[BYPASS] IKEv2 Fortinet Misconfiguration

Overview

The target was a Fortinet Next-Generation Firewall (NGFW), a formidable system relying on aggressive Deep Packet Inspection (DPI). Its directive was simple: block all unauthorized tunneling. My objective was proving that its "threat intelligence" had exploitable policy blind spots.

Initial attempts with solutions like OpenVPN and WireGuard failed instantly. This was not simple port blocking. The FortiGate performed protocol fingerprinting, immediately recognizing the distinct traffic signatures of these protocols and dropping the connections. Any consumer VPN signature was a red flag. The challenge was confirming and exploiting a predictable policy exclusion.

The goal crystallized: Protocol-Based Bypass.

The IKEv2 Differential

Testing revealed the key vulnerability: IKEv2 (Internet Key Exchange version 2) traffic showed inconsistent signs of life while other protocols were instantly terminated.

IKEv2 is part of the IPsec suite and is specifically favored for enterprise-grade site-to-site VPNs. This was the technical distinction. The DPI engine was clearly trained to terminate common consumer tools, leaving the corporate standard—which the network itself might depend on—less scrutinized.

The specific protocol reliance confirmed the attack vector: IKEv2 traffic uses UDP Ports 500 (ISAKMP) and 4500 (NAT-T). I theorized these ports were either monitored with lower fidelity or excluded entirely from the aggressive "deny VPN" rule set that flagged OpenVPN's default ports.

The PoC and Execution

My strategy was explicit: Leverage the enterprise protocol's traffic pattern to bypass policy.

I deployed a Virtual Private Server (VPS) and utilized Strongswan, an open-source IPsec/IKEv2 implementation. This setup ensured clean, standard IKEv2 traffic.

The connection attempt was made on UDP 500/4500. The connection established immediately. The tunnel was reliable, confirming sustained data flow without DPI interruption. The FortiGate, with all its deep inspection capabilities, completely ignored the custom IKEv2 tunnel.

This result confirmed the practical blind spot: the NGFW was misconfigured for signature-based termination of common consumer tools but maintained a permissive stance on standard enterprise protocols. Leveraging IKEv2 traffic successfully bypassed the entire DPI mechanism.