HTTP Working Group M. Nottingham
Internet-Draft
Intended status: Experimental M. Thomson
Expires: September 18, 2016 Mozilla
March 17, 2016
Opportunistic Security for HTTP
draft-ietf-httpbis-http2-encryption-04
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
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This Internet-Draft will expire on September 18, 2016.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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
3. Server Authentication . . . . . . . . . . . . . . . . . . . . 4
4. Interaction with "https" URIs . . . . . . . . . . . . . . . . 5
5. Requiring Use of TLS . . . . . . . . . . . . . . . . . . . . 5
5.1. Opportunistic Commitment . . . . . . . . . . . . . . . . 6
5.2. Client Handling of A Commitment . . . . . . . . . . . . . 6
5.3. Operational Considerations . . . . . . . . . . . . . . . 7
6. The "http-opportunistic" well-known URI . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8.1. Security Indicators . . . . . . . . . . . . . . . . . . . 8
8.2. Downgrade Attacks . . . . . . . . . . . . . . . . . . . . 8
8.3. Privacy Considerations . . . . . . . . . . . . . . . . . 9
8.4. Confusion Regarding Request Scheme . . . . . . . . . . . 9
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
9.1. Normative References . . . . . . . . . . . . . . . . . . 9
9.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
This document describes a use of HTTP Alternative Services
[I-D.ietf-httpbis-alt-svc] to decouple the URI scheme from the use
and configuration of underlying encryption, allowing a "http" URI
[RFC7230] to be accessed using TLS [RFC5246] opportunistically.
Serving "https" URIs require acquiring and configuring a valid
certificate, which means that some deployments find supporting TLS
difficult. This document describes a usage model whereby sites can
serve "http" URIs over TLS without being required to support strong
server authentication.
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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.
Normally, users will not be able to tell that it is in use (i.e.,
there will be no "lock icon").
By its nature, this technique is vulnerable to active attacks. A
mechanism for partially mitigating them is described in Section 5.
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 goal is to limit the potential for active attacks. It is
not intended to offer the same level of protection as afforded to
"https" URIs, but instead to increase the likelihood that an active
attack can be detected.
A final (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.
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 [I-D.ietf-httpbis-alt-svc] for a protocol
identifier that uses TLS, such as "h2" [RFC7540].
A client that receives such an advertisement MAY make future requests
intended for the associated origin ([RFC6454]) to the identified
service (as specified by [I-D.ietf-httpbis-alt-svc]).
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.
3. Server Authentication
[I-D.ietf-httpbis-alt-svc] requires that an alternative service only
be used when there are "reasonable assurances" that it is under
control of and valid for the whole origin.
As defined in that specification, one way of establishing this is
using a TLS-based protocol with the certificate checks defined in
[RFC2818]. Clients MAY impose additional criteria for establishing
reasonable assurances.
For the purposes of this specification, an additional way of
establishing reasonable assurances is available when the alternative
is on the same host as the origin, using the "http-opportunistic"
well-known URI defined in Section 6.
This allows deployment without the use of valid certificates, to
encourage deployment of opportunistic security. When it is in use,
the alternative service can provide any certificate, or even select
TLS cipher suites that do not include authentication.
When the client has a valid http-opportunistic response for an
origin, it MAY consider there to be reasonable assurances when:
o The origin and alternative service's hostnames are the same when
compared in a case-insensitive fashion, and
o The chosen alternative service returns the same response as above.
For example, this request/response pair would constitute reasonable
assurances for the origin "http://www.example.com" for any
alternative service also on "www.example.com":
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GET /.well-known/http-opportunistic HTTP/1.1
Host: www.example.com
HTTP/1.1 200 OK
Content-Type: application/json
Connection: close
{
"origins": ["http://example.com", "http://www.example.com:81"]
}
Note that this mechanism is only defined to establish reasonable
assurances for the purposes of this specification; it does not apply
to other uses of alternative services unless they explicitly invoke
it.
4. 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.
Since "https" URIs rely on server authentication, a connection that
is initially created for "http" URIs without authenticating the
server cannot be used for "https" URIs until the server certificate
is successfully authenticated. Section 3.1 of [RFC2818] describes
the basic mechanism, though the authentication considerations in
[I-D.ietf-httpbis-alt-svc] also apply.
Connections that are established without any means of server
authentication (for instance, the purely anonymous TLS cipher
suites), cannot be used for "https" URIs.
5. Requiring Use of TLS
Even when the alternative service is strongly authenticated,
opportunistically upgrading cleartext HTTP connections to use TLS is
subject to active attacks. In particular:
o Because the original HTTP connection is in cleartext, it is
vulnerable to man-in-the-middle attacks, and
o By default, if clients cannot reach the alternative service, they
will fall back to using the original cleartext origin.
Given that the primary goal of this specification is to prevent
passive attacks, these are not critical failings (especially
considering the alternative - HTTP over cleartext). However, a
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modest form of protection against active attacks can be provided for
clients on subsequent connections.
When an origin is able to commit to providing service for a
particular origin over TLS for a bounded period of time, clients can
choose to rely upon its availability, failing when it cannot be
contacted. Effectively, this makes the choice to use a secured
protocol "sticky".
5.1. Opportunistic Commitment
An origin can reduce the risk of attacks on opportunistically secured
connections by committing to provide an secured, authenticated
alternative service. This is done by including the optional "commit"
member in the http-opportunistic well-known resource (see Section 6).
This feature is optional due to the requirement for server
authentication and the potential risk entailed (see Section 5.3).
The value of the "commit" member is a number ([RFC7159], Section 6)
indicating the duration of the commitment interval in seconds.
