WEBSEC                                                           D. Ross
Internet-Draft                                                 Microsoft
Intended status: Informational                                T. Gondrom
Expires: April 26, August 29, 2013                                  Thames Stanley
                                                       February 25, 2013                                 October 23, 2012

                   HTTP Header Field X-Frame-Options


   To improve the protection of web applications against Clickjacking Clickjacking,
   this standard defines an http specification describes the X-Frame-Options HTTP response header
   field that declares a policy communicated from a host the server to the
   client browser on whether the browser
   must not may display the transmitted
   content in frames that are part of other web pages.  This drafts
   informational document serves to document the existing use and
   specification of X-Frame-Options. 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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   This Internet-Draft will expire on April 26, August 29, 2013.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Requirements Language  . . . . . . . . . . . . . . . . . . .  3
   2.  X-Frame-Options Header . . . . . . . . . . . . . . . . . . . .  3
     2.1.  Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . 3  4
     2.2.  Augmented Backus-Naur Form (BNF) (ABNF)  . . . . . . . . . . . .  5
       2.2.1.  Examples of X-Frame-Options  . . . . . . 4 . . . . . . .  5
       2.2.2.  Variations of the ALLOW-FROM field . . . . . . . . . .  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  6  Violation of X-Frame-Options Headers . . . . . . . . . . .  6  Variation in current browser behaviour . . . . . .  6
       2.4.1.  Example  Usage design pattern and example scenario for
                   the ALLOW-FROM parameter . . . . . 6 . . . . . . . .  7
   3.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 6  7
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . . 6  8
     4.1.  Registration Template  . . . . . . . . . . . . . . . . . . . 7  8
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
     5.1.  Privacy Considreations . . 7 . . . . . . . . . . . . . . . .  9
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 7  9
     6.1.  Normative References . . . . . . . . . . . . . . . . . . . 7  9
     6.2.  Informative References . . . . . . . . . . . . . . . . . . 7  9
   Appendix A.  Browsers that support X-Frame-Options . . . . . . . . 10
   Appendix B.  Description of a Clickjacking attack  . . . . . . . . . 8
     A.1. 10
     B.1.  Shop . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
     A.2. 10
     B.2.  Online Shop Confirm Purchase Page  . . . . . . . . . . . . . . . . . . . 9
     A.3. 11
     B.3.  Flash Configuration  . . . . . . . . . . . . . . . . . . . . 9 11
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9 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 header RFC 2616 HTTP [RFC2616] header field "X-Frame-Options" to
   protect against Clickjacking Clickjacking [Clickjacking].  This draft  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
   document 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
   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 and shall 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 [FRAME-OPTIONS] standard. directive in the Content Security Policy Version 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 obeyed enforced by conform conforming browser implementations.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   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 a the browser MUST NOT allow to should render a page in the transmitted resource within a
   <frame> or <iframe> .
   Hosts <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

2.1.  Syntax

   The header field name is:

   There are three different values for the header field.  These values
   are mutually exclusive, that is NOT more than exactly one of the three values MUST
   be set.

         A browser receiving content with this header MUST NOT display
         this content in any frame.

         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 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 to the origin of the content using the X-FRAME-
         OPTIONS directive, in other others it compares with may compare to the origin of
         the framing page.) 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.
         For example: X-FRAME-OPTIONS: ALLOW-FROM:


   If the ALLOW-FROM URI value is used, it MUST be valid. 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.

   Wildcards or lists to declare multiple domains in one ALLOW-FROM
   statement are not permitted.

   Please note that  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 (BNF) (ABNF)

   The RFC 822 [RFC0822] EBNF 2234 [RFC2234] ABNF of the X-Frame-Options header is:

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

   With URI as defined in [RFC3986]
   [TBD] Or 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: ALLOW-FROM https://example.com/

2.2.2.  Variations of the ALLOW-FROM field

   Regarding the syntax, it should we use be noted that existing
   implementations have variations in whether a colon (":") should be
   between "ALLOW-FROM" and the ABNF (RFC 2234) alternatively to EBNF or URI.  E.g. in addition? IE8+ the colon ":" is not
   needed, while Firefox and Chrome implementations at the time of
   writing of this document support both forms.

