WDR201A WiFi Extender (HW V2.1, FW LFMZX28040922V1.02) contains an OS command injection vulnerability in the firewall.cgi binary across five request handlers that apply insufficient input validation. Attackers can inject arbitrary shell commands through vulnerable parameters like websURLFilter, websHostFilter, portForward, singlePortForward, and ipportFilter using subshell syntax or unfiltered parameters, with payloads persisting in NVRAM and re-executing on every subsequent firewall.cgi request.