Zyxel Firewall ZTP Unauthenticated Command Injection

This module exploits CVE-2022-30525, an unauthenticated remote
command injection vulnerability affecting Zyxel firewalls with zero
touch provisioning (ZTP) support. By sending a malicious setWanPortSt
command containing an mtu field with a crafted OS command to the
/ztp/cgi-bin/handler page, an attacker can gain remote command execution
as the nobody user.

Affected Zyxel models are:

* USG FLEX 50, 50W, 100W, 200, 500, 700 using firmware 5.21 and below
* USG20-VPN and USG20W-VPN using firmware 5.21 and below
* ATP 100, 200, 500, 700, 800 using firmware 5.21 and below