This affects all VPNs and is a consequence of using "loose" reverse path filtering for anti-spoofing. The default CentOS setting is strict filtering but you may have changed this to loose for some unusual routing situations. Check that the value of /proc/sys/net/ipv4/conf/all/rp_filter is still set to 1. If it's set to 2 (loose filtering), you're vulnerable.
Technical details:
https://seclists.org/oss-sec/2019/q4/122
According to the report, systemd changed the default to 2 in November 2018 so many distros are vulnerable.
Here's Red Hat's explanation of why you might want to use a value of 2. "When RHEL has multiple IPs configured, only one is reachable from a remote network. Or why does RHEL ignore packets when the route for outbound traffic differs from the route of incoming traffic?"
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Kenneth Porter