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<a href="https://gitlab.com/kmiller">Kenneth J. Miller</a>
commented:
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<p dir="auto">The ticket name should be changed to describe the underlying issue, the failing validation of certificates chaining back to the expired Sectigo root/intermediate is a symptom of deeper issue with the way GnuTLS currently validates certificate paths.</p>
<p dir="auto">I recreated the issue locally with <strong>GnuTLS 3.6.13</strong>, Nginx, and some test certificates. From my observation, GnuTLS fails if the first path validated contains an expired certificate, even if there exist possible valid alternative paths.</p>
<p dir="auto">Here the X.509 PKI I tested with an intermediate cross-signed by a valid and expired root certificate respectively:</p>
<pre class="code highlight js-syntax-highlight plaintext" lang="plaintext" v-pre="true"><code><span id="LC1" class="line" lang="plaintext"> ROOT 1 ROOT 2</span>
<span id="LC2" class="line" lang="plaintext">(valid) (expired)</span>
<span id="LC3" class="line" lang="plaintext"> \ /</span>
<span id="LC4" class="line" lang="plaintext"> \ /</span>
<span id="LC5" class="line" lang="plaintext"> INTERMEDIATE 1</span>
<span id="LC6" class="line" lang="plaintext"> (valid)</span>
<span id="LC7" class="line" lang="plaintext"> |</span>
<span id="LC8" class="line" lang="plaintext"> |</span>
<span id="LC9" class="line" lang="plaintext"> LEAF 1</span>
<span id="LC10" class="line" lang="plaintext"> (valid)</span></code></pre>
<p dir="auto">Note that this is a slightly different hierarchy than the issue presented by Sectigo's certificates where the public keys of an intermediate certificate were used to re-create a self-signed root. However, the underlying issue presents itself in the same way.</p>
<p dir="auto">Depending on the order in which Nginx serves the above cross-signed INTERMEDIATE 1 certificates after the LEAF 1 certificate, a request will either succeed or fail.</p>
<p dir="auto">When the certificate bundle supplied by Nginx contains the INTERMEDIATE 1 signed by the valid ROOT 1 <em>before</em> the one signed by ROOT 2, the TLS handshake succeeds:</p>
<pre class="code highlight js-syntax-highlight plaintext" lang="plaintext" v-pre="true"><code><span id="LC1" class="line" lang="plaintext">asd# gnutls-cli example.test</span>
<span id="LC2" class="line" lang="plaintext">Processed 2 CA certificate(s).</span>
<span id="LC3" class="line" lang="plaintext">Resolving 'example.test:443'...</span>
<span id="LC4" class="line" lang="plaintext">Connecting to '127.0.1.1:443'...</span>
<span id="LC5" class="line" lang="plaintext">- Certificate type: X.509</span>
<span id="LC6" class="line" lang="plaintext">- Got a certificate list of 3 certificates.</span>
<span id="LC7" class="line" lang="plaintext">- Certificate[0] info:</span>
<span id="LC8" class="line" lang="plaintext"> - subject `CN=example.test', issuer `CN=INTERMEDIATE 1', serial 0x6ce0398c3f993ad4f5509763fc2ed69286df6631, RSA key 3072 bits, signed using RSA-SHA256, activated `2020-05-31 03:20:16 UTC', expires `2020-06-28 03:20:17 UTC', pin-sha256="ySlPmQ0lv9y4f7hwsopslzgEdiUnU3/5u4nFya2WwSo="</span>
<span id="LC9" class="line" lang="plaintext"> Public Key ID:</span>
<span id="LC10" class="line" lang="plaintext"> sha1:66fc0194e47a524774be3c64c1fcf11dab218ebd</span>
<span id="LC11" class="line" lang="plaintext"> sha256:c9294f990d25bfdcb87fb870b28a6c973804762527537ff9bb89c5c9ad96c12a</span>
<span id="LC12" class="line" lang="plaintext"> Public Key PIN:</span>
<span id="LC13" class="line" lang="plaintext"> pin-sha256:ySlPmQ0lv9y4f7hwsopslzgEdiUnU3/5u4nFya2WwSo=</span>
<span id="LC14" class="line" lang="plaintext"></span>
<span id="LC15" class="line" lang="plaintext">- Certificate[1] info:</span>
