On Sep 30, 2021, Let's Encrypt had one of their intermediate certificates expire (an old DST Root CA X3 certificate). This certificate is still present in the public certificates that they issue. Normally, this would not be a problem, because Let's Encrypt offers another valid signature. However, older SSL implementations will reject the certificate. This includes OpenBSD 6.9 release and older and older versions of mIRC.

Switching to another certificate authority would normally help. However, mIRC users have complained about validation errors. It seems they are missing one of the certificate authorities used by buypass. For this reason, do not use buypass for your TLS certificates.

The best solution is to use Let's Encrypt issued certificates while also deleting the extra intermediate certificate that has expired.

Go to /etc/ssl/ where your public certificates are stored and edit /etc/ssl/example.com.fullchain.pem. Delete the lines of the third (and last) certificate by running this command as root:

# awk '/END CERTIFICATE/ { cert++; } { print $0; if (cert == 2) exit;}' /etc/ssl/example.com.fullchain.pem > /etc/ssl/example.com.fullchain.pem.fixed
# mv /etc/ssl/example.com.fullchain.pem.fixed /etc/ssl/example.com.fullchain.pem

You should repeat this for every single SSL cert you have. Then, if the daemon that serves the cert is running inside a chroot, make sure to copy the SSL cert into the chroot. For example, for ngircd:

$ doas cp /etc/ssl/example.com.fullchain.pem /etc/ssl/private/example.com.key /etc/ngircd/
$ doas chown _ngircd:_ngircd /etc/ngircd/example.com.{fullchain.pem,key}
$ doas pkill -HUP ngircd

We make sure to set the proper permissions as well as send a HUP signal to ngircd to cause it to reload its cert.

For ZNC, we would run:

$ doas cp /etc/ssl/example.com.fullchain.pem /etc/ssl/private/example.com.key /home/znc/home/znc/.znc/
$ doas chown -R znc:znc /home/znc/home/znc/.znc/

Make sure that certs are properly copied into place for all your services.

Test to see if every one of your SSL certs work. It's best to use a wide variety of web browsers, email clients, and IRC clients on preferably different operating systems. For example, an SSL cert might validate on Firefox on Debian but not on lynx on OpenBSD or mIRC on Windows.

Recommended Testing:

Try testing with mIRC from Windows if you have it, or irssi on unpatched OpenBSD 6.9 release to your IRC server/ZNC bouncer. Also, try lynx/w3m on OpenBSD to your website, and mutt on OpenBSD to your mail server. Try to see if you can trigger the error before deleting the certificate, and if you have a valid certificate after it's deleted.

Patching OpenBSD

In Errata for OpenBSD 6.9, a patch is provided so that OpenBSD will verify trusted certificates first:

$ doas syspatch

See also http://undeadly.org/cgi?action=article;sid=20211001073034