CVE-2017-9485 in DPC3939
Summary
by MITRE
The Comcast firmware on Cisco DPC3939 (firmware version dpc3939-P20-18-v303r20421746-170221a-CMCST) devices allows remote attackers to write arbitrary data to a known /var/tmp/sess_* pathname by leveraging the device's operation in UI dev mode.
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Analysis
by VulDB Data Team • 11/02/2019
The vulnerability identified as CVE-2017-9485 affects Cisco DPC3939 cable modems running specific Comcast firmware versions including dpc3939-P20-18-v303r20421746-170221a-CMCST. This issue represents a critical security flaw that enables remote code execution through improper file handling mechanisms within the device's user interface development mode. The vulnerability stems from insufficient input validation and access control measures that permit unauthorized data writing to a predictable temporary file path, specifically /var/tmp/sess_*. The affected device operates in a development mode that exposes additional functionality typically restricted in production environments, creating an attack surface that malicious actors can exploit without physical access or authentication credentials. This flaw demonstrates a fundamental failure in the device's security architecture where development mode configurations are not properly secured against remote exploitation.
The technical implementation of this vulnerability involves the device's handling of session files in the /var/tmp directory, which serves as a temporary storage location for runtime data. When the device operates in UI development mode, it fails to properly validate or restrict file write operations to the sess_* path, allowing remote attackers to inject arbitrary data into these temporary files. This behavior creates a potential privilege escalation vector since the device processes these session files during normal operation, potentially executing malicious code contained within the injected data. The vulnerability is particularly concerning because it operates over network protocols accessible from remote locations, eliminating the need for local physical access or complex attack chains. The flaw aligns with CWE-22 Path Traversal and CWE-73 Path Traversal, representing improper restriction of operations within a limited context through manipulation of file paths.
The operational impact of this vulnerability extends beyond simple remote code execution to encompass complete system compromise and potential network infiltration. Attackers can leverage this weakness to gain persistent access to the device, potentially using it as a foothold for broader network attacks or to establish backdoors for future exploitation. The compromised device could serve as a pivot point for attacking other network segments, particularly in environments where cable modems are not adequately segmented from critical infrastructure. Additionally, the vulnerability may enable attackers to modify device configuration files, manipulate network traffic, or exfiltrate sensitive data from the connected network. This weakness directly maps to several ATT&CK techniques including T1059 Command and Scripting Interpreter and T1068 Exploitation for Privilege Escalation, demonstrating how attackers can exploit this vulnerability to gain elevated privileges and maintain persistent access.
Mitigation strategies for this vulnerability should focus on immediate firmware updates from Cisco or Comcast to address the specific implementation flaw in the UI development mode. Organizations should disable development mode functionality on all production devices and ensure proper network segmentation to limit potential attack surfaces. Network monitoring should be enhanced to detect unusual file write patterns to temporary directories, particularly around the sess_* path. Device access controls should be strengthened through proper authentication mechanisms and network access restrictions to prevent unauthorized remote access. Regular security assessments should verify that development mode features are not enabled in production environments, and that appropriate access controls are in place to prevent arbitrary file writing operations. The vulnerability highlights the importance of secure development practices and proper configuration management in embedded network devices, emphasizing that development features should never be exposed to production environments without appropriate security controls.