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A vulnerability has been discovered that enables an attacker to discover the CM MAC of a Comcast customer's gateway by connecting to the "xfinitywifi" hotspot running on the gateway.
Affected Platforms
Cisco DPC3939, firmware version dpc3939-P20-18-v303r20421733-160420a-CMCST
Cisco DPC3939, firmware version dpc3939-P20-18-v303r20421746-170221a-CMCST
Proof-of-Concept
When a user connects to an "xfinitywifi" hotspot on an affected gateway, the CM MAC, which is not normally known, is included in the DHCP ACK.
Test Environment
Cisco DPC3939, firmware version dpc3939-P20-18-v303r20421733-160420a-CMCST
Cisco DPC3939, firmware version dpc3939-P20-18-v303r20421746-170221a-CMCST
Mitigation
Comcast customers can prevent this information leakage by disabling the "xfinitywifi" hotspot on their gateway.
Recommended Remediation
Configure the "xfinitywifi" DHCP server so that it does not leak the CM MAC.