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 draft-ietf-httpbis-cookie-alone-00 Abstract 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 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 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 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 West Expires August 26, 2016 [Page 1] Internet-Draft Leave Secure Cookies Alone February 2016 described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology and notation . . . . . . . . . . . . . . . . . . . 3 3. Recommendations . . . . . . . . . . . . . . . . . . . . . . . . 3 4. Security Considerations . . . . . . . . . . . . . . . . . . . . 4 5. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5 5.1. Normative References . . . . . . . . . . . . . . . . . . . 5 5.2. Informative References . . . . . . . . . . . . . . . . . . 5 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . . 5 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 6 West Expires August 26, 2016 [Page 2] Internet-Draft Leave Secure Cookies Alone February 2016 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", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 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), 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 criteria: 1. Their "name" matches the "name" of the newly created cookie. 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" West Expires August 26, 2016 [Page 3] Internet-Draft Leave Secure Cookies Alone February 2016 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 well? 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 cookies. 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 [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 [HSTS-PRELOADING]. West Expires August 26, 2016 [Page 4] Internet-Draft Leave Secure Cookies Alone February 2016 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 [COOKIE-INTEGRITY] Zheng, X., Jiang, J., Liang, J., Duan, H., Chen, S., Wan, T., and N. Weaver, "Cookies Lack Integrity: Real-World Implications", August 2015, . [COOKIE-PREFIXES] West, M., "Cookie Prefixes", 2016, . [HSTS-PRELOADING] "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. West Expires August 26, 2016 [Page 5] Internet-Draft Leave Secure Cookies Alone February 2016 Author's Address Mike West Google, Inc Email: mkwst@google.com URI: https://mikewest.org/ West Expires August 26, 2016 [Page 6]