alt Hacker NewsCertificate expiry is less severe than an untrusted issuer or a host mismatch.
The former is most likely an administrative error (ie: someone forgot to renew, or the auto-renew is failing). The latter is more likely to be an MTM attack.
I'm not sure how you would use an expired cert as an attack vector. By loading in an old cert into an expired domain so you could spoof older content?
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Revocation information may not be available for expired certificates. Not that it matters much because the last time I checked revocation didn't really work for non-expired certificates either, but I think that (+ the risk of people treating expired certificates as worthless and thus increasing the risk of exposure) is the main reason.
Also of course domains changing owners, but again... I don't think we have good monitoring for that during the current long lifetime, so maybe a grace period where a warning is shown but it's easier to click through would be a good idea. Perhaps combined with a requirement to keep revocation information (and keep revoking expired certificates) X days past expiry.
If a key is breached, the certificate can be revoked, but that revocation goes away once the certificate is expired.
Expiry is a pretty fundamental part of the security model of certificates.