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Versions: (draft-nottingham-http2-encryption)
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RFC 8164
HTTP Working Group M. Nottingham
Internet-Draft
Intended status: Experimental M. Thomson
Expires: May 4, 2017 Mozilla
October 31, 2016
Opportunistic Security for HTTP
draft-ietf-httpbis-http2-encryption-08
Abstract
This document describes how "http" URIs can be accessed using
Transport Layer Security (TLS) to mitigate pervasive monitoring
attacks.
Note to Readers
Discussion of this draft takes place on the HTTP working group
mailing list (ietf-http-wg@w3.org), which is archived at
https://lists.w3.org/Archives/Public/ietf-http-wg/ .
Working Group information can be found at http://httpwg.github.io/ ;
source code and issues list for this draft can be found at
https://github.com/httpwg/http-extensions/labels/opp-sec .
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/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 4, 2017.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Goals and Non-Goals . . . . . . . . . . . . . . . . . . . 3
1.2. Notational Conventions . . . . . . . . . . . . . . . . . 3
2. Using HTTP URIs over TLS . . . . . . . . . . . . . . . . . . 3
2.1. Alternative Server Opt-In . . . . . . . . . . . . . . . . 4
2.2. Interaction with "https" URIs . . . . . . . . . . . . . . 4
2.3. The "http-opportunistic" well-known URI . . . . . . . . . 5
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
4. Security Considerations . . . . . . . . . . . . . . . . . . . 5
4.1. Security Indicators . . . . . . . . . . . . . . . . . . . 6
4.2. Downgrade Attacks . . . . . . . . . . . . . . . . . . . . 6
4.3. Privacy Considerations . . . . . . . . . . . . . . . . . 6
4.4. Confusion Regarding Request Scheme . . . . . . . . . . . 6
4.5. Server Controls . . . . . . . . . . . . . . . . . . . . . 7
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
5.1. Normative References . . . . . . . . . . . . . . . . . . 7
5.2. Informative References . . . . . . . . . . . . . . . . . 8
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
This document describes a use of HTTP Alternative Services [RFC7838]
to decouple the URI scheme from the use and configuration of
underlying encryption, allowing a "http" URI [RFC7230] to be accessed
using Transport Layer Security (TLS) [RFC5246] opportunistically.
Serving "https" URIs requires avoiding Mixed Content
[W3C.CR-mixed-content-20160802], which is problematic in many
deployments. This document describes a usage model whereby sites can
serve "http" URIs over TLS, thereby avoiding these issues, while
still providing protection against passive attacks.
Opportunistic Security [RFC7435] does not provide the same guarantees
as using TLS with "https" URIs; it is vulnerable to active attacks,
and does not change the security context of the connection.
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Normally, users will not be able to tell that it is in use (i.e.,
there will be no "lock icon").
1.1. Goals and Non-Goals
The immediate goal is to make the use of HTTP more robust in the face
of pervasive passive monitoring [RFC7258].
A secondary (but significant) goal is to provide for ease of
implementation, deployment and operation. This mechanism is expected
to have a minimal impact upon performance, and require a trivial
administrative effort to configure.
Preventing active attacks (such as a Man-in-the-Middle) is a non-goal
for this specification. Furthermore, this specification is not
intended to replace or offer an alternative to "https", since it both
prevents active attacks and invokes a more stringent security model
in most clients.
1.2. Notational Conventions
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].
2. Using HTTP URIs over TLS
An origin server that supports the resolution of "http" URIs can
indicate support for this specification by providing an alternative
service advertisement [RFC7838] for a protocol identifier that uses
TLS, such as "h2" [RFC7540], or "http/1.1" [RFC7301]. Note that
HTTP/1.1 requests MUST use the absolute form (see Section 5.3.2 of
[RFC7230]).
A client that receives such an advertisement MAY make future requests
intended for the associated origin ([RFC6454]) to the identified
service (as specified by [RFC7838]), provided that the alternative
service opts in as described in Section 2.1.
