The Libreswan Project received a report about an issue in the libreswan 5.3 series related to badly formed received certificates while running in FIPS mode. While being rejected, the certifiates were not deleted from the NSS cache, resulting in any real certificates with the same serial number being rejected as well. This can be abused to lock out legitimate peers from connecting.

Sending bogus certificates with new serial numbers could prevent the real certificates with those serial numbers from connecting until the service has restarted. While there is no RFC that dictates Certificate Authority (CA) should use randomized serial numbers, it is considered best practise and it is a requirement of the CA/Browser Forum. If using a private Root CA for IPsec VPNs, it is recommended to verify it uses randomized serial numbers.  However, an attacker could still trick a peer into connecting to them first, thus learning the serial number.

Vulnerable versions : 5.3, 5.3.1, 5.3.2
Not vulnerable      : 3.0 - 5.2, 5.4 and higher