{
"origins": ["http://example.com", "http://www.example.com:81"],
"commit": 86400
}
Including "commit" creates a commitment to provide a secured
alternative service for the advertised period. Clients that receive
this commitment can assume that a secured alternative service will be
available for the indicated period. Clients might however choose to
limit this time (see Section 5.3).
5.2. Client Handling of A Commitment
The value of the "commit" member MUST be ignored unless the
alternative service can be strongly authenticated. The same
authentication requirements that apply to "https://" resources SHOULD
be applied to authenticating the alternative. Minimum authentication
requirements for HTTP over TLS are described in Section 2.1 of
[I-D.ietf-httpbis-alt-svc] and Section 3.1 of [RFC2818]. As noted in
[I-D.ietf-httpbis-alt-svc], clients can impose other checks in
addition to this minimum set. For instance, a client might choose to
apply key pinning [RFC7469].
A client that receives a commitment and that successfully
authenticates the alternative service can assume that a secured
alternative will remain available for the commitment interval. The
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commitment interval starts when the commitment is received and
authenticated and runs for a number of seconds equal to value of the
"commit" member, less the current age of the http-opportunistic
response (as defined in Section 4.2.3 of [RFC7234]). A client SHOULD
avoid sending requests via cleartext protocols or to unauthenticated
alternative services for the duration of the commitment interval,
except to discover new potential alternatives.
A commitment only applies to the origin of the http-opportunistic
well-known resource that was retrieved; other origins listed in the
"origins" member MUST be independently discovered and authenticated.
A commitment is not bound to a particular alternative service.
Clients are able to use alternative services that they become aware
of. However, once a valid and authenticated commitment has been
received, clients SHOULD NOT use an unauthenticated alternative
service. Where there is an active commitment, clients SHOULD ignore
advertisements for unsecured alternative services. A client MAY send
requests to an unauthenticated origin in an attempt to discover
potential alternative services, but these requests SHOULD be entirely
generic and avoid including credentials.
5.3. Operational Considerations
Errors in configuration of commitments has the potential to render
even the unsecured origin inaccessible for the duration of a
commitment. Initial deployments are encouraged to use short duration
commitments so that errors can be detected without causing the origin
to become inaccessible to clients for extended periods.
To avoid situations where a commitment causes errors, clients MAY
limit the time over which a commitment is respected for a given
origin. A lower limit might be appropriate for initial commitments;
the certainty that a site has set a correct value - and the
corresponding limit on persistence - might increase as a commitment
is renewed multiple times.
6. The "http-opportunistic" well-known URI
This specification defines the "http-opportunistic" well-known URI
[RFC5785]. An origin is said to have a valid http-opportunistic
resource when:
o The client has obtained a 200 (OK) response for the well-known URI
from the origin, or refreshed one in cache [RFC7234], and
o That response has the media type "application/json", and
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o That response's payload, when parsed as JSON [RFC7159], contains
an object as the root.
o The "origins" member of the root object has a value of an array of
strings, one of which is a case-insensitive character-for-
character match for the origin in question, serialised into
Unicode as per [RFC6454], Section 6.1, and
This specification defines one additional, optional member of the
root object, "commit" in Section 5. Unrecognised members MUST be
ignored.
7. IANA Considerations
This specification registers a Well-known URI [RFC5785]:
o URI Suffix: http-opportunistic
o Change Controller: IETF
o Specification Document(s): [this specification]
o Related Information:
8. Security Considerations
8.1. 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").
8.2. Downgrade Attacks
A downgrade attack against the negotiation for TLS is possible. With
commitment Section 5, this is limited to occasions where clients have
no prior information (see Section 8.3), or when persisted commitments
have expired.
For example, because the "Alt-Svc" header field
[I-D.ietf-httpbis-alt-svc] 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.
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Downgrade attacks can be partially mitigated using the "commit"
member of the http-opportunistic well-known resource, because when it
is used, a client can avoid using cleartext to contact a supporting
server. However, this only works when a previous connection has been
established without an active attacker present; a continuously
present active attacker can either prevent the client from ever using
TLS, or offer its own certificate.
8.3. 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).
8.4. Confusion Regarding Request Scheme
Many existing HTTP/1.1 implementations use the presence or absence of
TLS in the stack to determine whether requests are for "http" or
"https" resources. This is necessary in many cases because the most
common form of an HTTP/1.1 request does not carry an explicit
indication of the URI scheme.
HTTP/1.1 MUST NOT be used for opportunistically secured requests.
Some HTTP/1.1 implementations use ambient signals to determine if a
request is for an "https" resource. For example, implementations
might look for TLS on the stack or a port number of 443. An
implementation that supports opportunistically secured requests
SHOULD suppress these signals if there is any potential for
confusion.
9. References
9.1. Normative References
[I-D.ietf-httpbis-alt-svc]
mnot, m., McManus, P., and J. Reschke, "HTTP Alternative
Services", draft-ietf-httpbis-alt-svc-14 (work in
progress), March 2016.
[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>.
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[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>.
[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>.
9.2. 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>.
[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>.
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[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>.
Appendix A. Acknowledgements
Thanks to Patrick McManus, Eliot Lear, Stephen Farrell, Guy Podjarny,
Stephen Ludin, Erik Nygren, Paul Hoffman, Adam Langley, Eric Rescorla
and Richard Barnes for their feedback and suggestions.
Authors' Addresses
Mark Nottingham
Email: mnot@mnot.net
URI: http://www.mnot.net/
Martin Thomson
Mozilla
Email: martin.thomson@gmail.com
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