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<a href="https://gitlab.com/robertsuska">Robert Suska</a> created an issue: <a href="https://gitlab.com/gnutls/gnutls/-/issues/1223">#1223</a>
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<a id="user-content-description-of-problem" class="anchor" href="#description-of-problem" aria-hidden="true"></a>Description of problem:</h2>
<p dir="auto">I've noticed that when GnuTLS checks name constraints during validation of X.509 certificates, it performs a case-sensitive matching on DNS names. I think this is a bug, since <a href="https://www.rfc-editor.org/rfc/rfc5280.txt" rel="nofollow noreferrer noopener" target="_blank">RFC 5280</a> (article 7.2) says <em>"When evaluating name constraints, conforming implementations MUST perform a case-insensitive exact match on a label-by-label basis."</em>.</p>
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<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">3.6.14</p>
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<a id="user-content-distributor-of-gnutls-eg-ubuntu-fedora-rhel" class="anchor" href="#distributor-of-gnutls-eg-ubuntu-fedora-rhel" aria-hidden="true"></a>Distributor of gnutls (e.g., Ubuntu, Fedora, RHEL)</h2>
<p dir="auto">RHEL 8.3</p>
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<a id="user-content-how-reproducible" class="anchor" href="#how-reproducible" aria-hidden="true"></a>How reproducible:</h2>
<p dir="auto"><a href="https://gitlab.com/gnutls/gnutls/uploads/fb73fc696d5db4bf969a6e7556c5046d/example_chain.zip" data-link="true" class="gfm">example_chain.zip</a> contains three certificates, where</p>
<p dir="auto"><em>root_ca.pem</em> specifies a single name constraint:</p>
<pre class="code highlight js-syntax-highlight language-plaintext" lang="plaintext" v-pre="true"><code><span id="LC1" class="line" lang="plaintext">X509v3 Name Constraints: critical</span>
<span id="LC2" class="line" lang="plaintext"> Permitted:</span>
<span id="LC3" class="line" lang="plaintext"> DNS:example.com</span></code></pre>
<p dir="auto"><em>server_ok.pem</em> has an alternative name:</p>
<pre class="code highlight js-syntax-highlight language-plaintext" lang="plaintext" v-pre="true"><code><span id="LC1" class="line" lang="plaintext">X509v3 Subject Alternative Name:</span>
<span id="LC2" class="line" lang="plaintext"> DNS:test.example.com</span></code></pre>
<p dir="auto"><em>server_error.pem</em> has an alternative name:</p>
<pre class="code highlight js-syntax-highlight language-plaintext" lang="plaintext" v-pre="true"><code><span id="LC1" class="line" lang="plaintext">X509v3 Subject Alternative Name:</span>
<span id="LC2" class="line" lang="plaintext"> DNS:test.EXAMPLE.COM</span></code></pre>
<p dir="auto">a the following two commands need to be run:</p>
<pre class="code highlight js-syntax-highlight language-plaintext" lang="plaintext" v-pre="true"><code><span id="LC1" class="line" lang="plaintext">certtool --load-ca-certificate root_ca.pem --verify-profile low --verify --infile server_ok.pem</span></code></pre>
<pre class="code highlight js-syntax-highlight language-plaintext" lang="plaintext" v-pre="true"><code><span id="LC1" class="line" lang="plaintext">certtool --load-ca-certificate root_ca.pem --verify-profile low --verify --infile server_error.pem</span></code></pre>
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<a id="user-content-actual-results" class="anchor" href="#actual-results" aria-hidden="true"></a>Actual results:</h2>
<p dir="auto">The validation result for the first chain (with <em>server_ok.pem</em>) is</p>
<pre class="code highlight js-syntax-highlight language-plaintext" lang="plaintext" v-pre="true"><code><span id="LC1" class="line" lang="plaintext">Chain verification output: Verified. The certificate is trusted.</span></code></pre>
<p dir="auto">while the validation result for the second chain (with <em>server_error.pem</em>) is</p>
<pre class="code highlight js-syntax-highlight language-plaintext" lang="plaintext" v-pre="true"><code><span id="LC1" class="line" lang="plaintext">Chain verification output: Not verified. The certificate is NOT trusted. The certificate chain violates the signer's constraints.</span></code></pre>
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<a id="user-content-expected-results" class="anchor" href="#expected-results" aria-hidden="true"></a>Expected results:</h2>
<p dir="auto">It would be expected that both chains are validated successfully.</p>
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<a id="user-content-note" class="anchor" href="#note" aria-hidden="true"></a>Note:</h2>
<p dir="auto">For reference, I've also included a <a href="https://gitlab.com/gnutls/gnutls/uploads/8dd73b9f217b6f3539a8b21fd9919835/real_world_example_chain.zip" data-link="true" class="gfm">real_world_example_chain.zip</a>.</p>
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