<https://www.bleepingcomputer.com/news/security/new-linux-vulnerability-lets-attackers-hijack-vpn-connections/>
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?"
<https://access.redhat.com/solutions/53031>
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