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RFC 8164
HTTP Working Group M. Nottingham
Internet-Draft
Intended status: Experimental M. Thomson
Expires: December 23, 2016 Mozilla
June 21, 2016
Opportunistic Security for HTTP
draft-ietf-httpbis-http2-encryption-06
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 December 23, 2016.
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
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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
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 . . . . . . . . . . . . . . . . . . . . 6
5.1. Opportunistic Commitment . . . . . . . . . . . . . . . . 6
5.2. Client Handling of A Commitment . . . . . . . . . . . . . 7
5.3. Operational Considerations . . . . . . . . . . . . . . . 7
6. The "http-opportunistic" well-known URI . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8.1. Security Indicators . . . . . . . . . . . . . . . . . . . 9
8.2. Downgrade Attacks . . . . . . . . . . . . . . . . . . . . 9
8.3. Privacy Considerations . . . . . . . . . . . . . . . . . 9
8.4. Confusion Regarding Request Scheme . . . . . . . . . . . 9
8.5. Server Controls . . . . . . . . . . . . . . . . . . . . . 10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
9.1. Normative References . . . . . . . . . . . . . . . . . . 10
9.2. Informative References . . . . . . . . . . . . . . . . . 11
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
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 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").
A mechanism for partially mitigating active attacks 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 [RFC7838] 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 [RFC7838]).
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, and therefore clients creating new TLS connections to
alternative services for the purposes of this specification MUST NOT
present them. Established connections with client certificates MAY
be reused, however.
3. Server Authentication
[RFC7838] 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, a client can establish reasonable
assurances when using a TLS-based protocol with the certificate
checks defined in [RFC2818].
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 a client has a valid http-opportunistic response for an origin
(as per Section 6), it MAY consider there to be reasonable assurances
as long as:
o The origin and alternative service's hostnames are the same when
compared in a case-insensitive fashion, and
o The origin object of the http-opportunistic response has a `tls-
ports' member, whose value is an array of numbers, one of which
matches the port of the alternative service in question, and
o The chosen alternative service returns the same representation as
the origin did for the http-opportunistic resource.
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For example, this request/response pair would constitute reasonable
assurances for the origin "http://www.example.com" for an alternative
service on port 443 or 8000 of the host "www.example.com":
GET /.well-known/http-opportunistic HTTP/1.1
Host: www.example.com
HTTP/1.1 200 OK
Content-Type: application/json
Connection: close
{
"http://www.example.com": {
"tls-ports": [443, 8000],
"lifetime": 2592000
}
}
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
Section 2.1 of [RFC7838] 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.
Because of the risk of server confusion about individual requests'
schemes (see Section 8.4), clients MUST NOT mix "https" and "http"
requests on the same connection unless the http-opportunistic
response's origin object Section 6 has a "mixed-scheme" member whose
value is "true".
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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
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 a secured, authenticated
alternative service. This is done by including the optional "tls-
commit" member in the origin object of the http-opportunistic well-
known response (see Section 6).
This feature is optional due to the requirement for server
authentication and the potential risk entailed (see Section 5.3).
When the value of the "tls-commit" member is "true" ([RFC7159],
Section 3), it indicates that the origin makes such a commitment for
the duration of the origin object lifetime.
{
"http://www.example.com": {
"tls-ports": [443,8080],
"tls-commit": true,
"lifetime": 3600
}
}
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Including "tls-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 origin object lifetime. Clients might however
choose to limit this time (see Section 5.3).
5.2. Client Handling of A Commitment
The value of the "tls-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
[RFC7838] and Section 3.1 of [RFC2818]. As noted in [RFC7838],
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 origin object lifetime.
A client SHOULD avoid sending requests via cleartext protocols or to
unauthenticated alternative services for the duration of the origin
object lifetime, except to discover new potential alternatives.
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 alternative service without both
reasonable assurances (see Section 3) and strong authentication.
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;
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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]. 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 object as the root, and
o The root object contains a member whose name is a case-insensitive
character-for-character match for the origin in question,
serialised into Unicode as per Section 6.1 of [RFC6454], and whose
value is an object (hereafter, the "origin object"),
o The origin object has a "lifetime" member, whose value is a number
indicating the number of seconds which the origin object is valid
for (hereafter, the "origin object lifetime"), and
o The origin object lifetime is greater than the "current_age" (as
per [RFC7234], Section 4.2.3).
Note that origin object lifetime might differ from the freshness
lifetime of the response.
7. IANA Considerations
This specification registers a Well-Known URI [RFC5785]:
o URI Suffix: http-opportunistic
o Change Controller: IETF
o Specification Document(s): Section 6 of [this specification]
o Related Information:
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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 (see 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 [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.
Downgrade attacks can be partially mitigated using the "tls-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
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
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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.
8.5. Server Controls
Because this specification allows "reasonable assurances" to be
established by the content of a well-known URI, servers SHOULD take
suitable measures to assure that its content 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.
9. References
9.1. 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>.
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[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>.
[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>.
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>.
[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
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
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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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