Internet DRAFT - draft-thomson-http-replay
draft-thomson-http-replay
httpbis M. Thomson
Internet-Draft Mozilla
Intended status: Standards Track M. Nottingham
Expires: February 10, 2018 true
W. Tarreau
HAProxy Technologies
August 09, 2017
Using Early Data in HTTP
draft-thomson-http-replay-01
Abstract
This document explains the risks of using early data for HTTP and
describes techniques for reducing them. In particular, it defines a
mechanism that enables clients to communicate with servers about
early data, to assure correct operation.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on February 10, 2018.
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the Trust Legal Provisions and are provided without warranty as
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Conventions and Definitions . . . . . . . . . . . . . . . 3
2. Early Data in HTTP . . . . . . . . . . . . . . . . . . . . . 3
3. Supporting Early Data in HTTP Servers . . . . . . . . . . . . 3
4. Using Early Data in HTTP Clients . . . . . . . . . . . . . . 5
5. Extensions for Early Data in HTTP . . . . . . . . . . . . . . 6
5.1. The Early-Data Header Field . . . . . . . . . . . . . . . 6
5.2. The 4NN (Too Early) Status Code . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6.1. Gateways and Early Data . . . . . . . . . . . . . . . . . 8
6.2. Consistent Handling of Early Data . . . . . . . . . . . . 8
6.3. Denial of Service . . . . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative References . . . . . . . . . . . . . . . . . . 9
8.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
TLS 1.3 [TLS13] introduces the concept of early data (also known as
zero round trip data or 0-RTT data). Early data allows a client to
send data to a server in the first round trip of a connection,
without waiting for the TLS handshake to complete if the client has
spoken to the same server recently.
When used with HTTP [HTTP], early data allows clients to send
requests immediately, avoiding the one or two round trip delay needed
for the TLS handshake. This is a significant performance
enhancement; however, it has significant limitations.
The primary risk of using early data is that an attacker might
capture and replay the request(s) it contains. TLS [TLS13] describes
techniques that can be used to reduce the likelihood that an attacker
can successfully replay a request, but these techniques can be
difficult to deploy, and still leave some possibility of a successful
attack.
Note that this is different from automated or user-initiated retries;
replays are initiated by an attacker without the awareness of the
client.
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To help mitigate the risk of replays in HTTP, this document gives an
overview of techniques for controlling these risks in servers, and
defines requirements for clients when sending requests in early data.
The advice in this document also applies to use of 0-RTT in HTTP over
QUIC [HQ].
1.1. Conventions and Definitions
The words "MUST", "MUST NOT", "SHOULD", and "MAY" are used in this
document. It's not shouting; when they are capitalized, they have
the special meaning defined in [RFC2119].
2. Early Data in HTTP
Conceptually, early data is concatenated with other application to
form a single stream. This can mean that requests are entirely
contained within early data, or only part of a request is early. In
a multiplexed protocol, like HTTP/2 [RFC7540] or HTTP/QUIC [HQ],
multiple requests might be partially delivered in early data.
The model that this document assumes is that once the TLS handshake
completes, early data is not replayed. However, it is important to
note that this does not mean that early data will not be or has not
been replayed on another connection.
3. Supporting Early Data in HTTP Servers
A server decides whether or not to offer a client the ability to send
early data on future connections when sending the TLS session ticket.
When a server enables early data, there are a number of techniques it
can use to mitigate the risks of replay:
1. The server can choose whether it will process early data before
the TLS handshake completes. By deferring processing, it can
ensure that only a successfully completed connection is used for
the request(s) therein. Assuming that a replayed ClientHello
will not result in additional connections being made by the
client, this provides the server with some assurance that the
early data was not replayed.
2. If the server receives multiple requests in early data, it can
determine whether to defer HTTP processing on a per-request
basis. This may require buffering the responses to preserve
ordering in HTTP/1.1.
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3. The server can cause a client to retry a request and not use
early data by responding with the 4NN (Too Early) status code
(Section 5.2), in cases where the risk of replay is judged too
great.
4. Finally, TLS [TLS13] describes several mitigation strategies that
reduce the ability of an attacker to successfully replay early
data. Servers are strongly encouraged to implement these
techniques, but to also recognize that they are imperfect. These
anti-replay techniques can reduce the number of replays that will
be successful from being essentially unbounded to a fixed value.
For a given request, the level of tolerance to replay risk is
specific to the resource it operates upon (and therefore only known
to the origin server). In general, if processing a request does not
have state-changing side effects, the consequences of replay are not
significant.