<span id="LC16" class="line" lang="plaintext"> - subject `CN=INTERMEDIATE 1', issuer `CN=ROOT 1', serial 0x17662e3e54ea343b5e81205e0c69aafb94c2b29d, RSA key 3072 bits, signed using RSA-SHA256, activated `2020-05-31 03:25:19 UTC', expires `2020-06-29 03:25:22 UTC', pin-sha256="SbBuckO3jO2KQK2OTZ1s92tOYBU1Zh2cULbe6dZ+tK4="</span>
<span id="LC17" class="line" lang="plaintext">- Certificate[2] info:</span>
<span id="LC18" class="line" lang="plaintext"> - subject `CN=INTERMEDIATE 1', issuer `CN=ROOT 2', serial 0x1040f78089e97ffefc343f977eaeedd78fc15f87, RSA key 3072 bits, signed using RSA-SHA256, activated `2020-05-31 03:22:12 UTC', expires `2020-06-28 03:22:14 UTC', pin-sha256="SbBuckO3jO2KQK2OTZ1s92tOYBU1Zh2cULbe6dZ+tK4="</span>
<span id="LC19" class="line" lang="plaintext">- Status: The certificate is trusted. </span>
<span id="LC20" class="line" lang="plaintext">- Description: (TLS1.3-X.509)-(ECDHE-SECP256R1)-(RSA-PSS-RSAE-SHA256)-(AES-256-GCM)</span>
<span id="LC21" class="line" lang="plaintext">- Options:</span>
<span id="LC22" class="line" lang="plaintext">- Handshake was completed</span></code></pre>
<p dir="auto">When the certificate bundle supplied by Nginx contains the INTERMEDIATE 1 signed by the ROOT 1 <em>after</em> the one signed by the expired ROOT 1, the TLS handshake fails:</p>
<pre class="code highlight js-syntax-highlight plaintext" lang="plaintext" v-pre="true"><code><span id="LC1" class="line" lang="plaintext"># gnutls-cli example.test</span>
<span id="LC2" class="line" lang="plaintext">Processed 2 CA certificate(s).</span>
<span id="LC3" class="line" lang="plaintext">Resolving 'example.test:443'...</span>
<span id="LC4" class="line" lang="plaintext">Connecting to '127.0.1.1:443'...</span>
<span id="LC5" class="line" lang="plaintext">- Certificate type: X.509</span>
<span id="LC6" class="line" lang="plaintext">- Got a certificate list of 3 certificates.</span>
<span id="LC7" class="line" lang="plaintext">- Certificate[0] info:</span>
<span id="LC8" class="line" lang="plaintext"> - subject `CN=example.test', issuer `CN=INTERMEDIATE 1', serial 0x6ce0398c3f993ad4f5509763fc2ed69286df6631, RSA key 3072 bits, signed using RSA-SHA256, activated `2020-05-31 03:20:16 UTC', expires `2020-06-28 03:20:17 UTC', pin-sha256="ySlPmQ0lv9y4f7hwsopslzgEdiUnU3/5u4nFya2WwSo="</span>
<span id="LC9" class="line" lang="plaintext"> Public Key ID:</span>
<span id="LC10" class="line" lang="plaintext"> sha1:66fc0194e47a524774be3c64c1fcf11dab218ebd</span>
<span id="LC11" class="line" lang="plaintext"> sha256:c9294f990d25bfdcb87fb870b28a6c973804762527537ff9bb89c5c9ad96c12a</span>
<span id="LC12" class="line" lang="plaintext"> Public Key PIN:</span>
<span id="LC13" class="line" lang="plaintext"> pin-sha256:ySlPmQ0lv9y4f7hwsopslzgEdiUnU3/5u4nFya2WwSo=</span>
<span id="LC14" class="line" lang="plaintext"></span>
<span id="LC15" class="line" lang="plaintext">- Certificate[1] info:</span>
<span id="LC16" class="line" lang="plaintext"> - subject `CN=INTERMEDIATE 1', issuer `CN=ROOT 2', serial 0x1040f78089e97ffefc343f977eaeedd78fc15f87, RSA key 3072 bits, signed using RSA-SHA256, activated `2020-05-31 03:22:12 UTC', expires `2020-06-28 03:22:14 UTC', pin-sha256="SbBuckO3jO2KQK2OTZ1s92tOYBU1Zh2cULbe6dZ+tK4="</span>
<span id="LC17" class="line" lang="plaintext">- Certificate[2] info:</span>
<span id="LC18" class="line" lang="plaintext"> - subject `CN=INTERMEDIATE 1', issuer `CN=ROOT 1', serial 0x17662e3e54ea343b5e81205e0c69aafb94c2b29d, RSA key 3072 bits, signed using RSA-SHA256, activated `2020-05-31 03:25:19 UTC', expires `2020-06-29 03:25:22 UTC', pin-sha256="SbBuckO3jO2KQK2OTZ1s92tOYBU1Zh2cULbe6dZ+tK4="</span>
<span id="LC19" class="line" lang="plaintext">- Status: The certificate is NOT trusted. The certificate chain uses expired certificate. The signature in the certificate is invalid. </span>
<span id="LC20" class="line" lang="plaintext">*** PKI verification of server certificate failed...</span>
<span id="LC21" class="line" lang="plaintext">*** Fatal error: Error in the certificate.</span></code></pre>
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