========================================================================== CVE-2026-77205: Denial of Service via sending bogus X.509 certificates ========================================================================== Release date: Monday, Oct 5, 2026 Contact: security@libreswan.org PGP key: 907E790F25C1E8E561CD73B585FF4B43B30FC6F9 This alert (and any updates) are available at the following URLs: https://libreswan.org/security/CVE-2026-77205 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. Severity: Low Vulnerable versions : 5.3, 5.3.1, 5.3.2 Not vulnerable : 3.0 - 5.2, 5.4 and higher Vulnerability details ===================== The add_decoded_cert() function missed a call to CERT_DestroyCertificate(). This was due to an incorrect fix for CVE-2026-14957, see https://libreswan.org/security/CVE-2026-14957/CVE-2026-14957.txt Exploitation ============ 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. Workaround ========== Disabling FIPS mode will prevent this attack vector. If this is not possible, apply the patch below or upgrade to a later libreswan version. History ======= * 30-07-2026 Libreswan was notified of the issue via security@libreswan.org. * 24-09-2026 Advanced notice given to supported customers and distributions. * 05-10-2026 Public announcement and release of libreswan 5.3.3. Credits ======= Reported by Wojciech Tatarski (TuxCare)