The request method's safety ([RFC7231], Section 4.2.1) is one way to
determine this. However, some resources do elect to associate side
effects with safe methods, so this cannot be universally relied upon.
It is RECOMMENDED that origin servers allow resources to explicitly
configure whether early data is appropriate in requests. Absent such
explicit information, they SHOULD mitigate against early data in
requests that have unsafe methods, using the techniques outlined
above.
A request might be sent partially in early data with the remainder of
the request being sent after the handshake completes. This does not
necessarily affect handling of that request; what matters is when the
server starts acting upon the contents of a request. Any time a
server might initiate processing prior to completion of the handshake
needs to consider how a possible replay of early data could affect
that processing (see also Section 6.2).
A server can partially process requests that are incomplete. Parsing
header fields - without acting on the values - and determining
request routing is likely to be safe from side-effects, but other
actions might not be.
Intermediary servers do not have sufficient information to make this
determination, so Section 5.2 describes a way for the origin to
signal to them that a particular request isn't appropriate for early
data. Intermediaries that accept early data MUST implement that
mechanism.
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Note that a server cannot choose to selectively reject early data at
the TLS layer. TLS only permits a server to accept all early data,
or none of it. Once a server has decided to accept early data, it
MUST process all requests in early data, even if the server rejects
the request by sending a 4NN (Too Early) response.
A server can limit the amount of early data with the
"max_early_data_size" field of the "early_data" TLS extension. This
can be used to avoid committing an arbitrary amount of memory for
deferred requests. A server SHOULD ensure that when it accepts early
data, it can defer processing of requests until after the TLS
handshake completes.
4. Using Early Data in HTTP Clients
A client that wishes to use early data commences sending HTTP
requests immediately after sending the TLS ClientHello.
By their nature, clients have control over whether a given request is
sent in early data - thereby giving the client control over risk of
replay. Absent other information, clients MAY send requests with
safe HTTP methods (see [RFC7231], Section 4.2.1) in early data when
it is available, and SHOULD NOT send unsafe methods (or methods whose
safety is not known) in early data.
If the server rejects early data at the TLS layer, a client MUST
start sending again as though the connection was new. For HTTP/2,
this means re-sending the connection preface. Any requests sent in
early data MUST be sent again, unless the client decides to abandon
those requests.
This automatic retry exposes the request to a potential replay
attack. An attacker sends early data to one server instance that
accepts and processes the early data, but allows that connection to
proceed no further. The attacker then forwards the same messages
from the client to another server instance that will reject early
data. The client the retries the request, resulting in the request
being processed twice. Replays are also possible if there are
multiple server instances that will accept early data, or if the same
server accepts early data multiple times (though this would be in
violation of requirements in TLS).
Clients that use early data MUST retry requests upon receipt of a 4NN
(Too Early) status code; see Section 5.2.
An intermediary MUST NOT use early data when forwarding a request
unless early data was used on a previous hop, or it knows that the
request can be retried safely without consequences (typically, using
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out-of-band configuration). Absent better information, that means
that an intermediary can only use early data if the request either
arrived in early data or arrived with the "Early-Data" header field
set to "1".
5. Extensions for Early Data in HTTP
Because HTTP requests can span multiple "hops", it is necessary to
explicitly communicate whether a request has been sent in early data
on a previous connection. Likewise, some means of explicitly
triggering a retry when early data is not desirable is necessary.
Finally, it is necessary to know whether the client will actually
perform such a retry.
To meet these needs, two signalling mechanisms are defined:
o The "Early-Data" header field is included in requests that are
received in early data.
o The 4NN (Too Early) status code is defined for an server to
indicate that a request could not be processed due to the
consequences of a possible replay attack.
They are designed to enable better coordination of the use of early
data between the user agent and origin server, and also when a
gateway (also "reverse proxy", "Content Delivery Network", or
"surrogate") is present.
Gateways typically don't have specific information about whether a
given request can be processed safely when it is sent in early data.
In many cases, only the origin server has the necessary information
to decide whether the risk of replay is acceptable. These extensions
allow coordination between a gateway and its origin server.
5.1. The Early-Data Header Field
The "Early-Data" request header field indicates that the request has
been conveyed in early data, and additionally indicates that a client
understands the 4NN (Too Early) status code.
It has just one valid value: "1". Its syntax is defined by the
following ABNF [ABNF]:
Early-Data = "1"
For example:
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GET /resource HTTP/1.0
Host: example.com
Early-Data: 1
An intermediary that forwards a request received in TLS early data
MUST send it with the "Early-Data" header field set to "1" (i.e., it
adds it if not present in the request).
