By design, the DHCP protocol does not authenticate messages, including for example the classless static route option (121). An attacker with the ability to send DHCP messages can manipulate routes to redirect VPN traffic, allowing the attacker to read, disrupt, or possibly modify network traffic that was expected to be protected by the VPN. Many, if not most VPN systems based on IP routing are susceptible to such attacks.

References:
http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2024-3661
https://www.cve.org/CVERecord?id=CVE-2024-3661
https://datatracker.ietf.org/doc/html/rfc2131#section-7
https://datatracker.ietf.org/doc/html/rfc3442#section-7
https://www.leviathansecurity.com/blog/tunnelvision
https://tunnelvisionbug.com/
https://www.leviathansecurity.com/research/tunnelvision
https://news.ycombinator.com/item?id=40279632
https://arstechnica.com/security/2024/05/novel-attack-against-virtually-all-vpn-apps-neuters-their-entire-purpose/
https://issuetracker.google.com/issues/263721377
https://krebsonsecurity.com/2024/05/why-your-vpn-may-not-be-as-secure-as-it-claims/
https://lowendtalk.com/discussion/188857/a-rogue-dhcp-server-within-your-network-can-and-will-hijack-your-vpn-traffic
https://mullvad.net/en/blog/evaluating-the-impact-of-tunnelvision
https://news.ycombinator.com/item?id=40284111
https://www.agwa.name/blog/post/hardening_openvpn_for_def_con
https://www.zscaler.com/blogs/security-research/cve-2024-3661-k-tunnelvision-exposes-vpn-bypass-vulnerability