DNS hijacking via shared VPN NAT is feasible because the full 16-bit TxID space (up to 65,536 values) can be brute-forced in an average of 4.27 seconds, well within a typical 10-second DNS request timeout; browser DNS cache windows range from 60 seconds (Chrome/Edge) to 660 seconds or more (Firefox), with longer windows enlarging the injection race window.
From 2026-yang-invisible-adversaries-systematic — Invisible Adversaries: A Systematic Study of Session Manipulation Attacks on VPNs
· §III-C, §IV-D, Table I
· 2026
· arXiv preprint
Implications
Circumvention tools that route user DNS through a shared VPN NAT expose all co-tenant users to response injection; deploying encrypted DNS (DoH/DoT) over a separate channel or per-user resolver eliminates the shared-session attack surface.
Circumvention clients should avoid relying on the VPN server's forwarded DNS and instead use a trusted local resolver to prevent TxID-based injection races.