Interesting read.

So, in short:

• The attacker needs to have access to your LAN and become the DHCP server, e.g. by a starvation attack or timing attacks
• The attacked host system needs to support DHCP option 121 (atm basically every OS except Android)
• by abusing DHCP option 121, the attacker can push routes to the attacked host system that supersede other rules in most network stacks by having a more specific prefix, e.g. a 192.168.1.1/32 will supersede 0.0.0.0/0
• The attacker can now force the attacked host system to route the traffic intended for a VPN virtual network interface (to be encrypted and forwarded to the VPN server) to the (physical) interface used for DHCP
• This leads to traffic intended to be sent over the VPN to not get encrypted and being sent outside the tunnel.
• This attack can be used before or after a VPN connection is established
• Since the VPN tunnel is still established, any implemented kill switch doesn’t get triggered

DHCP option 121 is still used for a reason, especially in business networks. At least on Linux, using network namespaces will fix this. Firewall mitigations can also work, but create other (very theoretical) attack surfaces.