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

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Copyright Notice

Copyright (c) 2014 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 1.1. Examples
• 2. Terminology and notation
• 2.1. First-party and Third-party Requests
• 3. Server Requirements
• 3.1. Grammar
• 3.2. Semantics of the “First-Party” Attribute (Non-Normative)
• 4. User Agent Requirements
• 4.1. The “First-Party” attribute
• 4.2. Monkey-patching the Storage Model
• 4.3. Monkey-patching the “Cookie” header
• 5. Authoring Considerations
• 5.1. Mashups and Widgets
• 6. Privacy Considerations
• 6.1. User Controls
• 7. Security Considerations
• 7.1. Limitations
• 8. Acknowledgements
• 9. References
• 9.1. Normative References
• 9.2. Informative References
• Author's Address

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.

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.
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:

• New windows create new first-party contexts.
• Full-page navigations create new first-party contexts. Notably, this includes both HTTP and <meta>-driven redirects.
• <iframe>s do not create new first-party contexts; their requests MUST be considered in the context of the origin of the URL the user actually sees in the user agent’s address bar.

3. Server Requirements

This section describes extensions to [RFC6265] necessary to implement the server-side requirements of the First-Party attribute.

3.1. Grammar

Add First-Party to the list of accepted attributes in the Set-Cookie header field’s value by replacing the cookie-av token definition in Section 4.1.1 of [RFC6265] with the following ABNF grammar:

cookie-av         = expires-av / max-age-av / domain-av /
                    path-av / secure-av / httponly-av /
                    first-party-av / extension-av
first-party-av    = "First-Party"

3.2. Semantics of the “First-Party” Attribute (Non-Normative)

The “First-Party” attribute limits the scope of the cookie such that it will only be attached to requests if those requests are “first-party”, as described in Section 2.1. For example, requests for https://example.com/sekrit-image will attach first-party cookies if and only if the top-level browsing context is currently displaying a document from https://example.com.

The changes to the Cookie header field suggested in Section 4.3 provide additional detail.

4. User Agent Requirements

This section describes extensions to [RFC6265] necessary in order to implement the client-side requirements of the First-Party attribute.

4.1. The “First-Party” attribute

The following attribute definition should be considered part of the the Set-Cookie algorithm as described in Section 5.2 of [RFC6265]:

If the attribute-name case-insensitively matches the string “First-Party”, the user agent MUST append an attribute to the cookie-attribute-list with an attribute-name of “First-Party” and an empty attribute-value.

4.2. Monkey-patching the Storage Model

Note: There’s got to be a better way to specify this. Until I figure out what that is, monkey-patching!

Alter Section 5.3 of [RFC6265] as follows:

1. Add firstparty-flag to the list of fields stored for each cookie.
2. Before step 11 of the current algorithm, add the following:
   1. If the cookie-attribute-list contains an attribute with an attribute-name of “First-Party”, set the cookie’s first-party-flag to true. Otherwise, set the cookie’s first-party-flag to false.
   2. If the cookie’s first-party-flag is set to true, and the request which generated the cookie is not a first-party request (as defined in Section 2.1), then abort these steps and ignore the newly created cookie entirely.

4.3. Monkey-patching the “Cookie” header

Note: There’s got to be a better way to specify this. Until I figure out what that is, monkey-patching!

Alter Section 5.4 of [RFC6265] as follows:

1. Add the following requirement to the list in step 1:
   • If the cookie’s first-party-flag is true, then exclude the cookie if the HTTP request is a third-party request (see Section 2.1).

Note that the modifications suggested here concern themselves only with the origin of the top-level browsing context and the origin of the resource being requested. The cookie’s domain, path, and secure attributes do not come into play for this comparison.

5. Authoring Considerations

5.1. Mashups and Widgets

The First-Party attribute is inappropriate for some important use-cases. In particular, note that content intended for embedding in a third-party context (social networking widgets or commenting services, for instance) will not have access to first-party cookies. Non-first-party cookies may be required in order to provide seamless functionality that relies on a user’s state.

Likewise, some forms of Single-Sign On might require authentication in a third-party context; these mechanisms will not function as intended with first-party cookies.

6. Privacy Considerations

6.1. User Controls

First-party cookies in and of themselves don’t do anything to address the general privacy concerns outlined in Section 7.1 of [RFC6265]. The attribute is set by the server, and serves to mitigate the risk of certain kinds of attacks that the server is worried about. The user is not involved in this decision.

User agents, however, could offer users the ability to toggle a cookie’s first-party-flag themselves, perhaps as part of a more general cookie management interface. This could provide an interesting middle-ground between the options (e.g. “Block all”, “Block third-party”, and “Allow all”) that many user agents offer to users.

7. Security Considerations

7.1. Limitations

It is possible to bypass the protection that first-party cookies offer against cross-site request forgery attacks by creating first-party contexts in which to execute the attack. Consider, for instance, the URL https://example.com/logout which logs the current user out of example.com. If the user’s session cookie is first-party cookie, then embedding the logout URL in an <iframe> element or an <img> element won’t log her out, as the cookie won’t be sent. Popping up a new window, or doing a top-level navigation, on the other hand, will create a first-party context, attach cookies, and perform the logout.

Note, though, that popping up a window, or doing a top-level navigation are both significantly more visible to the user than loading a subresource. Users will at least have the opportunity to notice that something strange is going on, which hopefully reduces an attacker’s ability to perform untargeted attacks.

Further, note that certain kinds of attacks are no longer possible if a first-party context is required. Information leakage attacks which rely on visible side-effects of loading a session-protected image, for example, can no longer access those side-effects if the image is loaded in a new window. Timing attacks like those Paul Stone outlines in [pixel-perfect] are no longer possible if the session cookie is first-party, as they rely on <iframes> to contain the protected content in a way the attacker can manipulate.

8. Acknowledgements

The first-party cookie concept documented here is similar to (but stricter than) Mark Goodwin’s and Joe Walker’s [samedomain-cookies].

9. References

9.1. Normative References

9.2. Informative References

Author's Address