Network Working Group E. Rescorla
Internet-Draft RTFM, Inc.
Intended status: Standards Track M. Ray
Expires: May 30, 2010 S. Dispensa
PhoneFactor
N. Oskov
Microsoft
November 26, 2009
Transport Layer Security (TLS) Renegotiation Indication Extension
draft-ietf-tls-renegotiation-01.txt
Abstract
SSL and TLS renegotiation are vulnerable to an attack in which the
attacker forms a TLS connection with the target server, injects
content of his choice, and then splices in a new TLS connection from
a client. The server treats the client's initial TLS handshake as a
renegotiation and thus believes that the initial data transmitted by
the attacker is from the same entity as the subsequent client data.
This draft defines a TLS extension to cryptographically tie
renegotiations to the TLS connections they are being performed over,
thus preventing this attack.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on May 30, 2010.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used In This Document . . . . . . . . . . . . . . . 4
3. Extension Definition . . . . . . . . . . . . . . . . . . . . . 4
4. Renegotiation Protection Request Cipher Suite . . . . . . . . . 5
5. Requirements for Sending and Receiving . . . . . . . . . . . . 5
6. Backward Compatibility . . . . . . . . . . . . . . . . . . . . 6
6.1. Client Considerations . . . . . . . . . . . . . . . . . . . 6
6.2. Server Considerations . . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
10.1. Normative References . . . . . . . . . . . . . . . . . . . 8
10.2. Informative References . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 8
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1. Introduction
TLS [RFC5246] allows either the client or the server to initiate
renegotiation--a new handshake which establishes new cryptographic
parameters. Unfortunately, although the new handshake is carried out
over the protected channel established by the original handshake,
there is no cryptographic connection between the two. This creates
the opportunity for an attack in which the attacker who can intercept
a client's transport layer connection can inject traffic of his own
as a prefix to the client's interaction with the server. The attack
proceeds as shown below:
Client Attacker Server
------ ------- ------
<----------- Handshake ---------->
<======= Initial Traffic ========>
<-------------------------- Handshake ============================>
<======================== Client Traffic ==========================>
To start the attack, the attacker forms a TLS connection to the
server (perhaps in response to an initial intercepted connection from
the client). He then sends any traffic of his choice to the server.
This may involve multiple requests and responses at the application
layer, or may simply be a partial application layer request intended
to prefix the client's data. This traffic is shown with == to
indicate it is encrypted. He then allows the client's TLS handshake
to proceed with the server. The handshake is in the clear to the
attacker but encrypted over the attacker's channel to the server.
Once the handshake has completed, the client communicates with the
server over the new channel. The attacker cannot read this traffic,
but the server believes that the initial traffic to and from the
attacker is the same as that to and from the client.
If certificate-based client authentication is used, the server will
believe that the initial traffic corresponds to the authenticated
client identity. Even without certificate-based authentication, a
variety of attacks may be possible in which the attacker convinces
the server to accept data from it as data from the client. For
instance, if HTTPS [RFC2818] is in use with HTTP cookies [REF], the
attacker may be able to generate a request of his choice validated by
the client's cookie.
This attack can be prevented by cryptographically binding
renegotiation handshakes to the enclosing TLS channel, thus allowing
the server to differentiate renegotiation from initial negotiation,
as well as preventing renegotiations from being spliced in between
connections. An attempt by an attacker to inject himself as
described above will result in a mismatch of the extension and can
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thus be detected This document defines an extension that performs
that cryptographic binding. The extension described here is similar
to that used for TLS Channel Bindings
[I-D.altman-tls-channel-bindings].
2. Conventions Used In This Document
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].
3. Extension Definition
This document defines a new TLS extension: "renegotiation_info",
which contains a cryptographic binding to the enclosing TLS
connection (if any) for which the renegotiation is being performed.
