WEBSEC D. Ross
Internet-Draft Microsoft
Intended status: Standards Track T. Gondrom
Expires: September 6, 2012 March 5, 2012
HTTP Header Frame Options
draft-gondrom-frame-options-02
Abstract
To improve the protection of web applications against Cross Site
Request Forgery (CSRF) and Clickjacking this standards defines a http
response header that declares a policy communicated from a host to
the client browser whether the transmitted content MUST NOT be
displayed in frames of other pages from different origins or a list
of trusted origins which are allowed to frame the content.
Status of this Memo
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This Internet-Draft will expire on September 6, 2012.
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . . 3
2. Frame-Options Header . . . . . . . . . . . . . . . . . . . . . 3
2.1. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Backus-Naur Form (BNF) . . . . . . . . . . . . . . . . . . 5
2.3. Design Issues . . . . . . . . . . . . . . . . . . . . . . . 5
2.3.1. Enable HTML content from other domains . . . . . . . . 5
2.3.2. Browser Behaviour and Processing . . . . . . . . . . . 5
2.4. Examples of 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
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 introduced the use of a non-
standard http header RFC 2616 [RFC2616] "X-Frame-Options" to protect
against Clickjacking [Clickjacking] and Cross Site Request Forgery
(CSRF) [CSRF]. This standard is to replace the non-standard header.
In some forms of Clickjacking and CSRF an attacker tricks a user into
clicking on a button or link to another page and by thus executing an
unintended command in the context of a different web application.
For example with Clickjacking the attacker might use transparent or
opaque layers to integrate and obscure a button to another page so
that the user may click on it in the expectation of a different
action. So, in this way the attacker is "hijacking" the "Click" on a
button meant by the user to be sent to host A, while clicking the
button in effect sends a message to host B. If the user does for
example also have an open session with host B this can lead to a CSRF
attack and executing a command in the session context of the user
(using the user's authentication and authorization) on host B without
his intention or knowledge.
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 by "Frame-Options" provided defense mechanism against
Clickjacking is to allow 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 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. Frame-Options Header
The 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
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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:
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.
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".
[TBD]current implementations do not display if the origin of
the top-level-browsing-context is different than the origin of
the page containing the FRAME-OPTIONS header.
ALLOW-FROM (followed by a list of URIs of trusted origins)
A browser receiving content with this header MUST NOT display
this content in any frame from a page of different origin than
any of the listed origins. This allows deployment with multi-
domain sites, as the webmaster can define a whitelist of
origins that are allowed to frame the page. While this can
expose the page to risks by the trusted origins, in some cases
it may be necessary to use content from other domains or more
than one origin (hostname).
for example: FRAME-OPTIONS: ALLOW-FROM https://www.domain.com/
In the case of SAMEORIGIN and ALLOW-FROM, there is also an optional
flag "AllAncestors". If this flag is set, it means that browsers
MUST validate the URL of each hosting frame up to the top level and
only allow the framing if all ancestor frames' origins are either the
same as in SAMEORIGIN or included in the ALLOW-FROM list.
The URIs listed for ALLOW-FROM must be valid.
Any data beyond the domain address (i.e. any data after the "/"
separator) is to be ignored and to verify a referring page is of the
same origin as the content or that the referring page is listed in
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the ALLOW-FROM list of URI, the algorithm to compare origins from
[ORIGIN] should be used.
Wildcards to declare multiple domains in one statement are not
permitted.
[TBD] Current Implementations do not consider the port a component of
the origin - conflicting with [ORIGIN].
2.2. Backus-Naur Form (BNF)
The RFC 822 [RFC0822] EBNF of the Frame-Options header is:
Frame-Options = "Frame-Options" ":" "DENY"/ "SAMEORIGIN" /
("ALLOW-FROM" ":" Origin-List) : flags
Origin-List = 1*URI
flags = token [ "=" ( token | quoted-string ) ]
[TBD] with URI as defined in the websec-origin draft
[TBD] Or should we use the ABNF (RFC 2234) alternatively 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. To allow a
conform security configuration those plugins MUST be conform to the
FRAME-OPTIONS directive as specified in this draft as well.
2.3.2. Browser Behaviour and Processing
To allow secure implementations browser implementations MUST behave
in a consistent and reliable way conform to thsi specition.
If a HTTP Header prohibits framing, the user-agent of the browser MAY
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immediately abort downloading or parsing of the document.
When a browser discovers loaded content with the 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 FRAME-OPTIONS header an
accroding 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.
[TBD] The NoFrame page MAY provide the user with an option to open
the target URL in a new window.
2.4. Examples of 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: Frame-Option
Applicable protocol: http [RFC2616]
Status: Standard
Author/Change controller: IETF
Specification document(s): draft-gondrom-frame-options
Related information:
Figure 1
5. Security Considerations
The introduction of the http header 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 possibilities 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>.
[CSRF] OWASP (Open Web Application Security Project), "OWASP
Top-10: Cross-Site Request Forgery (CSRF)", 2010,
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<http://www.owasp.org/index.php/Top_10_2010-A5>.
[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/>.
[ORIGIN] IETF, "The Web Origin Concept", December 2010,
<http://tools.ietf.org/id/
draft-ietf-websec-origin-00.txt>.
[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.
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454,
December 2011.
Appendix A. Description of a Clickjacking attack
More detailed explanation of Clickjacking scenarios
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 CSRF-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-CSRF/anti-CJ protections.
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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 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 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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