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<span style="color: rgb(0, 0, 0); font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt;">The "Old Let’s Encrypt Root Certificate Expiration and OpenSSL 1.0.2" blog article [1] doesn't cover OpenSSL versions prior to 1.0.2, presumably because
they've been unsupported for a long time. However, no doubt there are still some users and applications that are stuck using even older versions of OpenSSL. Here are some solutions...</span></div>
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<span style="color: rgb(0, 0, 0); font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt;">Firstly, it's worth noting that Workaround 1 (see [1])
</span><span style="color: rgb(0, 0, 0); font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt;">also works with OpenSSL 1.0.1n and above, which seems to be because the 1.0.2 "alternative chains" behaviour was backported (see [2]).</span><br>
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<span style="color: rgb(0, 0, 0); font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt;">Secondly, I've come up with a variant of Workaround 1 (see "Workaround 0" below) that also works for even older OpenSSL releases. I thought I'd share it
here in case it helps anyone out...</span></div>
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<span style="color: rgb(0, 0, 0); font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt;">Workaround 0 (on clients with OpenSSL >=0.9.8m)</span></div>
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<u>Modify</u> the "notAfter" date in the expired root certificate (DST Root CA X3) from "Sep 30 14:01:15 2021 GMT" to "Sep 30 18:14:03 2024 GMT" (which is when the new DST->ISRG cross-certificate is due to expire - see [3] and [4]); then, in the OpenSSL-based
client application's trust store, r<span style="background-color:rgb(255, 255, 255);display:inline !important">eplace the expired root certificate with the modified version. </span><span style="color: rgb(0, 0, 0); font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt;">Here's
one way to create the modified certificate:</span></div>
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wget -O DSTRootCAX3_Extended.crt "https://crt.sh/?d=8395"</div>
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sed -i "s/xMDkzMDE0MDExNVow/0MDkzMDE4MTQwM1ow/g" <span style="background-color:rgb(255, 255, 255);display:inline !important">DSTRootCAX3_Extended.crt</span><br>
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You should find that "openssl s_client" with "-CAfile DSTRootCAX3_Extended.crt" will return "Verify return code: 0 (ok)" when connecting to any server that serves Let's Encrypt's preferred certificate chain (i.e., the chain that includes [4]). For example:</div>
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openssl s_client -connect letsencrypt.org:443 -CAfile ../DSTRootCAX3_Extended.crt</div>
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Why does this work?</div>
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Modifying the expired root certificate obviously invalidates its self-signed signature. However, OpenSSL
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0.9.8m and above don't check self-signed signatures by default (due to [5]), which is one factor in why this workaround works (whereas 0.9.8l and below do check self-signed signatures). The other factor is that OpenSSL treats the modified certificate as a
root certificate, because the Issuer and Subject Names are identical; since it's not treated as an intermediate certificate, older versions of OpenSSL are able to treat it as a trust anchor even though they lack the "partial chains" functionality that was
added in 1.0.2 (see [6]).</span></div>
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<div>It is not necessary to add the new ISRG Root X1 self-signed certificate to the client's trust store for Workaround 0 to work. Note however that Workaround 0 won't work beyond Sep 30 18:14:03 2024 GMT.<br>
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[1] <a href="https://www.openssl.org/blog/blog/2021/09/13/LetsEncryptRootCertExpire/)">https://www.openssl.org/blog/blog/2021/09/13/LetsEncryptRootCertExpire/</a></div>
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[2] <a href="https://github.com/openssl/openssl/commit/f7bf8e02dfcb2c02bc12a59276d0a3ba43e6c204" style="font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt;">https://github.com/openssl/openssl/commit/f7bf8e02dfcb2c02bc12a59276d0a3ba43e6c204</a></div>
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[3] <a href="https://letsencrypt.org/2020/12/21/extending-android-compatibility.html">https://letsencrypt.org/2020/12/21/extending-android-compatibility.html</a></div>
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[4] <a href="https://crt.sh/?id=3958242236">https://crt.sh/?id=3958242236</a></div>
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[5] <span style="background-color:rgb(255, 255, 255);display:inline !important"><a href="https://github.com/openssl/openssl/commit/1e53b797f65ef6d3c2eb1052797683fec27a9ff5">https://github.com/openssl/openssl/commit/1e53b797f65ef6d3c2eb1052797683fec27a9ff5</a></span></div>
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[6] <a href="https://github.com/openssl/openssl/commit/9a1f59cd3128ddac73d3e0721ecd55935f53ba8b">https://github.com/openssl/openssl/commit/9a1f59cd3128ddac73d3e0721ecd55935f53ba8b</a></div>
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<div>--</div>
<div>Rob Stradling</div>
<div>Senior Research & Development Scientist</div>
<div>Sectigo Limited<br>
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