An intermediary MUST NOT remove this header field if it is present in
a request.
The "Early-Data" header field is not intended for use by user agents
(that is, the original initiator of a request). Sending a request in
early data implies that the client understands this specification and
is willing to retry a request in response to a 4NN (Too Early) status
code. A user agent that sends a request in early data does not need
to include the "Early-Data" header field.
A server cannot make a request that contains the Early-Data header
field safe for processing by waiting for the handshake to complete.
A request that is marked with Early-Data was sent in early data on a
previous hop. Requests that contain the Early-Data field and cannot
be safely processed MUST be rejected using the 4NN (Too Early) status
code.
5.2. The 4NN (Too Early) Status Code
A 4NN (Too Early) status code indicates that the server is unwilling
to risk processing a request that might be replayed.
Clients (user-agents and intermediaries) that sent the request in
early data MUST automatically retry the request when receiving a 4NN
(Too Early) response status code. Such retries MUST NOT be sent in
early data, and SHOULD NOT be sent if the TLS handshake on the
original connection does not successfully complete.
Intermediaries that receive a 4NN (Too Early) status code MAY
automatically retry requests after allowing the handshake to complete
unless the original request contained the "Early-Data" header field
when it was received. Otherwise, an intermediary MUST forward the
4NN (Too Early) status code.
The server cannot assume that a client is able to retry a request
unless the request is received in early data or the "Early-Data"
header field is set to "1". A server SHOULD NOT emit the 4NN status
code unless one of these conditions is met.
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The 4NN (Too Early) status code is not cacheable by default. Its
payload is not the representation of any identified resource.
6. Security Considerations
Using early data exposes a client to the risk that their request is
replayed. A retried or replayed request can produce different side
effects on the server. In addition to those side effects, replays
and retries might be used for traffic analysis to recover information
about requests or the resources those requests target.
6.1. Gateways and Early Data
A gateway that forwards requests that were received in early data
MUST only do so if it knows that the server that receives those
requests understands the "Early-Data" header field and will correctly
generate a 4NN (Too Early) status code. A gateway that isn't certain
about server support SHOULD either delay forwarding the request until
the TLS handshake completes, or send a 4NN (Too Early) status code in
response. A gateway that is uncertain about whether an origin server
supports the "Early-Data" header field SHOULD disable early data.
6.2. Consistent Handling of Early Data
Consistent treatment of a request that arrives in - or partially in -
early data is critical to avoiding inappropriate processing of
replayed requests. If a request is not safe to process before the
TLS handshake completes, then all instances of the server need to
agree and either reject the request or delay processing.
6.3. Denial of Service
Accepting early data exposes a server to potential denial of service
through the replay of requests that are expensive to handle. A
server that is under load SHOULD prefer rejecting TLS early data as a
whole rather than accepting early data and selectively processing
requests. Generating a 503 (Service Unavailable) or 4NN (Too Early)
status code often leads to clients retrying requests, which could
result in increased load.
7. IANA Considerations
This document registers the "Early-Data" header field in the "Message
Headers" registry [HEADERS].
Header field name: Early-Data
Applicable protocol: http
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Status: standard
Author/Change controller: IETF
Specification document(s): This document
Related information: (empty)
This document registers the 4NN (Too Early) status code in the
"Hypertext Transfer Protocol (HTTP) Status Code" registry established
in [RFC7231].
Value: 4NN
Description: Too Early
Reference: This document
8. References
8.1. Normative References
[ABNF] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<http://www.rfc-editor.org/info/rfc5234>.
[HEADERS] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
DOI 10.17487/RFC3864, September 2004,
<http://www.rfc-editor.org/info/rfc3864>.
[HTTP] 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>.
[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>.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
<http://www.rfc-editor.org/info/rfc7231>.
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[TLS13] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", draft-ietf-tls-tls13-21 (work in progress),
July 2017.
8.2. Informative References
[HQ] Bishop, M., "Hypertext Transfer Protocol (HTTP) over
QUIC", draft-ietf-quic-http-04 (work in progress), June
2017.
[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>.
Appendix A. Acknowledgments
This document was not easy to produce. The following people made
substantial contributions to the quality and completeness of the
document: Subodh Iyengar, Benjamin Kaduk, Ilari Liusavaara, Kazuho
Oku, and Victor Vasiliev.
Authors' Addresses
Martin Thomson
Mozilla
Email: martin.thomson@gmail.com
Mark Nottingham
true
Email: mnot@mnot.net
Willy Tarreau
HAProxy Technologies
Email: willy@haproxy.org
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