Improper Certificate Validation vulnerability in Erlang OTP publickey (pubkeyocsp module) allows forged OCSP responses signed with an expired responder certificate to be accepted as valid.

OCSP response verification in pubkeyocsp:verifyresponse/5 and pubkeyocsp:isauthorizedresponder/3 in lib/publickey/src/pubkeyocsp.erl does not check the validity period (notBefore/notAfter) of the OCSP responder certificate. An attacker who has obtained the private key of an expired CA-designated OCSP responder certificate can forge OCSP responses that Erlang/OTP accepts as valid.

This affects TLS clients using OCSP stapling via the ssl application: a malicious or compromised server can present a revoked TLS certificate together with a forged OCSP response signed by an expired responder key, and the client will accept the revoked certificate as valid. It also affects applications calling publickey:pkixocspvalidate/5 directly, where the impact depends on the use case — server-side client certificate validation using this API may allow authentication bypass with a revoked client certificate.

This issue affects OTP from OTP 27.0 before OTP 29.0.1, OTP 28.5.0.1 and OTP 27.3.4.12, corresponding to publickey from 1.16 before 1.21.1, 1.20.3.1 and 1.17.1.3.