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.¶

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This Internet-Draft will expire on March 9, 2017.¶

Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved.¶

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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.¶

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].¶

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.
      4. The path of the newly created cookie path-matches the path of the existing cookie.
      Note: The path comparison is not symmetric, ensuring only that a newly-created non-secure cookie does not overlay an existing secure cookie, providing some mitigation against cookie fixing attacks. That is, given an existing secure cookie named a with a path of /login, a non-secure cookie named a could be set for a path of / or /foo, but not for a path of /login or /login/en.
      
      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 evicted, 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.

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].¶

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