OpenVPN 2.1 until v2.4.12 and v2.5.6 may enable authentication bypass in external authentication plug-ins when more than one of them makes use of deferred authentication replies, which allows an external user to be granted access with only partially correct credentials.
https://openvpn.net/community-downloads/
https://lists.debian.org/debian-lts-announce/2022/05/msg00002.html
https://community.openvpn.net/openvpn/wiki/SecurityAnnouncements