Network protocol normalization and reassembly is the basis of traffic inspection performed by NGFW and IPS devices. Even common network protocols are complex with multiple possible interpretations for the same traffic sequence. We present a novel method for automated discovery of errors in traffic normalization by targeted protocol stack fuzzing. These errors can be used by attackers to evade detection and bypass security devices. We will demonstrate the techniques against up-to-date security devices and show that many security devices still have basic evasion vulnerabilities. The tools used will be publicly available after this presentation.