HTTPbis M. West Internet-Draft Google, Inc Updates: 6265 (if approved) October 27, 2014 Intended status: Standards Track Expires: April 30, 2015 First-Party Cookies draft-west-first-party-cookies-00 Abstract This document updates RFC6265, defining the "First-Party" attribute for cookies, which allows servers to mitigate the risk of cross-site request forgery and related information leakage attacks by asserting that a particular cookie should only be sent in a "first-party" context. 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/. 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Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of West Expires April 30, 2015 [Page 1] Internet-Draft first-party-cookies October 2014 the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Examples . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology and notation . . . . . . . . . . . . . . . . . . 3 2.1. First-party and Third-party Requests . . . . . . . . . . 3 3. Server Requirements . . . . . . . . . . . . . . . . . . . . . 4 3.1. Grammar . . . . . . . . . . . . . . . . . . . . . . . . . 4 3.2. Semantics of the "First-Party" Attribute (Non-Normative) 4 4. User Agent Requirements . . . . . . . . . . . . . . . . . . . 5 4.1. The "First-Party" attribute . . . . . . . . . . . . . . . 5 4.2. Monkey-patching the Storage Model . . . . . . . . . . . . 5 4.3. Monkey-patching the "Cookie" header . . . . . . . . . . . 6 5. Authoring Considerations . . . . . . . . . . . . . . . . . . 6 5.1. Mashups and Widgets . . . . . . . . . . . . . . . . . . . 6 6. Privacy Considerations . . . . . . . . . . . . . . . . . . . 6 6.1. User Controls . . . . . . . . . . . . . . . . . . . . . . 6 7. Security Considerations . . . . . . . . . . . . . . . . . . . 7 7.1. Limitations . . . . . . . . . . . . . . . . . . . . . . . 7 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 9.1. Normative References . . . . . . . . . . . . . . . . . . 7 9.2. Informative References . . . . . . . . . . . . . . . . . 8 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8 1. Introduction Section 8.2 of [RFC6265] eloquently notes that cookies are a form of ambient authority, attached by default to requests the user agent sends on a user's behalf. Even when an attacker doesn't know the contents of a user's cookies, she can still execute commands on the user's behalf (and with the user's authority) by asking the user agent to send HTTP requests to unwary servers. Here, we update [RFC6265] with a simple mitigation strategy that allows servers to declare certain cookies as "First-party cookies" which should be attached to requests if and only if they occur in a first-party context. Note that the mechanism outlined here is backwards compatible with the existing cookie syntax. Servers may serve first-party cookies to all user agents; those that do not support the "First-Party" attribute will simply store a non-first-party cookie, just as they do today. West Expires April 30, 2015 [Page 2] Internet-Draft first-party-cookies October 2014 1.1. Examples First-party cookies are set via the "First-Party" attribute in the "Set-Cookie" header field. That is, given a server's response to a user agent which contains the following header field: Set-Cookie: SID=31d4d96e407aad42; First-Party Subsequent requests from that user agent can be expected to contain the following header field if and only if both the requested resource and the resource in the top-level browsing context match the cookie. 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]. This specification uses the Augmented Backus-Naur Form (ABNF) notation of [RFC5234]. Two sequences of octets are said to case-insensitively match each other if and only if they are equivalent under the "i;ascii-casemap" collation defined in [RFC4790]. The terms "active document", and "top-level browsing context" are defined in the HTML Living Standard. [HTML] The term "origin" and the mechanism of deriving an origin from a URI are defined in [RFC6454]. 2.1. First-party and Third-party Requests The URL displayed in a user agent's address bar is the only security context directly exposed to users, and therefore the only signal users can reasonably rely upon to determine who they're talking to. Broadly speaking, then, a "first-party" request is an HTTP request for a resource whose URL's origin matches the origin of the URL the user sees in the address bar. A "third-party" request is an HTTP request for a resource at any other origin. To be slightly more precise, given an HTTP request "request": 1. Let "context" be the top-level browsing context in the window responsible for "request". West Expires April 30, 2015 [Page 3] Internet-Draft first-party-cookies October 2014 2. Let "top-origin" be the origin of the location of the active document in "context". 3. If the origin of "request"'s URL is the same as "top-origin", "request" is a *first-party request*. Otherwise, "request" is a *third-party request*. Note that we deal with the document's _location_ in step 2 above, not with the document's origin. For example, a top-level document from "https://example.com" which has been sandboxed into a unique origin still creates a non-unique first-party context for subsequent requests. This definition has a few implications: o New windows create new first-party contexts. o Full-page navigations create new first-party contexts. Notably, this includes both HTTP and ""-driven redirects. o "