This vulnerability arises from improper certificate validation (CWE-295) in the Erlang OTP public_key module's OCSP response verification functions (pubkey_ocsp:verify_response/5 and pubkey_ocsp:is_authorized_responder/3). Specifically, the validity period (notBefore/notAfter) of the OCSP responder certificate is not checked, enabling attackers who have access to the private key of an expired OCSP responder certificate to forge OCSP responses. Affected OTP versions include 27.0 before 27.3.4.12, 28.5.0.1, and 29.0.1, with corresponding public_key versions prior to 1.17.1.3, 1.20.3.1, and 1.21.1. This flaw can cause TLS clients using OCSP stapling to accept revoked certificates as valid and may allow authentication bypass in server-side client certificate validation scenarios.

The vulnerability allows acceptance of forged OCSP responses signed by expired responder certificates, which can lead to acceptance of revoked TLS certificates by clients using OCSP stapling. This undermines the trust model of certificate revocation checking and can result in authentication bypass or acceptance of revoked certificates. The impact depends on the use case, but it affects TLS client validation and applications using the public_key:pkix_ocsp_validate/5 API for certificate validation.

Patch status is not yet confirmed — check the vendor advisory for current remediation guidance. Until an official fix is released, users should consider alternative certificate validation methods or avoid relying solely on OCSP stapling with affected Erlang OTP versions. Monitor Erlang's official channels for updates and apply patches promptly once available.