   Alternative ABNF of the X-Frame-Options header:

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

2.3.  Design Issues

2.3.1.  Enable HTML content from other domains

   There are three a number of main direct vectors that enable HTML content
   from other domains:

   o  IFRAME Tag 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 directive policy as specified in
   this draft document as well.

2.3.2.  Browser Behaviour and Processing

   To allow secure implementations, browsers MUST must behave in a consistent
   and reliable way.

   If an X-Frame-Options HTTP Header header field prohibits framing, the user-agent user-
   agent of the browser MAY immediately abort downloading or parsing of
   the document.  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 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.

   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.  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-
   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:
   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 frame-options [FRAME-OPTIONS] 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.

2.4.  Examples of X-Frame-Options Headers

2.4.1.  Example  Usage design pattern and example scenario for the ALLOW-FROM

   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

   As the hostname meets whatever criteria.  For
       example, for "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.

   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 hostname(s) expected for that Like
       "Like" button.

   3.  Server serves up  The server return the hostname in X-FRAME-OPTIONS: ALLOW-FROM if
       the proper criteria was met in step #2.

   4.  Browser  The 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 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


   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 X-FRAME-OPTIONS http header X-FRAME-OPTIONS field does
   improve the protection against Clickjacking, however Clickjacking.  However, it is not
   self-sufficient on its own own, but MUST must be used in conjunction with
   other security measures like secure coding and the Content Security
   Policy (CSP) [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

              IETF, "Hypertext Transfer Protocol (HTTP/1.1): Message
              Syntax and Routing", 2013, <http://tools.ietf.org/html/

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 2234, November 1997.

   [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.

6.2.  Informative References

              Microsoft, "Clickjacking Defense", 2009, <http://

   [CSP]      W3C, "Content Security Policy 1.0", November 2012,

   [CSP-1-1]  W3C, "Content Security Policy 1.1", December 2012,

              OWASP (Open Web Application Security Project),
              "Clickjacking", 2010,

              Stanford Web Security Research, "Busting frame busting: a
              study of clickjacking vulnerabilities at popular sites",
              2010, <http://seclab.stanford.edu/websec/framebusting/>.

              IETF, "The Web Origin Concept", July 2012, <http://

              Microsoft, "Combating ClickJacking With X-Frame-Options",
              2010, <http://blogs.msdn.com/b/ieinternals/archive/2010/

              Mozilla, "The X-Frame-Options response header", 2010, <htt

   [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.,

   [RFC6648]  Saint-Andre, P., Crocker, D., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

   [RFC6454]  Barth, A., "The Web Origin Concept", M. Nottingham,
              "Deprecating the "X-" Prefix and Similar Constructs in
              Application Protocols", BCP 178, RFC 6454,
              December 2011. 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 bad guys) malicious
   attackers) to be able to stick the "Buy such-and-such from XYZ"
   IFRAMES into their pages.  There is a ClickJack possibility 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-CJ anti-Clickjacking


B.2.  Online Shop Confirm Purchase Page

   Onlineshop "Confirm purchase" anti-CSRF page

   The Confirm Purchase "Confirm Purchase"" page of an online shop must be shown to the
   end user without
   possibility the risk of an overlay or misuse by an attacker.
   For that reason, the confirmation page uses anti-CSRF a combination of anti-
   CSRF tokens and contains the X-FRAME-
   OPTIONS directive, X-FRAME-OPTIONS HTTP header field, mitigating ClickJack
   ClickJacking attacks.


B.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


   Tobias Gondrom
   Thames Stanley
   Kruegerstr. 5A

   Phone: +44 7521003005
   Email: tobias.gondrom@gondrom.org