The "extension data" field of this extension contains a
"Renegotiation_Info" structure:
struct {
opaque renegotiated_connection<0..255>;
} Renegotiation_Info;
The contents of this extension are specified as follows.
o If this is the initial handshake for a connection, then the
"renegotiated_connection" field is of zero length in both the
ClientHello and the ServerHello. Thus, the entire encoding of the
extension is: ff 01 00 01 00. The first two octets represent the
extension type, the third and fourth octet the length of the
extension itself, and the final octet the zero length byte for the
"renegotiated_connection" field.
o For ClientHellos which are renegotiating, this field contains the
verify_data from the Finished message sent by the client on the
immediately previous handshake. For current versions of TLS, this
will be a 12-byte value. Note that this value is the "tls-unique"
channel binding from [I-D.altman-tls-channel-bindings]
o For ServerHellos which are renegotiating, this field contains the
concatenation of the verify_data values sent by the client and the
server (in that order) on the immediately previous handshake. For
current versions of TLS, this will be a 24-byte value.
The above rules apply even when TLS resumption is used.
Upon receipt of the "renegotiation_info" extension, both client and
server implementations which support the extension MUST verify that
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it contains the correct contents as specified above. If the contents
are incorrect, then it MUST generate a fatal "handshake_failure"
alert and terminate the connection. This allows two implementations
both of which support the extension to safely renegotiate without
fear of the above attack.
4. Renegotiation Protection Request Cipher Suite
Both the SSLv3 and TLS 1.0/TLS1.1 specifications require
implementations to ignore data following the ClientHello (i.e.,
extensions) if they do not understand it. However, some SSLv3 and
TLS 1.0 implementations incorrectly fail the handshake in such case.
This means that clients which offer "renegotiation_info" may find
handshake failures. In order to enhance compatibility with such
servers, this document defines a second signalling mechanism via a
special TLS cipher suite "TLS_RENEGO_PROTECTION_REQUEST", with code
point 0xNN, 0xMM. This cipher suite has exactly the same semantics
as an empty "renegotiation_info" extension. Because servers
ordinarily ignore unknown cipher suites, this cipher suite can be
added safely on any initial handshake, including SSLv2 backward
compatibility handshakes.
Servers MUST treat receipt of TLS_RENEGO_PROTECTION_REQUEST exactly
as if the client had sent an empty "renegotiation_info" extension and
respond with their own "renegotiation_info" extension. This is an
explicit exception to the RFC 5246 Section 7.4.1.4 prohibition on the
server sending unsolicited extensions and is only allowed because the
client is signaling its willingness to receive the extension via the
the TLS_RENEGO_PROTECTION_REQUEST cipher suite. TLS implementations
MUST continue to comply with 7.4.1.4 for all other extensions.
Servers MUST NOT select this cipher suite in any handshake, as it
does not correspond to any valid set of algorithms.
Because this cipher suite is equivalent to an empty
"renegotiation_info" extension, only renegotiation_info" may be used
rehandshakes.
Note that a minimal client which does not support renegotiation at
all can simply use this cipher suite in all initial handshakes. Any
compliant server will reject any (apparent) attempt at renegotiation
by such a client. Clients which do support renegotiation MUST
implement Section 3 as well.
5. Requirements for Sending and Receiving
TLS clients which support this draft MUST generate either the
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"renegotiation_info" extension or the TLS_RENEGO_PROTECTION_REQUEST
cipher suite with every ClientHello.
TLS servers which support this draft MUST generate the
"renegotiation_info" extension in the ServerHello in response to any
client which offers either "renegotiation_info" or
TLS_RENEGO_PROTECTION_REQUEST in the ClientHello.
6. Backward Compatibility
Existing implementations which do not support this extension are
widely deployed and in general must interoperate with newer
implementations which do support it. This section describes
considerations for backward compatible interoperation.
6.1. Client Considerations
If a client offers the "renegotiation_info" extension or the
TLS_RENEGO_PROTECTION_REQUEST cipher suite and the server does not
reply with "renegotiation_info" in the ServerHello, then this
indicates that the server either does not support secure
renegotiation or is unwilling to use it. Because the above attack
looks like a single handshake to the client, the client cannot
determine whether the connection is under attack or not. Note,
however, that merely because the server does not acknowledge the
extension does not mean that it is vulnerable; it might choose to
reject all rehandshakes and simply not signal it. However, it is not
possible for the client to determine purely via TLS mechanisms
whether this is the case or not.
