Internet DRAFT - draft-ietf-httpbis-cookie-prefixes
draft-ietf-httpbis-cookie-prefixes
HTTP Working Group M. West
Internet-Draft Google, Inc
Updates: 6265 (if approved) February 23, 2016
Intended status: Standards Track
Expires: August 26, 2016
Cookie Prefixes
draft-ietf-httpbis-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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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
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology and notation . . . . . . . . . . . . . . . . . . 2
3. Prefixes . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. The "__Secure-" prefix . . . . . . . . . . . . . . . . . 3
3.2. The "__Host-" prefix . . . . . . . . . . . . . . . . . . 3
4. User Agent Requirements . . . . . . . . . . . . . . . . . . . 4
5. Aesthetic Considerations . . . . . . . . . . . . . . . . . . 4
5.1. Not pretty. . . . . . . . . . . . . . . . . . . . . . . . 4
5.2. Why "__"? . . . . . . . . . . . . . . . . . . . . . . . . 4
6. Security Considerations . . . . . . . . . . . . . . . . . . . 4
6.1. Secure Origins Only . . . . . . . . . . . . . . . . . . . 5
6.2. Limitations . . . . . . . . . . . . . . . . . . . . . . . 5
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
7.1. Normative References . . . . . . . . . . . . . . . . . . 5
7.2. Informative References . . . . . . . . . . . . . . . . . 5
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6
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].
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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:
1. Set with a "Secure" attribute
2. Set from a URI whose "scheme" is considered "secure" by the user
agent.
The following cookie would be rejected when set from any origin, as
the "Secure" flag is not set
Set-Cookie: __Secure-SID=12345; Domain=example.com
While the following would be accepted if set from a secure origin
(e.g. "https://example.com/"), and rejected otherwise:
Set-Cookie: __Secure-SID=12345; Secure; Domain=example.com
3.2. The "__Host-" prefix
If a cookie's name begins with "__Host-", the cookie MUST be:
1. Set with a "Secure" attribute
2. Set from a URI whose "scheme" is considered "secure" by the user
agent.
3. Sent only to the host which set the cookie. That is, a cookie
named "__Host-cookie1" set from "https://example.com" MUST NOT
contain a "Domain" attribute (and will therefore be sent only to
"example.com", and not to "subdomain.example.com").
4. Sent to every request for a host. That is, a cookie named
"__Host-cookie1" MUST contain a "Path" attribute with a value of
"/".
The following cookies would always be rejected:
Set-Cookie: __Host-SID=12345
Set-Cookie: __Host-SID=12345; Secure
Set-Cookie: __Host-SID=12345; Domain=example.com
Set-Cookie: __Host-SID=12345; Domain=example.com; Path=/
Set-Cookie: __Host-SID=12345; Secure; Domain=example.com; Path=/
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While the following would be accepted if set from a secure origin
(e.g. "https://example.com/"), and rejected otherwise:
Set-Cookie: __Host-SID=12345; Secure; Path=/
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 "__Secure-" or
"__Host-", abort these steps and ignore the cookie entirely
unless both of the following conditions are true:
* The cookie's "secure-only-flag" is "true"
* "request-uri"'s "scheme" component denotes a "secure" protocol
(as determined by the user agent)
2. If the "cookie-name" begins with the string "__Host-", abort
these steps and ignore the cookie entirely unless the following
conditions are true:
* The cookie's "host-only-flag" is "true"
* The cookie's "path" is "/"
5. Aesthetic Considerations
5.1. Not pretty.
Prefixes are ugly. :(
5.2. Why "__"?
We started with "$", but ran into issues with servers that had
implemented [RFC2109]-style cookies. "__" is a prefix used for a
number of well-known cookies in the wild (notably Google Analytics's
"__ut*" cookies, and CloudFlare's "__cfduid"), and so is unlikely to
produce such compatibility issues, while being uncommon enough to
mitigate the risk of collisions.
6. Security Considerations
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6.1. Secure Origins Only
It would certainly be possible to extend this scheme to non-secure
origins (and an earlier draft of this document did exactly that).
User agents, however, are slowly moving towards a world where
features with security implications are available only over secure
transport (see [SECURE-CONTEXTS], [POWERFUL-FEATURES], and
[DEPRECATING-HTTP]). This document follows that trend, limiting
exciting new cookie properties to secure transport in order to ensure
that user agents can make claims which middlemen will have a hard
time violating.
To that end, note that the requirements listed above mean that
prefixed cookies will be rejected entirely if a non-secure origin
attempts to set them.
6.2. Limitations
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. References
7.1. 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,
<http://www.rfc-editor.org/info/rfc2119>.
[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,
<http://www.rfc-editor.org/info/rfc3986>.
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
DOI 10.17487/RFC6265, April 2011,
<http://www.rfc-editor.org/info/rfc6265>.
7.2. Informative References
[DEPRECATING-HTTP]
Barnes, R., "Deprecating Non-Secure HTTP", April 2015,
<https://blog.mozilla.org/security/2015/04/30/deprecating-
non-secure-http/>.
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[Lawrence2015]
Lawrence, E., "Duct Tape and Baling Wire -- Cookie
Prefixes", October 2015,
<http://textslashplain.com/2015/10/09/
duct-tape-and-baling-wirecookie-prefixes/>.
[POWERFUL-FEATURES]
Palmer, C., "Prefer Secure Origins for Powerful New
Features", 2015, <https://www.chromium.org/Home/chromium-
security/prefer-secure-origins-for-powerful-new-features>.
[RFC2109] Kristol, D. and L. Montulli, "HTTP State Management
Mechanism", RFC 2109, DOI 10.17487/RFC2109, February 1997,
<http://www.rfc-editor.org/info/rfc2109>.
[SECURE-CONTEXTS]
West, M., "Secure Contexts", 2016, <https://w3c.github.io/
webappsec-secure-contexts/>.
Appendix A. Acknowledgements
Eric Lawrence had this idea a million years ago, and wrote about its
genesis in [Lawrence2015]. Devdatta Akhawe helped justify the
potential impact of the scheme on real-world websites. Thomas Broyer
pointed out the issues with a leading "$" in the prefixes, and Brian
Smith provided valuable contributions to the discussion around a
replacement (ISO C indeed).
Author's Address
Mike West
Google, Inc
Email: mkwst@google.com
URI: https://mikewest.org/
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