CVE-2017-9478 in DPC3939
Summary
by MITRE
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 sets the CM MAC address to a value with a two-byte offset from the MTA/VoIP MAC address, which indirectly allows remote attackers to discover hidden Home Security Wi-Fi networks by leveraging the embedding of the MTA/VoIP MAC address into the DNS hostname.
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Analysis
by VulDB Data Team • 11/02/2019
The vulnerability identified as CVE-2017-9478 affects Comcast firmware versions running on Cisco DPC3939 cable modem devices, specifically versions dpc3939-P20-18-v303r20421733-160420a-CMCST and dpc3939-P20-18-v303r20421746-170221a-CMCST. This flaw represents a significant security weakness in the device's MAC address assignment mechanism that creates predictable relationships between different network interfaces. The vulnerability stems from the firmware's practice of setting the cable modem's MAC address with a two-byte offset from the embedded MTA/VoIP MAC address, creating a direct correlation that can be exploited by remote attackers to infer information about network topology and hidden services.
The technical implementation of this vulnerability occurs through the DNS hostname generation process where the MTA/VoIP MAC address is embedded into the device's hostname. This embedding creates a predictable pattern that allows attackers to determine the MAC address of the cable modem by analyzing the DNS records of the VoIP interface. The relationship between the two MAC addresses follows a mathematical offset pattern that can be reverse-engineered, effectively breaking the intended network segmentation between voice services and wireless security domains. This type of information disclosure vulnerability maps directly to CWE-200, which covers "Information Exposure" and specifically addresses scenarios where system information is inadvertently revealed through predictable patterns or embedded data.
The operational impact of this vulnerability extends beyond simple information disclosure to create potential pathways for more sophisticated attacks against home security networks. When attackers can determine the cable modem's MAC address through the MTA/VoIP interface, they gain insight into the device's network configuration and can potentially use this information to craft more targeted attacks against the wireless network. The vulnerability particularly affects home security Wi-Fi networks that rely on the assumption that network interfaces are properly segmented and that MAC addresses are not directly correlatable. This weakness creates an indirect attack vector where remote adversaries can leverage legitimate network discovery mechanisms to uncover hidden network components, effectively bypassing network segmentation controls that should protect sensitive services.
The attack surface for this vulnerability is particularly concerning given the prevalence of cable modem deployments in residential and small business environments where network security assumptions may be less stringent. The implementation of this flaw demonstrates a failure in the principle of least privilege and proper network isolation between different service interfaces on the same device. From an ATT&CK framework perspective, this vulnerability aligns with techniques involving reconnaissance and credential access through information discovery, where attackers can use the embedded MAC address information to map network topology and identify potential attack targets. The vulnerability also represents a failure in secure configuration management, as the firmware does not properly isolate the different MAC address spaces that should remain independent for security purposes.
Mitigation strategies for CVE-2017-9478 should focus on firmware updates from Cisco and Comcast that address the MAC address assignment algorithm to eliminate the predictable offset pattern. Network administrators should implement additional segmentation controls such as VLAN isolation between MTA/VoIP and wireless interfaces, even when the underlying firmware vulnerability exists. The use of network access control lists and firewall rules to restrict DNS query access can help limit the exposure of embedded MAC address information in DNS records. Organizations should also consider implementing network monitoring to detect unusual DNS query patterns that might indicate attempts to extract MAC address information through the hostname embedding mechanism. Regular security assessments of network infrastructure should include verification of MAC address assignment patterns to ensure that no predictable relationships exist between different network service interfaces that could be exploited by remote attackers.