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Versions: (draft-west-leave-secure-cookies-alone) 00 01 draft-ietf-httpbis-rfc6265bis

HTTP Working Group                                               M. West
Internet-Draft                                               Google, Inc
Updates: 6265 (if approved)                            February 23, 2016
Intended status: Standards Track
Expires: August 26, 2016

   Deprecate modification of 'secure' cookies from non-secure origins


   This document updates RFC6265 by removing the ability for a non-
   secure origin to set cookies with a 'secure' flag, and to overwrite
   cookies whose 'secure' flag is set.  This deprecation improves the
   isolation between HTTP and HTTPS origins, and reduces the risk of
   malicious interference.

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 August 26, 2016.

Copyright Notice

   Copyright (c) 2016 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
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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology and notation  . . . . . . . . . . . . . . . . . .   2
   3.  Recommendations . . . . . . . . . . . . . . . . . . . . . . .   2
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   3
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   4
     5.2.  Informative References  . . . . . . . . . . . . . . . . .   4
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .   5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   Section 8.5 and Section 8.6 of [RFC6265] spell out some of the
   drawbacks of cookies' implementation: due to historical accident,
   non-secure origins can set cookies which will be delivered to secure
   origins in a manner indistinguishable from cookies set by that origin
   itself.  This enables a number of attacks, which have been recently
   spelled out in some detail in [COOKIE-INTEGRITY].

   We can mitigate the risk of these attacks by making it more difficult
   for non-secure origins to influence the state of secure origins.
   Accordingly, this document recommends the deprecation and removal of
   non-secure origins' ability to write cookies with a 'secure' flag,
   and their ability to overwrite cookies whose 'secure' flag is set.

2.  Terminology and notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   The "scheme" component of a URI is defined in Section 3 of [RFC3986].

3.  Recommendations

   This document updates Section 5.3 of [RFC6265] as follows:

   1.  After step 8 of the current algorithm, which sets the cookie's
       "secure-only-flag", execute the following step:

       1.  If the "scheme" component of the "request-uri" does not
           denote a "secure" protocol (as defined by the user agent),

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           and the cookie's "secure-only-flag" is "true", then abort
           these steps and ignore the newly created cookie entirely.
   2.  Before step 11, execute the following step:

       1.  If the newly created cookie's "secure-only-flag" is not set,
           and the "scheme" component of the "request-uri" does not
           denote a "secure" protocol, then abort these steps and ignore
           the newly created cookie entirely if the cookie store
           contains one or more cookies that meet all of the following

           1.  Their "name" matches the "name" of the newly created
           2.  Their "secure-only-flag" is set.
           3.  Their "domain" domain-matches the "domain" of the newly
               created cookie, or vice-versa.

           Note: This comparison intentionally ignores the "path"
           component.  The intent is to allow the "secure" flag to
           supercede the "path" restrictions to protect sites against
           cookie fixing attacks.

           Note: This allows "secure" pages to override "secure" cookies
           with non-secure variants.  Perhaps we should restrict that as
   3.  In order to ensure that a non-secure site can never cause a
       "secure" cookie to be evisted, adjust the "remove excess cookies"
       priority order at the bottom of Section 5.3 to be the following:

       1.  Expired cookies.
       2.  Cookies whose "secure-only-flag" is not set and which share a
           "domain" field with more than a predetermined number of other
       3.  Cookies that share a "domain" field with more than a
           predetermined number of other cookies.
       4.  All cookies.

       Note that the eviction algorithm specified here is triggered only
       after insertion of a cookie which causes the user agent to exceed
       some predetermined upper bound.  Conforming user agents MUST
       ensure that inserting a non-secure cookie does not cause a secure
       cookie to be removed.

4.  Security Considerations

   This specification increases a site's confidence that secure cookies
   it sets will remain unmodified by insecure pages on hosts which it
   domain-matches.  Ideally, sites would use HSTS as described in

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   [RFC6797] to defend more robustly against the dangers of non-secure
   transport in general, but until adoption of that protection becomes
   ubiquitous, this deprecation this document recommends will mitigate a
   number of risks.

   The mitigations in this document do not, however, give complete
   confidence that a given cookie was set securely.  If an attacker is
   able to impersonate a response from "http://example.com/" before a
   user visits "https://example.com/", the user agent will accept any
   cookie that the insecure origin sets, as the "secure" cookie won't
   yet be present in the user agent's cookie store.  An active network
   attacker may still be able to use this ability to mount an attack
   against "example.com", even if that site uses HTTPS exclusively.

   The proposal in [COOKIE-PREFIXES] could mitigate this risk, as could
   "preloading" HSTS for "example.com" into the user agent

5.  References

5.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,

   [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              DOI 10.17487/RFC6265, April 2011,

5.2.  Informative References

              Zheng, X., Jiang, J., Liang, J., Duan, H., Chen, S., Wan,
              T., and N. Weaver, "Cookies Lack Integrity: Real-World
              Implications", August 2015,

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              West, M., "Cookie Prefixes", 2016,

              "HSTS Preload Submission", n.d.,

   [RFC6797]  Hodges, J., Jackson, C., and A. Barth, "HTTP Strict
              Transport Security (HSTS)", RFC 6797,
              DOI 10.17487/RFC6797, November 2012,

Appendix A.  Acknowledgements

   Richard Barnes encouraged a formalization of the deprecation
   proposal.  [COOKIE-INTEGRITY] was a useful exploration of the issues
   [RFC6265] described.

Author's Address

   Mike West
   Google, Inc

   Email: mkwst@google.com
   URI:   https://mikewest.org/

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