The Comcast firmware on Cisco DPC3939 (firmware version dpc3939-P20-18-v303r20421733-160420a-CMCST) and DPC3939 (firmware version dpc3939-P20-18-v303r20421746-170221a-CMCST) devices allows remote attackers to discover a CM MAC address by sniffing Wi-Fi traffic and performing simple arithmetic calculations.