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<a href="https://gitlab.com/pfg666">Paul</a> created an issue: <a href="https://gitlab.com/gnutls/gnutls/-/issues/1233">#1233</a>
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<h2 dir="auto">
<a id="user-content-description-of-problem" class="anchor" href="#description-of-problem" aria-hidden="true"></a>Description of problem:</h2>
<p dir="auto">GnuTLS does not validate message sequence numbers in ClientHello messages.
According to <a href="https://www.rfc-editor.org/rfc/rfc6347.html#page-18" rel="nofollow noreferrer noopener" target="_blank">DTLS RFC</a>:</p>
<blockquote dir="auto">
<p>The first message each side transmits in each handshake always has message_seq = 0. Whenever each new message is generated, the message_seq value is incremented by one.</p>
</blockquote>
<p dir="auto">We found that GnuTLS does not check for message_seq to be 0 in a ClientHello delivered in the middle of an on-going handshake.</p>
<h2 dir="auto">
<a id="user-content-version-of-gnutls-used" class="anchor" href="#version-of-gnutls-used" aria-hidden="true"></a>Version of gnutls used:</h2>
<p dir="auto">3.7.1</p>
<h2 dir="auto">
<a id="user-content-operating-system" class="anchor" href="#operating-system" aria-hidden="true"></a>Operating System</h2>
<p dir="auto">Ubuntu 20</p>
<h2 dir="auto">
<a id="user-content-how-reproducible" class="anchor" href="#how-reproducible" aria-hidden="true"></a>How reproducible:</h2>
<p dir="auto">I attached files necessary for reproduction using <a href="https://github.com/assist-project/dtls-fuzzer/" rel="nofollow noreferrer noopener" target="_blank">DTLS-fuzzer</a>, a Java-based tool for testing DTLS libraries, whose .jar is included in the archive. Also included is a capture of the interaction, generated on my machine. DTLS-fuzzer requires the JDK for Java 8. On Ubuntu, this can be installed by running:
<code>sudo apt-get install openjdk-8-jdk</code></p>
<p dir="auto">Unpack the archive at the end of this post, <code>cd</code> to resulting folder, download to this folder the .jar of DTLS-fuzzer available <a href="https://github.com/pfg666/reproduction/blob/main/dtls-fuzzer.jar" rel="nofollow noreferrer noopener" target="_blank">here</a>, and run <code>bash reproduce.sh </code>, while running an instance of Wireshark on the side. The reproduction script will:</p>
<ul dir="auto">
<li>launch a gnutls-serv server instance</li>
<li>execute a test exposing the behavior using DTLS-fuzzer.</li>
</ul>
<p dir="auto">It assumes <code>gnutls-serv</code> is present in the PATH.</p>
<h2 dir="auto">
<a id="user-content-actual-results" class="anchor" href="#actual-results" aria-hidden="true"></a>Actual results:</h2>
<p dir="auto">If everything works as planned, Wireshark should show an interaction similar to that in the image below:
<a class="no-attachment-icon gfm" href="https://gitlab.com/gnutls/gnutls/uploads/cba60d807a4f7486b7ec81926c02ffb3/handshake_restart.png" target="_blank" rel="noopener noreferrer" data-link="true"><img src="https://gitlab.com/gnutls/gnutls/uploads/cba60d807a4f7486b7ec81926c02ffb3/handshake_restart.png" alt="handshake_restart" class="gfm" style="max-width: 100%; height: auto;"></a></p>
<p dir="auto">Therein, if we check the value of the highlighted restarting ClientHello message, we see:
<a class="no-attachment-icon gfm" href="https://gitlab.com/gnutls/gnutls/uploads/31b4c6a9e60dc748d4bf89a065a7a788/handshake_restart_mseq.png" target="_blank" rel="noopener noreferrer" data-link="true"><img src="https://gitlab.com/gnutls/gnutls/uploads/31b4c6a9e60dc748d4bf89a065a7a788/handshake_restart_mseq.png" alt="handshake_restart_mseq" class="gfm" style="max-width: 100%; height: auto;"></a></p>
<p dir="auto">BTW, I had to pack dtls-fuzzer separately since its .jar is too large.</p>
<h2 dir="auto">
<a id="user-content-expected-results" class="anchor" href="#expected-results" aria-hidden="true"></a>Expected results:</h2>
<p dir="auto">The server should not have restarted the handshake using this message.</p>
<p dir="auto">Thanks!
<a href="https://gitlab.com/gnutls/gnutls/uploads/848784142be5fb0e425d95f3f33e4e09/handshake_restarted.tar.gz" data-link="true" class="gfm">handshake_restarted.tar.gz</a></p>
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