The Improper Certificate Validation vulnerability in Erlang OTP’s public_key pubkey_ocsp module permits an OCSP designated‑responder authorization bypass because the pkix_ocsp_validate/5 function does not verify that a CA‑designated responder certificate was cryptographically signed by its issuing CA. It only checks that the responder certificate’s issuer name matches the CA’s subject name and that the certificate has the OCSPSigning extended key usage. Consequently an attacker who can intercept or otherwise control OCSP responses could generate a self‑signed certificate with a matching issuer name and the required EKU and use it to forge OCSP responses that mark revoked certificates as valid. This can cause SSL/TLS clients that use OCSP stapling to accept connections to servers with revoked certificates, potentially leading to the transmission of sensitive data to malicious servers. Applications that call public_key:pkix_ocsp_validate/5 directly are similarly affected, depending on how the API is used.

The flaw affects Erlang: OTP versions from 27.0 through 28.4.2 and 27.3.4.10, which correspond to public_key releases 1.16 through 1.20.3 and 1.17.1.2, as well as ssl releases 11.2 through 11.5.4 and 11.2.12.7. No other vendors or products are listed in the CNA data.

With a CVSS base score of 7.6 the vulnerability is considered high risk. The EPSS score of < 1% indicates a very low probability of exploitation, and the vulnerability is not listed in the CISA KEV catalog. Based on the description, exploitation likely requires the attacker to control or observe OCSP traffic, such as by operating a malicious OCSP responder or performing a man‑in‑the‑middle attack. If successful, the attacker can forge responses and cause TLS clients to accept revoked certificates, potentially exposing confidentiality and integrity of the transported data.