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rfc:rfc7034

Internet Engineering Task Force (IETF) D. Ross Request for Comments: 7034 Microsoft Category: Informational T. Gondrom ISSN: 2070-1721 Thames Stanley

                                                          October 2013
                 HTTP Header Field X-Frame-Options

Abstract

 To improve the protection of web applications against clickjacking,
 this document describes the X-Frame-Options HTTP header field, which
 declares a policy, communicated from the server to the client
 browser, regarding whether the browser may display the transmitted
 content in frames that are part of other web pages.

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for informational purposes.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Not all documents
 approved by the IESG are a candidate for any level of Internet
 Standard; see Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7034.

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
 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.

Ross & Gondrom Informational [Page 1] RFC 7034 X-Frame-Options October 2013

Table of Contents

 1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   1.1.  Requirements Language  . . . . . . . . . . . . . . . . . .  3
 2.  X-Frame-Options Header . . . . . . . . . . . . . . . . . . . .  4
   2.1.  Syntax . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.2.  Augmented Backus-Naur Form (ABNF)  . . . . . . . . . . . .  5
     2.2.1.  Examples of X-Frame-Options  . . . . . . . . . . . . .  6
   2.3.  Design Issues  . . . . . . . . . . . . . . . . . . . . . .  6
     2.3.1.  Enable HTML Content from Other Domains . . . . . . . .  6
     2.3.2.  Browser Behavior and Processing  . . . . . . . . . . .  6
       2.3.2.1.  Violation of X-Frame-Options . . . . . . . . . . .  6
       2.3.2.2.  Variation in Current Browser Behavior  . . . . . .  7
       2.3.2.3.  Usage Design Pattern and Example Scenario for
                 the ALLOW-FROM Parameter . . . . . . . . . . . . .  8
       2.3.2.4.  No Caching of the X-Frame-Options Header . . . . .  8
 3.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
   3.1.  Registration Template  . . . . . . . . . . . . . . . . . .  9
 4.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   4.1.  Privacy Considerations . . . . . . . . . . . . . . . . . . 10
 5.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
   5.1.  Normative References . . . . . . . . . . . . . . . . . . . 10
   5.2.  Informative References . . . . . . . . . . . . . . . . . . 11
 Appendix A.  Browsers That Support X-Frame-Options . . . . . . . . 13
 Appendix B.  Description of a Clickjacking Attack  . . . . . . . . 13
   B.1.  Shop . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
   B.2.  Online Shop Confirm Purchase Page  . . . . . . . . . . . . 13
   B.3.  Flash Configuration  . . . . . . . . . . . . . . . . . . . 13
 Appendix C.  Acknowledgements  . . . . . . . . . . . . . . . . . . 13

Ross & Gondrom Informational [Page 2] RFC 7034 X-Frame-Options October 2013

1. Introduction

 In 2009 and 2010, many browser vendors ([Microsoft-X-Frame-Options],
 [CLICK-DEFENSE-BLOG], and [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 page A
 and routing them to page B.  The attacker is tricking the user (who
 sees the overlaying user interface content from page A) 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.  In addition, 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.
 As described in Section 2.3.2.2, not all browsers implement
 X-Frame-Options in exactly the same way, which can lead to unintended
 results.  And, given that the "X-" construction is deprecated
 [RFC6648], the X-Frame-Options header field will be replaced in the
 future by the Frame-Options directive in the Content Security Policy
 (CSP) version 1.1 [CSP-1-1].
 A study [FRAME-BUSTING] demonstrated that existing anti-clickjacking
 measures, e.g., frame-breaking JavaScript, have weaknesses that allow
 their protection to be circumvented.
 Short of configuring the browser to disable frames and scripts
 entirely, which massively impairs browser utility, browser users are
 vulnerable to this type of attack.
 The use of "X-Frame-Options" allows a 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].

Ross & Gondrom Informational [Page 3] RFC 7034 X-Frame-Options October 2013

2. X-Frame-Options Header

 The X-Frame-Options HTTP header field indicates a policy that
 specifies whether the browser should render the transmitted resource
 within a <frame> or an <iframe>.  Servers can declare this policy in
 the header of their HTTP responses to prevent clickjacking attacks,
 which ensures 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, the header field MUST be set to
 exactly one of the three values.
 DENY
    A browser receiving content with this header field 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 cannot reliably determine whether or not
    the origin of the content and the frame are the same, 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.  Also see Section 2.3.2.2 for more details on the
    nesting of frames and variations in the handling of this header
    field by different browsers.  In addition, refer to Section 4,
    paragraph 2 for the resulting potential security problems.
 ALLOW-FROM  (followed by a serialized-origin [RFC6454])
    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

