Virtual private networking (VPN) companies market their services as a way to prevent anyone from snooping on your Internet usage. But new research suggests this is a dangerous assumption when connecting to a VPN via an untrusted network, because attackers on the same network could force a target’s traffic off of the protection provided by their VPN without triggering any alerts to the user.
Short version of this attack, it involves split routing for the tunnels. A lot of clients will have a default route-all to send traffic through the VPN. There is however a limitation to this because the tunnel itself needs a route from the local nic to connect to the VPN endpoint and establish the tunnel, otherwise you end up with a chicken and egg where you can’t establish the VPN. By taking advantage of the DHCP option to set preferred routes (really anything more specific than 0.0.0.0/0) it can tell the host system to send the specified traffic through the local gateway rather than the tunnel’s virtual adapter.
One relatively simple fix if you happen to have a fancy router/firewall on the edge of the network that handles the VPN would be to use policy based routing rather than relying on the underlying network configuration. Static route tables would be possible too, but in theory that could be overridden by just sending a more specific route again than what was set statically.