The SSL certificate and key used by MySQL should be used only for MySQL and only for one instance. Use of default certificates can allow an attacker to impersonate the MySQL server. NOTE: Nessus has not performed this check. Please review the benchmark to ensure target compliance.
Solution
Check if the certificate is bound to one instance of MySQL. Generate a new certificate/key per MySQL instance. If a the key is used on multiple system then a compromise of one system leads to compromise of the network traffic of all servers which use the same key.