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Issue created by <a href="https://gitlab.com/rates37">Satya Jhaveri</a>: <a href="https://gitlab.com/gnutls/gnutls/-/work_items/1893">#1893</a>
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<h2 id="user-content-description" dir="auto" style="margin-top: 0px; margin-bottom: 10px;" align="initial">Description:<a href="#description" aria-label="Link to heading 'Description:'" data-heading-content="Description:" class="anchor" style="margin-top: 0px;"></a>
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<p dir="auto" style="color: #3a383f; margin: 0px 0px 1rem;" align="initial">The default of 16 for max verify depth is too low for some real-world chains.</p>
<p dir="auto" style="color: #3a383f; margin: 0px 0px 1rem;" align="initial">The constraint is triggered in <code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; margin-top: 0px; font-weight: inherit; overflow-wrap: break-word; white-space: break-spaces; font-family: "GitLab Mono", "JetBrains Mono", "Menlo", "DejaVu Sans Mono", "Liberation Mono", "Consolas", "Ubuntu Mono", "Courier New", "andale mono", "lucida console", monospace; font-variant-ligatures: none; word-break: keep-all; padding: 0.125rem 0.25rem;'>_gnutls_x509_cert_verify_peers</code>:</p>
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<pre data-canonical-lang="c" class="code highlight js-syntax-highlight language-c" v-pre="true" style='display: block; font-size: 14px; color: #3a383f; line-height: 1.6em; overflow-x: auto; border-radius: .25rem; position: relative; font-family: "GitLab Mono", "JetBrains Mono", "Menlo", "DejaVu Sans Mono", "Liberation Mono", "Consolas", "Ubuntu Mono", "Courier New", "andale mono", "lucida console", monospace; font-variant-ligatures: none; word-break: break-all; word-wrap: break-word; background-color: #fbfafd; margin: 0px 0 1rem; padding: 12px; border: 1px solid #dcdcde;'><code style='font-size: inherit; color: inherit; word-wrap: normal; word-break: keep-all; background-color: inherit; border-radius: .25rem; white-space: pre; margin-top: 0px; font-family: "GitLab Mono", "JetBrains Mono", "Menlo", "DejaVu Sans Mono", "Liberation Mono", "Consolas", "Ubuntu Mono", "Courier New", "andale mono", "lucida console", monospace; font-variant-ligatures: none; overflow-wrap: normal; padding: unset;'><span id="LC1" class="line" lang="c" style="margin-top: 0px;"><span class="k" style="margin-top: 0px;">if</span> <span class="p">(</span><span class="n">info</span><span class="o">-></span><span class="n">ncerts</span> <span class="o">></span> <span class="n">cred</span><span class="o">-></span><span class="n">verify_depth</span> <span class="o">&&</span> <span class="n">cred</span><span class="o">-></span><span class="n">verify_depth</span> <span class="o">></span> <span class="mi">0</span><span class="p">)</span> <span class="p">{</span></span>
<span id="LC2" class="line" lang="c"> <span class="n" style="margin-top: 0px;">gnutls_assert</span><span class="p">();</span></span>
<span id="LC3" class="line" lang="c"> <span class="k" style="margin-top: 0px;">return</span> <span class="n">GNUTLS_E_CONSTRAINT_ERROR</span><span class="p">;</span></span>
<span id="LC4" class="line" lang="c"><span class="p" style="margin-top: 0px;">}</span></span></code></pre>
<copy-code></copy-code><insert-code-snippet></insert-code-snippet>
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<p dir="auto" style="color: #3a383f; margin: 0px 0px 1rem;" align="initial">A chain with more than 16 certificates causes <code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; margin-top: 0px; font-weight: inherit; overflow-wrap: break-word; white-space: break-spaces; font-family: "GitLab Mono", "JetBrains Mono", "Menlo", "DejaVu Sans Mono", "Liberation Mono", "Consolas", "Ubuntu Mono", "Courier New", "andale mono", "lucida console", monospace; font-variant-ligatures: none; word-break: keep-all; padding: 0.125rem 0.25rem;'>GNUTLS_E_CONSTRAINT_ERROR</code> (-101),which surfaces to applications as a verification failure.</p>
<p dir="auto" style="color: #3a383f; margin: 0px 0px 1rem;" align="initial">The value of 16 is used as a defense against DOS attacks. I'm asking what the motivation for the choice of 16 as a threshold is. OpenSSL defaults to 100, and there are legitimate chains that exceed 16 certificates in length. A limit of 100 still provides meaningful DoS protection, while avoiding breaking real-world deployments. I'd welcome the maintainer's view on where the right balance for this lies.</p>
<h2 id="user-content-version-of-gnutls-used-373" dir="auto" style="margin-top: 20px; margin-bottom: 10px;" align="initial">Version of gnutls used: 3.73<a href="#version-of-gnutls-used-373" aria-label="Link to heading 'Version of gnutls used: 3.73'" data-heading-content="Version of gnutls used: 3.73" class="anchor" style="margin-top: 0px;"></a>
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<h2 id="user-content-distributor-of-gnutls-ubuntu-22045-lts" dir="auto" style="margin-top: 20px; margin-bottom: 10px;" align="initial">Distributor of gnutls: Ubuntu (22.04.5 LTS)<a href="#distributor-of-gnutls-ubuntu-22045-lts" aria-label="Link to heading 'Distributor of gnutls: Ubuntu (22.04.5 LTS)'" data-heading-content="Distributor of gnutls: Ubuntu (22.04.5 LTS)" class="anchor" style="margin-top: 0px;"></a>
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<h2 id="user-content-how-reproducible" dir="auto" style="margin-top: 20px; margin-bottom: 10px;" align="initial">How reproducible:<a href="#how-reproducible" aria-label="Link to heading 'How reproducible:'" data-heading-content="How reproducible:" class="anchor" style="margin-top: 0px;"></a>
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<p dir="auto" style="color: #3a383f; margin: 0px 0px 1rem;" align="initial">Steps to Reproduce:</p>
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<li style="margin-top: 0px; line-height: 1.6em; margin-left: 25px; padding-left: 3px;">Generate a root CA, an intermediate CA cross-signed by over 16 alternate roots, and a leaf certificate</li>
<li style="line-height: 1.6em; margin-left: 25px; padding-left: 3px;">Serve the full chain and attempt to connect via any client using GnuTLS</li>
<li style="line-height: 1.6em; margin-left: 25px; padding-left: 3px;">The connection will fail with error -101</li>
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