Defeating VPN Always-On

The talk examines how the always-on mode of three VPN agents can be defeated before the tunnel is established, letting an unprivileged user reach the network. It covers an on-network attacker and an insider on a hardened laptop, whose settings and logs are readable by the user.

Two checks are targeted. Trusted network detection can be fooled by impersonating expected DNS and DHCP values. Captive portal detection can be abused with a fake portal that opens a channel, though one vendor disables it by default. Some agents also permit unexpected UDP ports. Windows Filtering Platform rules explain the differences: one vendor hides its filters behind access control, and another shadows a restrictive rule with a higher-priority permit.

Results vary by vendor and version, and the vendors' responses are described. Red teams should expect at least one working path per product. Defenders should avoid trusting network values an attacker can spoof, prefer certificate-based server checks, and protect configuration files.