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IETF RFC 8164
Last modified on Thursday, May 11th, 2017
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Internet Engineering Task Force (IETF) M. Nottingham
Request for Comments: 8164
Category: Experimental M. Thomson
ISSN: 2070-1721 Mozilla
May 2017
Opportunistic Security for HTTP/2
 Abstract
This document describes how "http" URIs can be accessed using
Transport Layer Security (TLS) and HTTP/2 to mitigate pervasive
monitoring attacks. This mechanism not a replacement for "https"
URIs; it is vulnerable to active attacks.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.
This document defines an Experimental Protocol for the Internet
community. This document is a product of the Internet Engineering
Task Force (IETF). It represents the consensus of the IETF
community. It has received public review and has been approved for
publication by the Internet Engineering Steering Group (IESG). Not
all documents approved by the IESG are a candidate for any level of
Internet Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/RFC 8164.
Copyright Notice
Copyright (c) 2017 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
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Nottingham & Thomson Experimental PAGE 1 
RFC 8164 Opportunistic HTTP/2 Security May 2017
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 ..............................5
2.3. The "http-opportunistic" Well-Known URI ....................5
3. IANA Considerations .............................................6
4. Security Considerations .........................................7
4.1. Security Indicators ........................................7
4.2. Downgrade Attacks ..........................................7
4.3. Privacy Considerations .....................................7
4.4. Confusion regarding Request Scheme .........................7
4.5. Server Controls ............................................8
5. References ......................................................8
5.1. Normative References .......................................8
5.2. Informative References .....................................9
Acknowledgements ...................................................9
Authors' Addresses ................................................10
1. Introduction
This document describes a use of HTTP Alternative Services [RFC 7838]
to decouple the URI scheme from the use and configuration of
underlying encryption. It allows an "http" URI [RFC 7230] to be
accessed using HTTP/2 and Transport Layer Security (TLS) [RFC 5246]
with Opportunistic Security [RFC 7435].
This document describes a usage model whereby sites can serve "http"
URIs over TLS, thereby avoiding the problem of serving Mixed Content
(described in [W3C.CR-mixed-content-20160802]) while still providing
protection against passive attacks.
Opportunistic Security does not provide the same guarantees as using
TLS with "https" URIs, because it is vulnerable to active attacks,
and does not change the security context of the connection.
Normally, users will not be able to tell that it is in use (i.e.,
there will be no "lock icon").
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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 [RFC 7258].
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
trivial administrative effort to configure.
Preventing active attacks (such as man-in-the-middle attacks) is a
non-goal for this specification. Furthermore, this specification is
not intended to replace or offer an alternative to "https", since
"https" 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 [RFC 2119].
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 [RFC 7838] for a protocol identifier that uses
TLS, such as "h2" [RFC 7540]. Such a protocol MUST include an
explicit indication of the scheme of the resource. This excludes
HTTP/1.1; HTTP/1.1 clients are forbidden from including the absolute
form of a URI in requests to origin servers (see Section 5.3.1 of
[RFC 7230]).
A client that receives such an advertisement MAY make future requests
intended for the associated origin [RFC 6454] to the identified
service (as specified by [RFC 7838]), 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.
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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
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; therefore, clients creating new TLS connections to
alternative services for the purposes of this specification MUST NOT
present them. A server that also provides "https" resources on the
same port can request a certificate during the TLS handshake, but it
MUST NOT abort the handshake if the client does not provide one.
2.1. Alternative Server Opt-In
For various reasons, it is possible that the server might become
confused about whether requests' URLs have an "http" or "https"
scheme (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 defined in [RFC 2818]. Using an authenticated
alternative service establishes "reasonable assurances" for the
purposes of [RFC 7838]. In addition to authenticating the server, the
client MUST have obtained a valid "http-opportunistic" response for
an origin (as per Section 2.3) using the authenticated connection.
An exception to the latter restriction is made for requests for the
"http-opportunistic" well-known URI.
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:
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RFC 8164 Opportunistic HTTP/2 Security May 2017
HEADERS
+ END_STREAM
+ END_HEADERS
:method = GET
:scheme = http
:authority = example.com
:path = /.well-known/http-opportunistic
HEADERS
:status = 200
content-type = application/json
DATA
+ END_STREAM
[ "http://www.example.com", "http://example.com" ]
This document describes multiple origins, but only for operational
convenience. Only a request made to an origin (over an authenticated
connection) can be used to acquire the "http-opportunistic" resource
for that origin. Thus, in the example, the request to
"http://example.com" cannot be assumed to also provide a
representation of the "http-opportunistic" resource for
"http://www.example.com".
2.2. Interaction with "https" URIs
Clients MUST NOT send "http" and "https" requests on the same
connection. Similarly, clients MUST NOT send "http" requests for
multiple origins on the same connection.