A client that places the importance of protection against passive
attacks over performance might choose to withhold requests until an
encrypted connection is available. However, if such a connection
cannot be successfully established, the client can resume its use of
the cleartext connection.
A client can also explicitly probe for an alternative service
advertisement by sending a request that bears little or no sensitive
information, such as one with the OPTIONS method. Likewise, clients
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with existing alternative services information could make such a
request before they expire, in order minimize the delays that might
be incurred.
Client certificates are not meaningful for URLs with the "http"
scheme, and therefore clients creating new TLS connections to
alternative services for the purposes of this specification MUST NOT
present them. Connections that use client certificates for other
reasons MAY be reused, though client certificates MUST NOT affect the
responses to requests for "http" resources.
2.1. Alternative Server Opt-In
It is possible that the server might become confused about whether
requests' URLs have a "http" or "https" scheme, for various reasons;
see Section 4.4. To ensure that the alternative service has opted
into serving "http" URLs over TLS, clients are required to perform
additional checks before directing "http" requests to it.
Clients MUST NOT send "http" requests over a secured connection,
unless the chosen alternative service presents a certificate that is
valid for the origin - as per [RFC2818] (this also establishes
"reasonable assurances" for the purposes of {RFC7838}}) - and they
have obtained a valid http-opportunistic response for an origin (as
per Section 2.3).
For example, assuming the following request is made over a TLS
connection that is successfully authenticated for those origins, the
following request/response pair would allow requests for the origins
"http://www.example.com" or "http://example.com" to be sent using a
secured connection:
GET http://example.com/.well-known/http-opportunistic HTTP/1.1
Host: example.com
HTTP/1.1 200 OK
Content-Type: application/json
Connection: close
[ "http://www.example.com", "http://example.com" ]
2.2. Interaction with "https" URIs
When using alternative services, requests for resources identified by
both "http" and "https" URIs might use the same connection, because
HTTP/2 permits requests for multiple origins on the same connection.
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Because of the potential for server confusion about the scheme of
requests (see Section 4.4), clients MUST NOT send "http" requests on
a connection prior to successfully retrieving a valid http-
opportunistic resource that contains the origin (see Section 2.3).
The primary purpose of this check is to provide a client with some
assurance that a server understands this specification and has taken
steps to avoid being confused about request scheme.
2.3. The "http-opportunistic" well-known URI
This specification defines the "http-opportunistic" well-known URI
[RFC5785]. A client is said to have a valid http-opportunistic
response for a given origin when:
o The client has obtained a 200 (OK) response for the well-known URI
from the origin, and it is fresh [RFC7234] (potentially through
revalidation [RFC7232]), and
o That response has the media type "application/json", and
o That response's payload, when parsed as JSON [RFC7159], contains
an array as the root, and
o The array contains a string that is a case-insensitive character-
for-character match for the origin in question, serialised into
Unicode as per Section 6.1 of [RFC6454].
A client MAY treat an "http-opportunistic" resource as invalid if the
contains values that are not strings.
3. IANA Considerations
This specification registers a Well-Known URI [RFC5785]:
o URI Suffix: http-opportunistic
o Change Controller: IETF
o Specification Document(s): Section 2.3 of [this specification]
o Related Information:
4. Security Considerations
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4.1. Security Indicators
User Agents MUST NOT provide any special security indicia when an
"http" resource is acquired using TLS. In particular, indicators
that might suggest the same level of security as "https" MUST NOT be
used (e.g., a "lock device").
4.2. Downgrade Attacks
A downgrade attack against the negotiation for TLS is possible.
For example, because the "Alt-Svc" header field [RFC7838] likely
appears in an unauthenticated and unencrypted channel, it is subject
to downgrade by network attackers. In its simplest form, an attacker
that wants the connection to remain in the clear need only strip the
"Alt-Svc" header field from responses.
4.3. Privacy Considerations
Cached alternative services can be used to track clients over time;
e.g., using a user-specific hostname. Clearing the cache reduces the
ability of servers to track clients; therefore clients MUST clear
cached alternative service information when clearing other origin-
based state (i.e., cookies).
