Network Working Group E. Rescorla
Internet-Draft RTFM, Inc.
Intended status: Standards Track M. Ray
Expires: May 23, 2010 S. Dispensa
PhoneFactor
N. Oskov
Microsoft
November 19, 2009
Transport Layer Security (TLS) Renegotiation Indication Extension
draft-ietf-tls-renegotiation-00.txt
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Copyright Notice
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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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Conventions Used In This Document . . . . . . . . . . . . . . . 5
3. Extension Definition . . . . . . . . . . . . . . . . . . . . . 5
4. Backward Compatibility . . . . . . . . . . . . . . . . . . . . 6
4.1. Client Considerations . . . . . . . . . . . . . . . . . . . 6
4.1.1. Renegotiation Info Support Cipher Suite . . . . . . . . 7
4.2. Server Considerations . . . . . . . . . . . . . . . . . . . 7
4.3. Broken Servers . . . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 9
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative References . . . . . . . . . . . . . . . . . . . 9
8.2. Informative References . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
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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;
All TLS implementations SHOULD support this extension. TLS clients
SHOULD generate it with every handshake and TLS servers SHOULD
generate it in response to any client which offers it.
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.
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The above rules apply even when TLS resumption is used.
Upon receipt of the "renegotiation_info" extension, implementations
which support the extension MUST verify that 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. 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.
4.1. Client Considerations
If a client offers the "renegotiation_info" extension and the server
does not respond, then this indicates that the server either does not
support the extension 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. 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.
While this specification does not require the client to send RI on
initial handshakes, clients which choose not to do so have no
mechanism for determining whether the server is operating in a
vulnerable mode (and provide no such indication to the server) and
are therefore relying entirely on the server refusing to renegotiate
in the absence of the extension as opposed to explicitly indicating
to the server that the initial handshake is in fact the first one on
the connection.
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4.1.1. Renegotiation Info Support Cipher Suite
[[OPEN ISSUE: Should this feature be added?]]
This document defines a special TLS cipher suite
"RENEGOTIATE_WITH_RI", with code point 0xNN, 0xMM. Clients which
support this extension MUST include this cipher suite in every
handshake. When the server receives a Client Hello message which (1)
contains this cipher suite (2) does not include RI and (3) the server
thinks is doing renegotiation it MUST reject it with a fatal
"handshake_failure" alert. Because servers ordinarily ignore unknown
cipher suites, this cipher suite can be added safely on any
handshake, thus allowing detection and prevention of the MITM attack
described above. Servers MUST NOT select this cipher suite in any
handshake, as it does not correspond to any valid cipher suite.
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.
4.2. Server Considerations
If the client does not offer the "renegotiation_info" extension, then
this indicates that the client does not support the extension or is
unwilling to use it. Note that TLS does not permit servers to offer
unsolicited extensions. 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
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.
4.3. Broken Servers
SSLv3 does not support extensions and thus it is not possible to Both
SSLv3 (at least later drafts) and TLS 1.0/1.1 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. Client
implementations which offer extensions sometimes will respond to such
failures by falling back to an extensionless mode. This practice can
be exploited by a MITM to cause a client which would ordinarily offer
the renegotiation extension not to do so. Note that this
consideration does not apply to implementations which ignore
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extensions since the ordinarily TLS Finished messages protect that
negotiation.
When combined with a server which allows renegotiation without the
extension (which, per Section 5, is NOT RECOMMENDED) this allows a
downgrade attack. Accordingly, clients which offer this extension
SHOULD NOT fall back to extensionless modes upon handshake errors.
Any SSLv3 or TLS implementation which chooses to address this issue
by refusing to renegotiate at all MUST at minimum ensure that the
extension is ignored (this is simply a restatement of existing
requirements). Thus, any such handshake failure can be assumed to
represent either an attack or a vulnerable server; in either case the
best practice is not to continue the connection. Even servers which
refuse to renegotiate SHOULD reply with an empty RI extension because
this signals that they have been upgraded.
5. 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 an identity has been authenticated within the TLS
handshake, it 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.
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6. 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
registry.
IANA [shall add/has added] TLS cipher suite number 0xNN,0xMM with
name RENEGOTIATE_WITH_RI to the TLS Cipher Suite registry.
7. 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. Comments
and refinements were received from Jesse Walker, the Project Mogul
team as well as Pasio Eronen.
8. References
8.1. 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.
8.2. 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.
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Authors' Addresses
Eric Rescorla
RTFM, Inc.
2064 Edgewood Drive
Palo Alto, CA 94303
USA
Email: ekr@rtfm.com
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
Microsoft
One Microsoft Way
Redmond, WA 98052
USA
Email: nasko.oskov@microsoft.com
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