HTTP Working Group M. Nottingham
Internet-Draft
Intended status: Experimental M. Thomson
Expires: December 17, 2015 Mozilla
June 15, 2015
Opportunistic Security for HTTP
draft-ietf-httpbis-http2-encryption-02
Abstract
This document describes how "http" URIs can be accessed using
Transport Layer Security (TLS) to mitigate pervasive monitoring
attacks.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on December 17, 2015.
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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 . . . . . . . . . . . . . . . . 4
5. Requiring Use of TLS . . . . . . . . . . . . . . . . . . . . 5
5.1. The HTTP-TLS Header Field . . . . . . . . . . . . . . . . 5
5.2. Operational Considerations . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6.1. Security Indicators . . . . . . . . . . . . . . . . . . . 7
6.2. Downgrade Attacks . . . . . . . . . . . . . . . . . . . . 7
6.3. Privacy Considerations . . . . . . . . . . . . . . . . . 8
6.4. Confusion Regarding Request Scheme . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1. Normative References . . . . . . . . . . . . . . . . . . 8
7.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
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 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.
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.
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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 [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 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.
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.
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3. Server Authentication
By their nature, "http" URIs do not require cryptographically strong
server authentication; that is only implied by "https" URIs.
Furthermore, doing so (as per [RFC2818]) creates a number of
operational challenges. For these reasons, server authentication is
not mandatory for "http" URIs when using the mechanism described in
this specification.
When connecting to an alternative service for an "http" URI, clients
are not required to perform the server authentication procedure
described in Section 3.1 of [RFC2818]. The server certificate, if
one is proffered by the alternative service, is not necessarily
checked for validity, expiration, issuance by a trusted certificate
authority or matched against the name in the URI. Therefore, the
alternative service can provide any certificate, or even select TLS
cipher suites that do not include authentication.
A client MAY perform additional checks on the offered certificate if
the server does not select an unauthenticated TLS cipher suite. This
document doesn't define any such checks, though clients could be
configured with a policy that defines what is acceptable.
As stipulated by [I-D.ietf-httpbis-alt-svc], clients MUST NOT use
alternative services with a host other than the origin's, unless the
alternative service itself is strongly authenticated (as the origin's
host); for example, using TLS with a certificate that validates as
per [RFC2818].
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.
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5. Requiring Use of TLS
Editors' Note: this is a very rough take on an approach that would
provide a limited form of protection against downgrade attack. It's
unclear at this point whether the additional effort (and modest
operational cost) is worthwhile.
The mechanism described in this specification is trivial to mount an
active attack against, for two reasons:
o A client that doesn't perform authentication is an easy victim of
server impersonation, through man-in-the-middle attacks.
o A client that is willing to use HTTP over cleartext to resolve the
resource will do so if access to any TLS-enabled alternative
services is blocked at the network layer.
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
modest form of protection against active attacks can be provided for
clients on subsequent connections.
When an alternative service 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" in the client.
5.1. The HTTP-TLS Header Field
A alternative service can make this commitment by sending a "HTTP-
TLS" header field, described here using the '#' ABNF extension
defined in Section 7 of [RFC7230]:
HTTP-TLS = 1#parameter
When it appears in a HTTP response from a strongly authenticated
alternative service, this header field indicates that the
availability of the origin through TLS-protected alternative services
is "sticky", and that the client MUST NOT fall back to cleartext
protocols while this information is considered fresh.
For example:
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GET /index.html HTTP/1.1
Host: example.com
HTTP/1.1 200 OK
Content-Type: text/html
Cache-Control: max-age=600
Age: 30
Date: Thu, 1 May 2014 16:20:09 GMT
HTTP-TLS: ma=3600
This header field creates a commitment from the origin [RFC6454] of
the associated resource (in the example, "http://example.com"). For
the duration of the commitment, clients SHOULD strongly authenticate
the server for all subsequent requests made to that origin, though
this creates some risks for clients (see Section 5.2).
Authentication for HTTP over TLS is described in Section 3.1 of
[RFC2818], noting the additional requirements in Section 2.1 of
[I-D.ietf-httpbis-alt-svc]. The header field MUST be ignored if
strong authentication fails; otherwise, an attacker could create a
persistent denial of service by falsifying a commitment.
The commitment to use authenticated TLS persists for a period
determined by the value of the "ma" parameter. See Section 4.2.3 of
[RFC7234] for details of determining response age.
ma-parameter = delta-seconds
The commitment made by the "HTTP-TLS" header field applies only to
the origin of the resource that generates the "HTTP-TLS" header
field.
Requests for an origin that has a persisted, unexpired value for
"HTTP-TLS" MUST fail if they cannot be made over an authenticated TLS
connection.
Note that the commitment is not bound to a particular alternative
service. Clients SHOULD use alternative services that they become
aware of. However, clients MUST NOT use an unauthenticated
alternative service for an origin with this commitment. Where there
is an active commitment, clients MAY instead ignore advertisements
for unsecured alternatives services.
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5.2. Operational Considerations
To avoid situations where a persisted value of "HTTP-TLS" causes a
client to be unable to contact a site, clients SHOULD limit the time
that a value is persisted for a given origin. A lower limit might be
appropriate for initial observations of "HTTP-TLS"; the certainty
that a site has set a correct value - and the corresponding limit on
persistence - can increase as the value is seen more over time.
Once a server has indicated that it will support authenticated TLS, a
client MAY use key pinning [RFC7469] or any other mechanism that
would otherwise be restricted to use with "https" URIs, provided that
the mechanism can be restricted to a single HTTP origin.
6. Security Considerations
6.1. 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., using a "lock device").
6.2. Downgrade Attacks
A downgrade attack against the negotiation for TLS is possible. With
the "HTTP-TLS" header field, this is limited to occasions where
clients have no prior information (see Section 6.3), 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.
Downgrade attacks can be partially mitigated using the "HTTP-TLS"
header field, 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.
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6.3. 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).
6.4. 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.
7. References
7.1. Normative References
[I-D.ietf-httpbis-alt-svc]
mnot, m., McManus, P., and J. Reschke, "HTTP Alternative
Services", draft-ietf-httpbis-alt-svc-07 (work in
progress), May 2015.
[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>.
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454, DOI 10
.17487/RFC6454, December 2011,
<http://www.rfc-editor.org/info/rfc6454>.
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[RFC7230] Fielding, R. and J. Reschke, "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., Nottingham, M., and J. Reschke, "Hypertext
Transfer Protocol (HTTP/1.1): Caching", RFC 7234, DOI 10
.17487/RFC7234, June 2014,
<http://www.rfc-editor.org/info/rfc7234>.
[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>.
[RFC7540] Belshe, M., Peon, R., and M. Thomson, "Hypertext Transfer
Protocol Version 2 (HTTP/2)", RFC 7540, DOI 10.17487/
RFC7540, May 2015,
<http://www.rfc-editor.org/info/rfc7540>.
7.2. 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>.
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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