WEBSEC D. Ross
Internet-Draft Microsoft
Intended status: Informational T. Gondrom
Expires: April 26, 2013 October 23, 2012
HTTP Header X-Frame-Options
draft-ietf-websec-x-frame-options-01
Abstract
To improve the protection of web applications against Clickjacking
this standard defines an http response header that declares a policy
communicated from a host to the client browser on whether the browser
must not display the transmitted content in frames of other web
pages. This drafts serves to document the existing use and
specification of X-Frame-Options.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on April 26, 2013.
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . . 3
2. X-Frame-Options Header . . . . . . . . . . . . . . . . . . . . 3
2.1. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2. Backus-Naur Form (BNF) . . . . . . . . . . . . . . . . . . 4
2.3. Design Issues . . . . . . . . . . . . . . . . . . . . . . . 5
2.3.1. Enable HTML content from other domains . . . . . . . . 5
2.3.2. Browser Behaviour and Processing . . . . . . . . . . . 5
2.4. Examples of X-Frame-Options Headers . . . . . . . . . . . . 6
2.4.1. Example scenario for the ALLOW-FROM parameter . . . . . 6
3. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 6
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Registration Template . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
6.1. Normative References . . . . . . . . . . . . . . . . . . . 7
6.2. Informative References . . . . . . . . . . . . . . . . . . 7
Appendix A. Description of a Clickjacking attack . . . . . . . . . 8
A.1. Shop . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
A.2. Confirm Purchase Page . . . . . . . . . . . . . . . . . . . 9
A.3. Flash Configuration . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
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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 header RFC 2616 [RFC2616] "X-Frame-Options" to
protect against Clickjacking [Clickjacking]. This draft is to
document the current use of X-Frame-Options header and shall in the
future be replaced by the Frame-Options [FRAME-OPTIONS] standard.
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.
The "X-Frame-Options" allows a secure 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). In principle this is done by a policy declared in the
HTTP header and obeyed by conform browser implementations.
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 indicates a policy whether a
browser MUST NOT allow to render a page in a <frame> or <iframe> .
Hosts can declare this policy in the header of their HTTP responses
to prevent clickjacking attacks, by ensuring that their content is
not embedded into other pages or frames.
2.1. Syntax
The header field name is:
X-Frame-Options
There are three different values for the header field. These values
are exclusive, that is NOT more than one of the three values MUST be
set.
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DENY
A browser receiving content with this header MUST NOT display
this content in any frame.
SAMEORIGIN
A browser receiving content with this header 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 may vary on the
interpretation of this criteria: In some it only allows to be
framed if the origin of the top-level browsing-context is
identical, in other it compares with to the origin of the
framing page.)
ALLOW-FROM (followed by a URI [RFC3986] of a trusted origin)
A browser receiving content with this header MUST NOT display
this content in a frame from any page with a top-level browsing
contex 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.
For example: X-FRAME-OPTIONS: ALLOW-FROM:
https://www.domain.com/
The ALLOW-FROM URI MUST be valid.
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 a referring page is of the
same origin as the content or that the referring page's origin is
identical with the ALLOW-FROM URI.
Wildcards or lists to declare multiple domains in one ALLOW-FROM
statement are not permitted.
Please note that in conflict with [RFC6454], current implementations
do not consider the port as a defining component of the origin.
2.2. Backus-Naur Form (BNF)
The RFC 822 [RFC0822] EBNF of the X-Frame-Options header is:
X-Frame-Options = "Frame-Options" ":" "DENY"/ "SAMEORIGIN" /
("ALLOW-FROM" ":" URI)
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With URI as defined in [RFC3986]
[TBD] Or should we use the ABNF (RFC 2234) alternatively to EBNF or
in addition?
2.3. Design Issues
2.3.1. Enable HTML content from other domains
There are three main direct vectors that enable HTML content from
other domains:
o IFRAME Tag
o Frame tag
o The Object tag (requires a redirect)
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 directive as specified in
this draft as well.
2.3.2. Browser Behaviour and Processing
To allow secure implementations, browsers MUST behave in a consistent
and reliable way.
If an HTTP Header prohibits framing, the user-agent of the browser
MAY immediately abort downloading or parsing of the document.
