A OS command injection vulnerability in the CGI program of Zyxel USG FLEX 100(W) firmware versions 5.00 through 5.21 Patch 1 USG FLEX 200 firmware versions 5.00 through 5.21 Patch 1 USG FLEX 500 firmware versions 5.00 through 5.21 Patch 1 USG FLEX 700 firmware versions 5.00 through 5.21 Patch 1 USG FLEX 50(W) firmware versions 5.10 through 5.21 Patch 1 USG20(W)-VPN firmware versions 5.10 through 5.21 Patch 1 ATP series firmware versions 5.10 through 5.21 Patch 1 VPN series firmware versions 4.60 through 5.21 Patch 1 which could allow an attacker to modify specific files and then execute some OS commands on a vulnerable device.