OpenVPN version 2.3.12 and newer is vulnerable to unauthenticated Denial of Service of server via received large control packet. Note that this issue is fixed in 2.3.15 and 2.4.2.
https://www.exploit-db.com/exploits/41993/
https://community.openvpn.net/openvpn/wiki/QuarkslabAndCryptographyEngineerAudits