WEBSEC D. Ross
Internet-Draft Microsoft
Intended status: Informational T. Gondrom
Expires: December 24, 2013 Thames Stanley
June 22, 2013
HTTP Header Field X-Frame-Options
draft-ietf-websec-x-frame-options-03
Abstract
To improve the protection of web applications against Clickjacking,
this specification 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.
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This Internet-Draft will expire on December 24, 2013.
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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) . . . . . . . . . . . 4
2.2.1. Examples of X-Frame-Options . . . . . . . . . . . . . 5
2.3. Design Issues . . . . . . . . . . . . . . . . . . . . . . 5
2.3.1. Enable HTML content from other domains . . . . . . . 5
2.3.2. Browser Behaviour and Processing . . . . . . . . . . 5
2.3.2.1. Violation of X-Frame-Options . . . . . . . . . . 5
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 . . . . . . . . . . . . . . 6
3. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
4.1. Registration Template . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
5.1. Privacy Considreations . . . . . . . . . . . . . . . . . 8
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
6.1. Normative References . . . . . . . . . . . . . . . . . . 8
6.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. Browsers that support X-Frame-Options . . . . . . . 9
Appendix B. Description of a Clickjacking attack . . . . . . . . 10
B.1. Shop . . . . . . . . . . . . . . . . . . . . . . . . . . 10
B.2. Online Shop Confirm Purchase Page . . . . . . . . . . . . 10
B.3. Flash Configuration . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
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 [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, possibly belonging
to another domain and thereby triggering actions on the second server
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B without the knowledge nor intention of the user and potentially
using an existing session context and login in that step.
This specification provides informational documentation about the
current use and definition of the X-Frame-Options HTTP header field.
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 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 enforced by conforming 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 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
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.
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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 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 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 compare 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
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.
If the ALLOW-FROM value is used, it MUST be followed by a valid URI.
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 or that the referring page's origin is
identical with the ALLOW-FROM URI. Though in conflict with
[RFC6454], current implementations do not consider the port as a
defining component of the origin.
Wildcards or lists to declare multiple domains in one ALLOW-FROM
statement are not permitted.
2.2. Augmented Backus-Naur Form (ABNF)
The RFC 5234 [RFC5234] ABNF of the X-Frame-Options header is:
X-Frame-Options = "DENY"
/ "SAMEORIGIN"
/ ( "ALLOW-FROM" RWS URI )
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With URI as defined in [RFC3986] and RWS and OWS as defined in
[HTTPbis-P1]. 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
2.3.1. Enable HTML content from other domains
There are a number of main direct vectors that enable HTML content
from other domains:
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
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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.
"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.
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
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 IE (Internet Explorer)
extension and at the time of writing has not been uniformly
implemented by other user agents.
And the criteria for the SAMEORIGIN option is not evaluated
unanimously either: 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 x-frame-options in the form of a frame-
options directive in the CSP 1.1[CSP-1-1] shall unify the behaviour
and replace this document in the future.
2.3.2.3. Usage design pattern and example scenario for the ALLOW-FROM
parameter
As the "ALLOW-FROM" field does support only one URI, in cases when
the server wishes to allow more than one resource to frame its
content, the following design pattern is recommended:
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.
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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 return 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.
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 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-Option
Applicable protocol: http [RFC2616]
Status: Standard
Author/Change controller: IETF
Specification document(s): draft-ietf-websec-x-frame-options
Related information:
Figure 1
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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, but must be used in conjunction with
other security measures like secure coding 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 multiply-
nested scenarios. For example, if a resource on 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 if the user agent only verifies the
top-level browsing context.
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 Considreations
The parameter ALLOW-FROM allows a page to guess who is framing it.
This is inherent by design, but may lead to data leakage or data
protection concerns.
6. References
6.1. Normative References
[HTTPbis-P1]
IETF, "Hypertext Transfer Protocol (HTTP/1.1): Message
Syntax and Routing", 2013, <http://tools.ietf.org/html/
draft-ietf-httpbis-p1-messaging-22>.
[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] W3C, "Content Security Policy 1.1", December 2012,
<http://www.w3.org/TR/CSP11/>.
[CSP] W3C, "Content Security Policy 1.0", November 2012,
<http://www.w3.org/TR/CSP/>.
[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/>.
[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>.
[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+
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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 tokens and the X-FRAME-OPTIONS HTTP header field, mitigating
ClickJacking attacks.
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.
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Tobias Gondrom
Thames Stanley
Kruegerstr. 5A
Unterschleissheim
Germany
Phone: +44 7521003005
Email: tobias.gondrom@gondrom.org
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