2.3. The "http-opportunistic" Well-Known URI
This specification defines the "http-opportunistic" well-known URI
[RFC 5785]. A client is said to have a valid "http-opportunistic"
response for a given origin when:
o The client has requested the well-known URI from the origin over
an authenticated connection and a 200 (OK) response was provided,
o That response is fresh [RFC 7234] (potentially through revalidation
[RFC 7232]),
o That response has the media type "application/json",
o That response's payload, when parsed as JSON [RFC 7159], contains
an array as the root, and
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o The array contains a string that is a case-insensitive, character-
for-character match for the origin in question, serialized into
Unicode as per Section 6.1 of [RFC 6454].
A client MAY treat an "http-opportunistic" resource as invalid if
values it contains are not strings.
This document does not define semantics for "http-opportunistic"
resources on an "https" origin, nor does it define semantics if the
resource includes "https" origins.
Allowing clients to cache the "http-opportunistic" resource means
that all alternative services need to be able to respond to requests
for "http" resources. A client is permitted to use an alternative
service without acquiring the "http-opportunistic" resource from that
service.
A client MUST NOT use any cached copies of an "http-opportunistic"
resource that was acquired (or revalidated) over an unauthenticated
connection. To avoid potential errors, a client can request or
revalidate the "http-opportunistic" resource before using any
connection to an alternative service.
Clients that use cached "http-opportunistic" responses MUST ensure
that their cache is cleared of any responses that were acquired over
an unauthenticated connection. Revalidating an unauthenticated
response using an authenticated connection does not ensure the
integrity of the response.
3. IANA Considerations
This specification registers the following well-known URI [RFC 5785]:
o URI Suffix: http-opportunistic
o Change Controller: IETF
o Specification Document(s): Section 2.3 of RFC 8164
o Related Information:
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4. Security Considerations
4.1. Security Indicators
User agents MUST NOT provide any special security indicators 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 [RFC 7838] 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 expected limitations in the protocol (the most
common HTTP/1.1 request form does not carry an explicit indication of
the URI scheme, and the resource might have been developed assuming
HTTP/1.1), or it may be because of 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 an HTTPS URI and operating in the security context implied
by HTTPS.
Therefore, server implementers and administrators 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
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<http://www.rfc-editor.org/info/RFC 2119>.
[RFC 2818] Rescorla, E., "HTTP Over TLS", RFC 2818,
DOI 10.17487/RFC 2818, May 2000,
<http://www.rfc-editor.org/info/RFC 2818>.
[RFC 5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC 5246, August 2008,
<http://www.rfc-editor.org/info/RFC 5246>.
[RFC 5785] Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
Uniform Resource Identifiers (URIs)", RFC 5785,
DOI 10.17487/RFC 5785, April 2010,
<http://www.rfc-editor.org/info/RFC 5785>.
[RFC 6454] Barth, A., "The Web Origin Concept", RFC 6454,
DOI 10.17487/RFC 6454, December 2011,
<http://www.rfc-editor.org/info/RFC 6454>.
[RFC 7159] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", RFC 7159, DOI 10.17487/RFC 7159, March
2014, <http://www.rfc-editor.org/info/RFC 7159>.
[RFC 7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC 7230, June 2014,
<http://www.rfc-editor.org/info/RFC 7230>.
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RFC 8164 Opportunistic HTTP/2 Security May 2017
[RFC 7232] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Conditional Requests", RFC 7232,
DOI 10.17487/RFC 7232, June 2014,
<http://www.rfc-editor.org/info/RFC 7232>.
[RFC 7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
RFC 7234, DOI 10.17487/RFC 7234, June 2014,
<http://www.rfc-editor.org/info/RFC 7234>.
[RFC 7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC 7540, May 2015,
<http://www.rfc-editor.org/info/RFC 7540>.
[RFC 7838] Nottingham, M., McManus, P., and J. Reschke, "HTTP
Alternative Services", RFC 7838, DOI 10.17487/RFC 7838,
April 2016, <http://www.rfc-editor.org/info/RFC 7838>.
5.2. Informative References
[RFC 7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, DOI 10.17487/RFC 7258, May
2014, <http://www.rfc-editor.org/info/RFC 7258>.
[RFC 7435] Dukhovni, V., "Opportunistic Security: Some Protection
Most of the Time", RFC 7435, DOI 10.17487/RFC 7435,
December 2014, <http://www.rfc-editor.org/info/RFC 7435>.
[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>.
Acknowledgements
Mike Bishop contributed significant text to this document.
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.
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RFC 8164 Opportunistic HTTP/2 Security May 2017
Authors' Addresses
Mark Nottingham
Email: mnot@mnot.net
URI: https://www.mnot.net/
Martin Thomson
Mozilla
Email: martin.thomson@gmail.com
Nottingham & Thomson Experimental PAGE 10
RFC TOTAL SIZE: 19620 bytes
PUBLICATION DATE: Thursday, May 11th, 2017
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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