The vulnerability in Erlang OTP's public_key module (pubkey_ocsp) arises because the OCSP response verification functions do not check the validity period (notBefore/notAfter) of the OCSP responder certificate. As a result, an attacker with the private key of an expired OCSP responder certificate can forge OCSP responses that are accepted as valid by Erlang/OTP. This can lead to acceptance of revoked TLS certificates when OCSP stapling is used, or authentication bypass in server-side client certificate validation scenarios using the public_key:pkix_ocsp_validate/5 API. Affected versions include OTP 27.0 up to but not including 29.0.1, OTP 28.5.0.1, and OTP 27.3.4.12, with corresponding public_key versions from 1.16 up to but not including 1.21.1, 1.20.3.1, and 1.17.1.3.

The vulnerability allows attackers who have access to the private key of an expired OCSP responder certificate to forge OCSP responses that Erlang OTP accepts as valid. This can cause TLS clients using OCSP stapling to accept revoked TLS certificates, undermining the trust model of certificate revocation. Additionally, server-side applications validating client certificates using the affected API may allow authentication bypass with revoked client certificates. This undermines the security of TLS connections and client authentication relying on OCSP validation.

Patch status is not yet confirmed — check the vendor advisory for current remediation guidance. Until a fix is available, users should consider alternative certificate validation methods or additional checks on OCSP responder certificate validity periods outside of Erlang OTP. Monitor Erlang's official channels for updates and apply official patches once released.