[systemd-devel] VPN connections subject to hijack attack

[Kenneth Porter]

<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.

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.

<https://github.com/systemd/systemd/commits/master/sysctl.d/50-default.conf>

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>

More about what the rp_filter setting does:

<https://www.theurbanpenguin.com/rp_filter-and-lpic-3-linux-security/>

[Please reply on the list. No need to cc me a copy.]