FreeBSD Security Advisory – During certificate verification, OpenSSL will attempt to find an alternative certificate chain if the first attempt to build such a chain fails, unless the application explicitly specifies X509_V_FLAG_NO_ALT_CHAINS. An error in the implementation of this logic could erroneously mark certificate as trusted when they should not. An attacker could cause certain checks on untrusted certificates, such as the CA (certificate authority) flag, to be bypassed, which would enable them to use a valid leaf certificate to act as a CA and issue an invalid certificate.