The vulnerability arises when Libreswan running in FIPS mode accepts a malformed certificate but fails to delete it from the NSS cache. The bad certificate remains cached and, because it shares a serial number, any legitimate certificate with that serial number is subsequently rejected by the server, effectively denying the peer. This manifests as a denial‑of‑service condition that prevents legitimate connections to the IPsec endpoint. The weakness is an instance of CWE‑459 (Improper Memory Management) and CWE‑772 (Improper Access Control).

Affected products are the Libreswan implementation of the IPsec VPN stack. Any Libreswan version before 5.3.3 that is configured with FIPS mode enabled is vulnerable. It is not restricted to a particular release series; the vulnerability exists in all pre‑5.3.3 builds that activate FIPS mode. The official advisory states that upgrading to version 5.3.3 or later resolves the issue.

The CVSS score is 3.7, indicating a low severity. The EPSS score of less than 1% indicates a very low probability of exploitation. The vulnerability is not featured in CISA’s KEV catalog. An attacker could exploit this by sending crafted X.509 certificates over the IPsec control channel while FIPS mode is enabled. Successful exploitation would lock out legitimate peers until the service is restarted, causing a denial of service. Because the attack requires network access to the vulnerable endpoint, it is a remote network attack when FIPS mode is enabled.