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<a href="https://gitlab.com/codesquid">Tim Kosse</a> created an issue: <a href="https://gitlab.com/gnutls/gnutls/-/issues/1303">#1303</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">If the preferred side (as per session->internals.priorities->server_precedence) only supports one algorithm and it is not the first in the other side's list of algorithms, psk_ke_modes_recv_params wrongly sets session->internals.hsk_flags to HSK_PSK_KE_MODE_INVALID.</p>
<p dir="auto">Observed in GnuTLS 3.7.2 compiled from the official source tarball without any special configure arguments.</p>
<p dir="auto">The issue was discovered while analyzing <a href="https://forum.filezilla-project.org/viewtopic.php?t=54333" rel="nofollow noreferrer noopener" target="_blank">https://forum.filezilla-project.org/viewtopic.php?t=54333</a></p>
<h2 dir="auto">
<a id="user-content-how-to-reproduce" class="anchor" href="#how-to-reproduce" aria-hidden="true"></a>How to reproduce:</h2>
<p dir="auto">You can easily reproduce this issue:</p>
<ul dir="auto">
<li>Applying the attached <a href="https://gitlab.com/gnutls/gnutls/uploads/e553e1da4aacce21eccca71f898fbe3f/reproducer.diff" data-canonical-src="/uploads/e553e1da4aacce21eccca71f898fbe3f/reproducer.diff" data-link="true" class="gfm">reproducer.diff</a>, which just swaps the algorithms in the psk_key_exchange_modes extension Client Hello.</li>
<li>Run <code>gnutls-serv -d9999</code>
</li>
<li>Connect to it with <code>gnutls-cli 127.0.0.1 -p 5556</code>
</li>
<li>In the output of gnutls-serv look for <code>|<3>| ASSERT: psk_ke_modes.c[psk_ke_modes_recv_params]:192</code> which is printed when HSK_PSK_KE_MODE_INVALID is set.</li>
</ul>
<p dir="auto">In this scenario the following happens in psk_ke_modes.c:</p>
<ul dir="auto">
<li>Line 156 is reached.</li>
<li>By line 174, the following values are held:
<ul>
<li>session->internals.priorities->server_precedence is false</li>
<li>dhpsk_pos is 0</li>
<li>psk_pos is MAX_POS</li>
<li>cli_dhpsk_pos is 1</li>
<li>cli_psk_pos is 0</li>
</ul>
</li>
<li>As result, neither mode is set in session->internals.hsk_flags and line 191 is reached.</li>
</ul>
<h2 dir="auto">
<a id="user-content-proposed-patch" class="anchor" href="#proposed-patch" aria-hidden="true"></a>Proposed patch:</h2>
<p dir="auto">I have attached a simple fix for the issue in <a href="https://gitlab.com/gnutls/gnutls/uploads/305bdcc93e5af97051116813e06dfbaa/psk_ke_modes_send_params.diff" data-canonical-src="/uploads/305bdcc93e5af97051116813e06dfbaa/psk_ke_modes_send_params.diff" data-link="true" class="gfm">psk_ke_modes_send_params.diff</a></p>
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