An issue was discovered in MikroTik RouterOS 6.41.4. Missing OpenVPN server certificate verification allows a remote unauthenticated attacker capable of intercepting client traffic to act as a malicious OpenVPN server. This may allow the attacker to gain access to the client's internal network (for example, at site-to-site tunnels).
https://janis-streib.de/2018/04/11/mikrotik-openvpn-security