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Versions: (draft-gondrom-x-frame-options) 00 01 02 03 04 05 06 07 08 09 10 11 12 RFC 7034

WEBSEC                                                           D. Ross
Internet-Draft                                                 Microsoft
Intended status: Informational                                T. Gondrom
Expires: February 12, 2014                                Thames Stanley
                                                         August 11, 2013


                   HTTP Header Field X-Frame-Options
                  draft-ietf-websec-x-frame-options-08

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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on February 12, 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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   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)  . . . . . . . . . . .   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  . . . . . . . . . .   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  . . . . . . . . . . . . . .   7
   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  . . . . . . . . . . . . . . . . . .   9
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Appendix A.  Browsers that support X-Frame-Options  . . . . . . .  10
   Appendix B.  Description of a Clickjacking attack . . . . . . . .  10
     B.1.  Shop  . . . . . . . . . . . . . . . . . . . . . . . . . .  11
     B.2.  Online Shop Confirm Purchase Page . . . . . . . . . . . .  11
     B.3.  Flash Configuration . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

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
   the user (who sees the overlaying user interface content from page A)



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   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.
   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).  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.

   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 (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.





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         X-Frame-Options = "DENY"
                   / "SAMEORIGIN"
                   / ( "ALLOW-FROM" RWS URI )

         RWS             = 1*( SP / HTAB )
                       ; required whitespace


   With URI as defined in [RFC3986] 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

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



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   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

   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.

   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



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   header.  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"

2.3.2.3.  Usage design pattern and example scenario for the ALLOW-FROM
          parameter

   As the "ALLOW-FROM" field only supports one URI, 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.

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]



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   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.  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.  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 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









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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.

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/>.



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   [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>.

   [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









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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.

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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