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    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.
 The meaning of the term "serialized-origin" is given in [RFC6454].
 If the ALLOW-FROM value is used, it MUST be followed by a valid
 origin [RFC6454] (as a subset of the URI [RFC3986]).
 Any data beyond the domain address (i.e., any data after the "/"
 separator) is to be ignored.  The algorithm to compare origins from
 [RFC6454] SHOULD be used to verify that a referring page is of the
 same origin as the content (in the case of SAMEORIGIN) or that the
 referring page's origin is identical with the ALLOW-FROM serialized-
 origin (in the case of ALLOW-FROM).  Though in conflict with
 [RFC6454], current implementations do not consider the port as a
 defining component of the origin; i.e., existing implementations
 differ with [RFC6454] in that origins with the same protocol but
 different port values are considered equivalent.
 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:
       X-Frame-Options = "DENY"
                 / "SAMEORIGIN"
                 / ( "ALLOW-FROM" RWS SERIALIZED-ORIGIN )
       RWS             = 1*( SP / HTAB )
                     ; required whitespace
 with serialized-origin as defined in [RFC6454] and required
 whitespace (RWS) as defined 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 SP or transformed to all SP octets before
 interpreting the field value or forwarding the message downstream.
 SP and horizontal tab (HTAB) are as defined in Appendix B.1 of RFC
 5234 [RFC5234].
 The values are specified as ABNF strings; therefore, they are case-
 insensitive.

Ross & Gondrom Informational [Page 5] RFC 7034 X-Frame-Options October 2013

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  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 Behavior 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 to a "NOFRAME" page
 as soon as possible.  For example, this can be a noframe.html page
 that also states the full URL and hostname of the protected page.

Ross & Gondrom Informational [Page 6] RFC 7034 X-Frame-Options October 2013

 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,
 whereas other implementations will simply render an empty frame.

2.3.2.2. Variation in Current Browser Behavior

 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.
 Furthermore, the criteria for the SAMEORIGIN (and ALLOW-FROM)
 directive may not be evaluated unanimously either: the known
 implementations in Appendix A evaluate the SAMEORIGIN directive based
 on the origin of the framed page and the top-level browsing context,
 while other implementations might evaluate it based on the framed
 page and the framing page, or the whole chain of nested frames in
 between.
 To illustrate the difference between the comparison of the "framing
 page" and the "top-level browsing context", consider the following
 scenario: web pages may embed frames with other pages that, in turn,
 embed frames with other pages as well, and so on.  In theory, this
 can result in an infinite nesting of framed pages.  For example, web
 page A may contain web page B in a frame, and web page B may contain
 web page C in a frame.
 Web page A
 <html>
 ....
 <frame src="https://URI_of_web_page_B" />
 </html>
 Web page B
 <html>
 ....
 <frame src="https://URI_of_web_page_C" />
 </html>
 and so forth.
 In this example, for the nested frames with the inner-framed web page
 C, the most outer web page A would be the "top-level browsing
 context", and web page B would be the "framing page".

Ross & Gondrom Informational [Page 7] RFC 7034 X-Frame-Options October 2013

 These potential variations in the evaluation of the header by
 different implementations impair the usage and reliability of this
 HTTP header and have security implications as described in Section 4.
 A revised version of X-Frame-Options in the form of a Frame-Options
 directive in CSP 1.1 [CSP-1-1] will unify the behavior, 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 serialized-origin, in
 cases when the server wishes to allow more than one resource to frame
 its content, the following design pattern can fulfill 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
     content to be framed via a query string parameter.
 2.  The server verifies that the hostname meets its criteria, so that
     the page is allowed to be framed by the target resource.  This
     may, for example, happen via a lookup of a whitelist 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.

2.3.2.4. No Caching of the X-Frame-Options Header

 Caching the X-Frame-Options header for a resource is not recommended.
 Caching the X-Frame-Options response could result in problems
 because:
 1.  For every http-request of the resource, the browser has to check
     whether the X-Frame-Options header has been set and then act
     accordingly, as a resource itself might be created dynamically
     and the header could change with it, too.
 2.  Also, as outlined in Section 2.3.2.3, servers may generate
     X-Frame-Options header responses depending on the request.
     Example case: Considering that we have only one serialized-origin
     in the ALLOW-FROM directive, imagine a user has multiple pages
     open in his browser tabs with web page 1 from domain A and web

Ross & Gondrom Informational [Page 8] RFC 7034 X-Frame-Options October 2013

     page 2 from domain B, and both frame the same page from domain C
     with the ALLOW-FROM directive.  In that case, the page needs to
     reply to both requests with different X-Frame-Options headers,
     with the first pointing to origin A and the second pointing to
     origin B.
 However, we found that none of the major browsers listed in
 Appendix A cache the responses.