When a browser discovers loaded content with the X-FRAME-OPTIONS
header would be displayed in a frame against the specified origin
orders of the header, the browser SHOULD redirect as soon as possible
to a "No-Frame" page.
"No-Frame" Page
If the display of content is denied by the X-FRAME-OPTIONS header an
error page SHOULD be displayed. For example this can be a
noframe.html page also stating the full URL of the protected page and
the hostname of the protected page.
The NoFrame page MAY provide the user with an option to open the
target URL in a new window.
Variation in current browser behaviour
There are currently variations in the implementation of the X-FRAME-
OPTIONS header. For example not all browsers may support the "ALLOW-
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FROM" option.
And the criteria for SAMEORIGIN option is not evaluated unanimously:
one implementation may evaluate the SAMEORIGIN option based on the
origin of the framed page and the framing page, while another may
evaluate based on the framed page and the top-level browsing-context
These variations in the evaluation of the header by different
implementations impair the useage and reliability of this http
header. A revised version of frame-options [FRAME-OPTIONS] shall
unify the behaviour and replace this document in the future.
2.4. Examples of X-Frame-Options Headers
2.4.1. Example scenario for the ALLOW-FROM parameter
1. Inner IFRAME suggests via a querystring parameter what site it
wants to be hosted by. This can obviously be specified by an
attacker, but that's OK.
2. Server verifies the hostname meets whatever criteria. For
example, for a Facebook "Like" button, the server can check to
see that the supplied hostname matches the hostname expected for
that Like button.
3. Server serves up the hostname in X-FRAME-OPTIONS: ALLOW-FROM if
the proper criteria was met in step #2.
4. Browser enforces the X-FRAME-OPTIONS: ALLOW-FROM domain.com
header.
3. Acknowledgements
This document was derived from input from specifications published by
various browser vendors like Microsoft (Eric Lawrence, David Ross),
Mozilla, Google, Opera and Apple.
4. IANA Considerations
This memo a request to IANA to include the specified HTTP header in
registry as outlined in Registration Procedures for Message Header
Fields [RFC3864]
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4.1. Registration Template
PERMANENT MESSAGE HEADER FIELD REGISTRATION TEMPLATE:
Header field name: X-Frame-Option
Applicable protocol: http [RFC2616]
Status: Standard
Author/Change controller: IETF
Specification document(s): draft-ietf-websec-x-frame-options
Related information:
Figure 1
5. Security Considerations
The introduction of the http header X-FRAME-OPTIONS does improve the
protection against Clickjacking, however it is not self-sufficient on
its own but MUST be used in conjunction with other security measures
like secure coding and Content Security Policy (CSP)
The parameter ALLOW-FROM allows a page to guess who is framing it.
This is by design, but may lead to data leakage or data protection
concerns.
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.
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>.
[Clickjacking]
OWASP (Open Web Application Security Project),
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"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/>.
[FRAME-OPTIONS]
IETF, "The Web Origin Concept", July 2012, <http://
tools.ietf.org/id/draft-ietf-websec-frame-options-00.txt>.
[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, <htt
ps://developer.mozilla.org/en-US/docs/
The_X-FRAME-OPTIONS_response_header>.
[RFC0822] Crocker, D., "Standard for the format of ARPA Internet
text messages", STD 11, RFC 822, August 1982.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
September 2004.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454,
December 2011.
Appendix A. Description of a Clickjacking attack
More detailed explanation of Clickjacking scenarios
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A.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 bad guys) to be
able to stick the "Buy such-and-such from XYZ" IFRAMES into their
pages. There is a ClickJack possibility 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-CJ protections.
A.2. Confirm Purchase Page
Onlineshop "Confirm purchase" anti-CSRF page
The Confirm Purchase page must be shown to the end user without
possibility of overlay or misuse by an attacker. For that reason,
the confirmation page uses anti-CSRF tokens and contains the X-FRAME-
OPTIONS directive, mitigating ClickJack attacks.
A.3. Flash Configuration
Macromedia Flash configuration page
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.
Phone:
Email:
Tobias Gondrom
Kruegerstr. 5A
Unterschleissheim,
Germany
Phone: +44 7521003005
Email: tobias.gondrom@gondrom.org
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