4.4. Confusion Regarding Request Scheme
HTTP implementations and applications sometimes use ambient signals
to determine if a request is for an "https" resource; for example,
they might look for TLS on the stack, or a server port number of 443.
This might be due to limitations in the protocol (the most common
HTTP/1.1 request form does not carry an explicit indication of the
URI scheme), or it may be because how the server and application are
implemented (often, they are two separate entities, with a variety of
possible interfaces between them).
Any security decisions based upon this information could be misled by
the deployment of this specification, because it violates the
assumption that the use of TLS (or port 443) means that the client is
accessing a HTTPS URI, and operating in the security context implied
by HTTPS.
Therefore, servers need to carefully examine the use of such signals
before deploying this specification.
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4.5. Server Controls
This specification requires that a server send both an Alternative
Service advertisement and host content in a well-known location to
send HTTP requests over TLS. Servers SHOULD take suitable measures
to ensure that the content of the well-known resource remains under
their control. Likewise, because the Alt-Svc header field is used to
describe policies across an entire origin, servers SHOULD NOT permit
user content to set or modify the value of this header.
5. References
5.1. 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>.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818,
DOI 10.17487/RFC2818, May 2000,
<http://www.rfc-editor.org/info/rfc2818>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>.
[RFC5785] Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
Uniform Resource Identifiers (URIs)", RFC 5785,
DOI 10.17487/RFC5785, April 2010,
<http://www.rfc-editor.org/info/rfc5785>.
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454,
DOI 10.17487/RFC6454, December 2011,
<http://www.rfc-editor.org/info/rfc6454>.
[RFC7159] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
2014, <http://www.rfc-editor.org/info/rfc7159>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<http://www.rfc-editor.org/info/rfc7230>.
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[RFC7232] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Conditional Requests", RFC 7232,
DOI 10.17487/RFC7232, June 2014,
<http://www.rfc-editor.org/info/rfc7232>.
[RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
RFC 7234, DOI 10.17487/RFC7234, June 2014,
<http://www.rfc-editor.org/info/rfc7234>.
[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015,
<http://www.rfc-editor.org/info/rfc7540>.
[RFC7838] Nottingham, M., McManus, P., and J. Reschke, "HTTP
Alternative Services", RFC 7838, DOI 10.17487/RFC7838,
April 2016, <http://www.rfc-editor.org/info/rfc7838>.
5.2. Informative References
[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
2014, <http://www.rfc-editor.org/info/rfc7258>.
[RFC7301] Friedl, S., Popov, A., Langley, A., and E. Stephan,
"Transport Layer Security (TLS) Application-Layer Protocol
Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
July 2014, <http://www.rfc-editor.org/info/rfc7301>.
[RFC7435] Dukhovni, V., "Opportunistic Security: Some Protection
Most of the Time", RFC 7435, DOI 10.17487/RFC7435,
December 2014, <http://www.rfc-editor.org/info/rfc7435>.
[RFC7469] Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April
2015, <http://www.rfc-editor.org/info/rfc7469>.
[W3C.CR-mixed-content-20160802]
West, M., "Mixed Content", World Wide Web Consortium CR
CR-mixed-content-20160802, August 2016,
<https://www.w3.org/TR/2016/CR-mixed-content-20160802>.
Appendix A. Acknowledgements
Mike Bishop contributed significant text to this document.
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Thanks to Patrick McManus, Stefan Eissing, Eliot Lear, Stephen
Farrell, Guy Podjarny, Stephen Ludin, Erik Nygren, Paul Hoffman, Adam
Langley, Eric Rescorla, Julian Reschke, Kari Hurtta, and Richard
Barnes for their feedback and suggestions.
Authors' Addresses
Mark Nottingham
Email: mnot@mnot.net
URI: https://www.mnot.net/
Martin Thomson
Mozilla
Email: martin.thomson@gmail.com
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