Improper certificate validation in Erlang OTP’s public_key module allows an attacker to forge OCSP responses signed with an expired OCSP responder certificate. Because the verifier does not check the responder’s notBefore/notAfter fields, revoked TLS certificates can be presented to the client as valid. This flaw enables authentication bypass for both TLS server certificates and client certificates validated via public_key:pkix_ocsp_validate/5, potentially leading to unauthorized access.

The vulnerability affects Erlang/OTP releases from 27.0 up to but not including 27.3.4.12, 28.5.0.1, and 29.0.1, along with corresponding public_key libraries 1.16 up to 1.17.1.3 (OTP 27), 1.20.3.1 (OTP 28), and 1.21.1 (OTP 29). Users running these versions of Erlang/OTP are at risk. All installations that rely on OCSP stapling via the ssl application or that call public_key:pkix_ocsp_validate/5 directly are potentially impacted.

With a CVSS score of 6.3, the vulnerability poses moderate severity. The EPSS score is unavailable, and it is not listed in the CISA KEV catalog, indicating no widespread exploitation yet. An attacker who has obtained the private key of an expired OCSP responder certificate can forge responses that an Erlang/OTP client will accept. Attack vector: a malicious or compromised server presenting a revoked certificate alongside a forged OCSP response.