Cookie Prefixes
draft-west-cookie-prefixes-00

Abstract

This document updates RFC6265 by adding a set of restrictions upon the names which may be used for cookies with specific properties. These restrictions enable user agents to smuggle cookie state to the server within the confines of the existing Cookie request header syntax, and limits the ways in which cookies may be abused in a conforming user agent.

Status of This Memo

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Copyright Notice

Copyright (c) 2015 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 2. Terminology and notation
• 3. Prefixes
• 3.1. The “$Secure-“ prefix
• 3.2. The “$Origin-“ prefix
• 4. User Agent Requirements
• 5. Aesthetic Considerations
• 6. Security Considerations
• 7. Normative References
• Appendix A. Acknowledgements
• Author's Address

1. Introduction

Section 8.5 and Section 8.6 of [RFC6265] spell out some of the drawbacks of cookies’ implementation: due to historical accident, it is impossible for a server to have confidence that a cookie set in a secure way (e.g., as a domain cookie with the Secure (and possibly HttpOnly) flags set) remains intact and untouched by non-secure subdomains.

We can’t alter the syntax of the Cookie request header, as that would likely break a number of implementations. This rules out sending a cookie’s flags along with the cookie directly, but we can smuggle information along with the cookie if we reserve certain name prefixes for cookies with certain properties.

This document describes such a scheme, which enables servers to set cookies which conforming user agents will ensure are Secure, and locked to a domain.

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. Prefixes

3.1. The “$Secure-“ prefix

If a cookie’s name begins with “$Secure-“, the cookie MUST be set with a Secure attribute.

The following cookie would be rejected:

Set-Cookie: $Secure-SID=12345; Domain=example.com

While the following would be accepted:

Set-Cookie: $Secure-SID=12345; Secure; Domain=example.com

3.2. The “$Origin-“ prefix

If a cookie’s name begins with “$Origin-“, the cookie MUST be:

1. Sent only to hosts which are identical to the host which set the cookie. That is, a cookie named “$Origin-cookie1” set from https://example.com MUST NOT contain a Domain attribute, and will therefore sent only to example.com, and not to subdomain.example.com.
2. Sent only to secure origins, if set from a secure origin. That is, a cookie named “$Origin-cookie1” set from https://example.com MUST contain a Secure attribute, as it was set from a URI whose scheme is “HTTPS”.

The following cookies would always be rejected:

Set-Cookie: $Origin-SID=12345; Domain=example.com
Set-Cookie: $Origin-SID=12345; Secure; Domain=example.com

The following would be rejected, if set from a secure origin, but accepted if set from a non-secure origin:

Set-Cookie: $Origin-SID=12345

While the following would be accepted, if set from a secure origin:

Set-Cookie: $Origin-SID=12345; Secure

4. User Agent Requirements

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

After step 10 of the current algorithm, the cookies flags are set. Insert the following steps to perform the prefix checks this document specifies:

1. If the cookie-name begins with the string “$Origin-“, then:
   1. If the scheme component of the request-uri is “HTTPS”, and the cookie’s secure-only-flag is false, abort these steps and ignore the cookie entirely.
   2. If the cookie’s host-only-flag is false, abort these steps and ignore the cookie entirely.
2. If the cookie-name begins with the string “$Secure-“, and the cookie’s secure-only-flag is false, abort these steps and ignore the cookie entirely.

5. Aesthetic Considerations

Prefixes are ugly. :(

6. Security Considerations

This scheme gives no assurance to the server that the restrictions on cookie names are enforced. Servers could certainly probe the user agent’s functionality to determine support, or sniff based on the User-Agent request header, if such assurances were deemed necessary.

7. Normative References

Appendix A. Acknowledgements

Eric Lawrence had this idea a million years ago. Devdatta Akhawe helped justify the potential impact of the scheme on real-world websites.

Author's Address