An unauthenticated command injection vulnerability exists in the D-Link DIR-878A1 router firmware FW101B04.bin. The vulnerability occurs in the 'SetDynamicDNSSettings' functionality, where the 'ServerAddress' and 'Hostname' parameters in prog.cgi are stored in NVRAM and later used by rc to construct system commands executed via twsystem(). An attacker can exploit this vulnerability remotely without authentication by sending a specially crafted HTTP request, leading to arbitrary command execution on the device.