.org/index.php/Clickjacking>.
[FRAME-BUSTING]
Stanford Web Security Research, "Busting frame busting: a
study of clickjacking vulnerabilities at popular sites",
2010, <http://seclab.stanford.edu/websec/framebusting/>.
[HTTPbis-P1]
IETF, "Hypertext Transfer Protocol (HTTP/1.1): Message
Syntax and Routing", 2013, <http://tools.ietf.org/html/
draft-ietf-httpbis-p1-messaging-23>.
[Microsoft-X-Frame-Options]
Microsoft, "Combating ClickJacking With X-Frame-Options",
2010, <http://blogs.msdn.com/b/ieinternals/archive/2010/03
/30/combating-clickjacking-with-x-frame-options.aspx>.
[Mozilla-X-Frame-Options]
Mozilla, "The X-Frame-Options response header", 2010,
<https://developer.mozilla.org/en-US/docs/The_X-FRAME-
OPTIONS_response_header>.
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[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.
[RFC6648] Saint-Andre, P., Crocker, D., and M. Nottingham,
"Deprecating the "X-" Prefix and Similar Constructs in
Application Protocols", BCP 178, RFC 6648, June 2012.
Appendix A. Browsers that support X-Frame-Options
o Internet Explorer 8+
o Firefox 3.6.9+
o Opera 10.5+
o Safari 4+
o Chrome 4.1+
Appendix B. Description of a Clickjacking attack
More detailed explanation of Clickjacking scenarios
B.1. Shop
An Internet Marketplace/Shop offering a feature with a link/button to
"Buy this" Gadget
The marketplace wants their affiliates (who could be malicious
attackers) to be able to stick the "Buy such-and-such from XYZ"
IFRAMES into their pages. There is a possible Clickjacking threat
here, which is why the marketplace/onlineshop needs to then
immediately navigate the main browsing context (or a new window) to a
confirmation page which is protected by anti-Clickjacking
protections.
B.2. Online Shop Confirm Purchase Page
The "Confirm Purchase" page of an online shop must be shown to the
end user without the risk of an overlay or misuse by an attacker.
For that reason, the confirmation page uses a combination of anti-
CSRF (Cross Site Request Forgery, [CSRF]) tokens and the X-FRAME-
OPTIONS HTTP header field, mitigating ClickJacking attacks.
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B.3. Flash Configuration
Macromedia Flash configuration settings are set by a Flash object
which can run only from a specific configuration page on Macromedia's
site. The object runs inside the page and thus can be subject to a
ClickJacking attack. In order to prevent ClickJacking attacks
against the security settings, the configuration page uses the X
-FRAME-OPTIONS directive.
Authors' Addresses
David Ross
Microsoft
U.S.
Tobias Gondrom
Thames Stanley
Kruegerstr. 5A
Unterschleissheim
Germany
Phone: +44 7521003005
Email: tobias.gondrom@gondrom.org
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