An incomplete X.509 implementation in GnuTLS 3.8.13 lets an attacker controlling a subordinate Certificate Authority bypass cross‑domain PKI restrictions and issue certificates for arbitrary domains. The failure to fully validate certificate chains effectively elevates a subordinate CA’s authority, allowing the forging of certificates that appear legitimate for any hostname. This flaw enables domain impersonation and man‑in‑the‑middle attacks, compromising the confidentiality and integrity of TLS connections.

The affected product is the GnuTLS cryptographic library, specifically version 3.8.13. No other products or versions are listed as impacted. Users running this version are therefore at risk.

The CVSS score indicates low severity impact. EPSS score of 0.00117 indicates a very low probability of exploitation and the vulnerability is not listed in CISA’s KEV catalog, suggesting limited or undocumented exploitation to date. Nonetheless, the danger lies in the requirement for an attacker to control a subordinate CA, which can occur if a CA that issues certificates used by GnuTLS‑based applications is compromised. Once that control is achieved, the attacker can issue certificates for any domain, undermining all TLS connections validated by GnuTLS.