If clients wish to ensure that such attacks are impossible, they MUST
terminate the connection immediately upon failure to receive the
extension without completing the handshake. However, it is expected
that many TLS servers that do not support renegotiation (and thus are
not vulnerable) will not support this extension either, so in
general, such behavior would not work well.
6.2. Server Considerations
If the client does not offer the "renegotiation_info" extension or
the TLS_RENEGO_PROTECTION_REQUEST cipher suite then this indicates
that the client does not support secure renegotiation or is unwilling
to use it. However, because the above attack looks like two
handshakes to the server, the server can safely continue the
connection as long as it does not allow the client to rehandshake.
If servers wish to ensure that such attacks are impossible they MUST
NOT allow clients who do not offer the "renegotiation_info" extension
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to renegotiate with them and SHOULD respond to such requests with a
"no_renegotiation" alert [RFC 5246 requires this alert to be at the
"warning" level.] Servers SHOULD follow this behavior.
In order to enable clients to probe, even servers which do not
support renegotiation SHOULD implement the minimal version of the
extension described in this document for initial handshakes, thus
signalling that they have been upgraded.
7. Security Considerations
The extension described in this document prevents an attack on TLS.
If this extension is not used, TLS renegotiation is subject to an
attack in which the attacker can inject their own conversation with
the TLS server as a prefix of the client's conversation. This attack
is invisible to the client and looks like an ordinary renegotiation
to the server. The extension defined in this document allows
renegotiation to be performed safely. Servers SHOULD NOT allow
clients to renegotiate without using this extension.
While this extension mitigates the man-in-the-middle attack described
in the overview, it does not resolve all possible problems an
application may face if it is unaware of renegotiation. It is
possible that the authenticated identity of the server or client may
change as a result of renegotiation.
By default, TLS implementations conforming to this document MUST
verify that once the peer has been identified and authenticated
within the TLS handshake, the identity does not change on subsequent
renegotiations. For certificate based cipher suites, this means
bitwise equality of the end-entity certificate. If the other end
attempts to authenticate with a different identity, the renegotiation
MUST fail. If the server_name extension is used, it MUST NOT change
when doing renegotiation.
A TLS library MAY provide a means for the application to allow
identity and/or server_name changes across renegotiations, in which
case the application is responsible for tracking the identity
associated with data it is processing. This may require additional
API facilities in the TLS library.
8. IANA Considerations
IANA [shall add/has added] the extension code point XXX [We request
0xff01, which has been used for prototype implementations] for the
"renegotiation_info" extension to the TLS ExtensionType values
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registry.
IANA [shall add/has added] TLS cipher suite number 0xNN,0xMM with
name TLS_RENEGO_PROTECTION_REQUEST to the TLS Cipher Suite registry.
9. Acknowledgements
This vulnerability was originally discovered by Marsh Ray. The
general concept behind the extension described here was independently
invented by Steve Dispensa, Nasko Oskov, and Eric Rescorla with
refinements from Nelson Bolyard, Pasi Eronen, Mike D'Errico, Bodo
Moeller, Martin Rex (who defined TLS_RENEGO_PROTECTION_REQUEST),
Jesse Walker, Nico Williams and other members of the the Project
Mogul team and the TLS WG. [Note: if you think your name should be
here, please speak up.]
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
10.2. Informative References
[I-D.altman-tls-channel-bindings]
Altman, J., Williams, N., and L. Zhu, "Channel Bindings
for TLS", draft-altman-tls-channel-bindings-07 (work in
progress), October 2009.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
Authors' Addresses
Eric Rescorla
RTFM, Inc.
2064 Edgewood Drive
Palo Alto, CA 94303
USA
Email: ekr@rtfm.com
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Marsh Ray
PhoneFactor
7301 W 129th Street
Overland Park, KS 66213
USA
Email: marsh@extendedsubset.com
Steve Dispensa
PhoneFactor
7301 W 129th Street
Overland Park, KS 66213
USA
Email: dispensa@phonefactor.com
Nasko Oskov
Microsoft
One Microsoft Way
Redmond, WA 98052
USA
Email: nasko.oskov@microsoft.com
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