OpenVPN 2.5.1 and earlier versions allows a remote attackers to bypass authentication and access control channel data on servers configured with deferred authentication, which can be used to potentially trigger further information leaks.
https://usn.ubuntu.com/usn/usn-4933-1
https://security.gentoo.org/glsa/202105-25
https://lists.debian.org/debian-lts-announce/2022/05/msg00002.html
https://community.openvpn.net/openvpn/wiki/SecurityAnnouncements