3. IANA Considerations

 IANA has included the specified HTTP header in the "Permanent Message
 Header Field Name" registry as outlined in "Registration Procedures
 for Message Header Fields" [RFC3864].

3.1. Registration Template

 Permanent Message Header Field Names Template:
 Header field name: X-Frame-Options
 Applicable protocol: http [RFC2616]
 Status: Informational
 Author/change controller: IETF
 Specification document(s): RFC 7034
 Related information: None

4. Security Considerations

 The introduction of the X-Frame-Options HTTP header field improves
 the protection against clickjacking.  However, it is not self-
 sufficient enough 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, etc.)
 and the Content Security Policy version 1.0 [CSP].
 It is important to note that current implementations do not check the
 origins of the framing resources' entire ancestor tree of frames, and
 this may expose the resource to attack in multiple-nested scenarios.
 The browser implementations evaluate based on the origin of the
 framed page and the top-level browsing context (i.e., the most outer
 frame):

Ross & Gondrom Informational [Page 9] RFC 7034 X-Frame-Options October 2013

 If a resource from 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
 when 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.

4.1. Privacy Considerations

 There are two kinds of potential data leakage to consider:
 1.  Using X-Frame-Options with the parameter ALLOW-FROM allows a page
     to guess or infer information about who is framing it.  A web
     server may answer requests with the "X-Frame-Options: ALLOW-FROM"
     header and thus determine which other page is framing it.  This
     is inherent by design, but it may lead to data-leakage or data-
     protection concerns.
 2.  The web server using the ALLOW-FROM directive effectively
     discloses the origin specified in the header.  If a web server
     wishes to reduce this leakage, it is recommended to generate the
     ALLOW-FROM header for each request based on the design pattern as
     described in Section 2.3.2.3.

5. References

5.1. 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.

Ross & Gondrom Informational [Page 10] RFC 7034 X-Frame-Options October 2013

5.2. Informative References

 [CLICK-DEFENSE-BLOG]
            Lawrence, E., "IE8 Security Part VII: Clickjacking
            Defenses", Microsoft Developer Network Blogs,
            January 2009, <http://blogs.msdn.com/b/ie/archive/2009/01/
            27/ie8-security-part-vii-clickjacking-defenses.aspx>.
 [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/>.
 [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/>.
 [CSRF]     OWASP (Open Web Application Security Project), "Top-10
            2013-A8-Cross-Site Request Forgery (CSRF)", June 2013,
            <https://www.owasp.org/index.php/
            Top_10_2013-A8-Cross-Site_Request_Forgery_%28CSRF%29>.
 [Clickjacking]
            OWASP (Open Web Application Security Project),
            "Clickjacking", April 2013,
            <http://www.owasp.org/index.php/Clickjacking>.
 [FRAME-BUSTING]
            Stanford Web Security Research, "Busting frame busting: a
            study of clickjacking vulnerabilities at popular sites",
            July 2010,
            <http://seclab.stanford.edu/websec/framebusting/>.
 [HTTPbis-P1]
            Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
            (HTTP/1.1): Message Syntax and Routing", Work in Progress,
            September 2013.
 [Microsoft-X-Frame-Options]
            Lawrence, E., "Combating ClickJacking With X-Frame-
            Options", Microsoft Developer Network Blogs, March 2010,
            <http://blogs.msdn.com/b/ieinternals/archive/2010/03/30/
            combating-clickjacking-with-x-frame-options.aspx>.
 [Mozilla-X-Frame-Options]
            Mozilla Developer Network, "The X-Frame-Options response
            header", August 2013, <https://developer.mozilla.org/
            en-US/docs/The_X-FRAME-OPTIONS_response_header>.

Ross & Gondrom Informational [Page 11] RFC 7034 X-Frame-Options October 2013

 [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.

Ross & Gondrom Informational [Page 12] RFC 7034 X-Frame-Options October 2013

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

 A more detailed explanation of clickjacking scenarios follows.

B.1. Shop

 An Internet marketplace/shop offering a feature with a link/button to
 "Buy this" gadget 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/online shop needs to then
 immediately navigate the main browsing context (or a new window) to a
 confirmation page that 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
 that 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.

Appendix C. Acknowledgements

 This document was derived from input from specifications published by
 various browser vendors such as Microsoft (Eric Lawrence and David
 Ross), Mozilla, Google, Opera, and Apple.

Ross & Gondrom Informational [Page 13] RFC 7034 X-Frame-Options October 2013

Authors' Addresses

 David Ross
 Microsoft
 EMail: dross@microsoft.com
 Tobias Gondrom
 Thames Stanley
 EMail: tobias.gondrom@gondrom.org

Ross & Gondrom Informational [Page 14]

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