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<title>GnuTLS 3.5.3: Safe renegotiation</title>
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<a name="Safe-renegotiation"></a>
<div class="header">
<p>
Next: <a href="OCSP-status-request.html#OCSP-status-request" accesskey="n" rel="next">OCSP status request</a>, Previous: <a href="HeartBeat.html#HeartBeat" accesskey="p" rel="prev">HeartBeat</a>, Up: <a href="TLS-Extensions.html#TLS-Extensions" accesskey="u" rel="up">TLS Extensions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Function-and-Data-Index.html#Function-and-Data-Index" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="Safe-renegotiation-1"></a>
<h4 class="subsection">3.6.5 Safe renegotiation</h4>
<a name="index-renegotiation"></a>
<a name="index-safe-renegotiation"></a>
<p>TLS gives the option to two communicating parties to renegotiate
and update their security parameters. One useful example of this feature
was for a client to initially connect using anonymous negotiation to a
server, and the renegotiate using some authenticated ciphersuite. This occurred
to avoid having the client sending its credentials in the clear.
</p>
<p>However this renegotiation, as initially designed would not ensure that
the party one is renegotiating is the same as the one in the initial negotiation.
For example one server could forward all renegotiation traffic to an other
server who will see this traffic as an initial negotiation attempt.
</p>
<p>This might be seen as a valid design decision, but it seems it was
not widely known or understood, thus today some application protocols use the TLS
renegotiation feature in a manner that enables a malicious server to insert
content of his choice in the beginning of a TLS session.
</p>
<p>The most prominent vulnerability was with HTTPS. There servers request
a renegotiation to enforce an anonymous user to use a certificate in order
to access certain parts of a web site. The
attack works by having the attacker simulate a client and connect to a
server, with server-only authentication, and send some data intended
to cause harm. The server will then require renegotiation from him
in order to perform the request.
When the proper client attempts to contact the server,
the attacker hijacks that connection and forwards traffic to
the initial server that requested renegotiation. The
attacker will not be able to read the data exchanged between the
client and the server. However, the server will (incorrectly) assume
that the initial request sent by the attacker was sent by the now authenticated
client. The result is a prefix plain-text injection attack.
</p>
<p>The above is just one example. Other vulnerabilities exists that do
not rely on the TLS renegotiation to change the client’s authenticated
status (either TLS or application layer).
</p>
<p>While fixing these application protocols and implementations would be
one natural reaction, an extension to TLS has been designed that
cryptographically binds together any renegotiated handshakes with the
initial negotiation. When the extension is used, the attack is
detected and the session can be terminated. The extension is
specified in [<em>RFC5746</em>].
</p>
<p>GnuTLS supports the safe renegotiation extension. The default
behavior is as follows. Clients will attempt to negotiate the safe
renegotiation extension when talking to servers. Servers will accept
the extension when presented by clients. Clients and servers will
permit an initial handshake to complete even when the other side does
not support the safe renegotiation extension. Clients and servers
will refuse renegotiation attempts when the extension has not been
negotiated.
</p>
<p>Note that permitting clients to connect to servers when the safe
renegotiation extension is not enabled, is open up for attacks.
Changing this default behavior would prevent interoperability against
the majority of deployed servers out there. We will reconsider this
default behavior in the future when more servers have been upgraded.
Note that it is easy to configure clients to always require the safe
renegotiation extension from servers.
</p>
<p>To modify the default behavior, we have introduced some new priority
strings (see <a href="Priority-Strings.html#Priority-Strings">Priority Strings</a>).
The <code>%UNSAFE_RENEGOTIATION</code> priority string permits
(re-)handshakes even when the safe renegotiation extension was not
negotiated. The default behavior is <code>%PARTIAL_RENEGOTIATION</code> that will
prevent renegotiation with clients and servers not supporting the
extension. This is secure for servers but leaves clients vulnerable
to some attacks, but this is a trade-off between security and compatibility
with old servers. The <code>%SAFE_RENEGOTIATION</code> priority string makes
clients and servers require the extension for every handshake. The latter
is the most secure option for clients, at the cost of not being able
to connect to legacy servers. Servers will also deny clients that
do not support the extension from connecting.
</p>
<p>It is possible to disable use of the extension completely, in both
clients and servers, by using the <code>%DISABLE_SAFE_RENEGOTIATION</code>
priority string however we strongly recommend you to only do this for
debugging and test purposes.
</p>
<p>The default values if the flags above are not specified are:
</p><dl compact="compact">
<dt><code>Server:</code></dt>
<dd><p>%PARTIAL_RENEGOTIATION
</p>
</dd>
<dt><code>Client:</code></dt>
<dd><p>%PARTIAL_RENEGOTIATION
</p>
</dd>
</dl>
<p>For applications we have introduced a new API related to safe
renegotiation. The <a href="Core-TLS-API.html#gnutls_005fsafe_005frenegotiation_005fstatus">gnutls_safe_renegotiation_status</a> function is
used to check if the extension has been negotiated on a session, and
can be used both by clients and servers.
</p>
<hr>
<div class="header">
<p>
Next: <a href="OCSP-status-request.html#OCSP-status-request" accesskey="n" rel="next">OCSP status request</a>, Previous: <a href="HeartBeat.html#HeartBeat" accesskey="p" rel="prev">HeartBeat</a>, Up: <a href="TLS-Extensions.html#TLS-Extensions" accesskey="u" rel="up">TLS Extensions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Function-and-Data-Index.html#Function-and-Data-Index" title="Index" rel="index">Index</a>]</p>
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