A command injection vulnerability exists in the D-Link DIR-823G router firmware DIR823G_V1.0.2B05_20181207.bin in the timelycheck and sysconf binaries, which process the /var/system/linux_vlan_reinit file. The vulnerability occurs because content read from this file is only partially validated for a prefix and then formatted using vsnprintf() before being executed with system(), allowing an attacker with write access to /var/system/linux_vlan_reinit to execute arbitrary commands on the device.