CVE-2017-12336 in NX-OS
Summary
by MITRE
A vulnerability in the TCL scripting subsystem of Cisco NX-OS System Software could allow an authenticated, local attacker to escape the interactive TCL shell and gain unauthorized access to the underlying operating system of the device. The vulnerability exists due to insufficient input validation of user-supplied files passed to the interactive TCL shell of the affected device. An attacker could exploit this vulnerability to escape the scripting sandbox and execute arbitrary commands on the underlying operating system with the privileges of the authenticated user. To exploit this vulnerability, an attacker must have local access and be authenticated to the targeted device with administrative or tclsh execution privileges. This vulnerability affects the following products running Cisco NX-OS System Software: Multilayer Director Switches, Nexus 2000 Series Fabric Extenders, Nexus 3000 Series Switches, Nexus 3500 Platform Switches, Nexus 5000 Series Switches, Nexus 5500 Platform Switches, Nexus 5600 Platform Switches, Nexus 6000 Series Switches, Nexus 7000 Series Switches, Nexus 7700 Series Switches, Nexus 9000 Series Switches in standalone NX-OS mode, Nexus 9500 R-Series Line Cards and Fabric Modules, Unified Computing System Manager. Cisco Bug IDs: CSCve93750, CSCve93762, CSCve93763, CSCvg04127.
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Analysis
by VulDB Data Team • 01/25/2021
The vulnerability described in CVE-2017-12336 represents a critical sandbox escape flaw within the TCL scripting subsystem of Cisco NX-OS software, fundamentally compromising the security boundaries designed to isolate user interactions from the underlying operating system. This issue arises from inadequate input validation mechanisms that fail to properly sanitize user-supplied files before processing them within the interactive TCL shell environment. The flaw specifically targets the privileged execution context where administrative users can access the tclsh shell, creating a pathway for authenticated attackers to bypass the intended security controls and execute arbitrary commands with elevated privileges. The vulnerability's impact extends across multiple Cisco networking platforms including high-end switches and unified computing systems, making it particularly concerning for enterprise network infrastructure.
The technical exploitation of this vulnerability requires an authenticated attacker with administrative or tclsh execution privileges who possesses local access to the target device, aligning with ATT&CK technique T1059.001 for command and scripting interpreter and T1068 for exploit for privilege escalation. The insufficient input validation creates a path for command injection attacks where maliciously crafted input can manipulate the TCL shell's parsing behavior to execute unintended system commands. This represents a classic sandbox escape scenario where the boundary between user-controlled input and system-level operations becomes compromised, allowing attackers to break out of the restricted scripting environment into the full operating system. The vulnerability's classification as a CWE-20 input validation weakness demonstrates the fundamental flaw in the system's trust model where user-supplied data is not adequately sanitized before being processed.
The operational impact of this vulnerability extends beyond simple privilege escalation to encompass complete system compromise of affected networking equipment. Attackers who successfully exploit this vulnerability can gain unauthorized access to sensitive network infrastructure, potentially enabling them to modify routing tables, intercept traffic, or establish persistent backdoors within the network environment. The affected products span Cisco's entire NX-OS ecosystem, including enterprise switches, fabric extenders, and unified computing systems, creating widespread exposure across different network segments and deployment scenarios. This vulnerability particularly threatens network security operations as it undermines the trust model of network devices that are expected to maintain secure isolation between different operational contexts. The attack vector requires local access, but the privilege escalation potential means that any administrative user with access to the device can become a significant threat vector.
Mitigation strategies for CVE-2017-12336 should focus on both immediate defensive measures and long-term architectural improvements. Cisco has released patches addressing the vulnerability through software updates that strengthen input validation within the TCL subsystem and improve sandbox boundary enforcement. Organizations should prioritize applying these patches across all affected devices and implement strict access controls limiting administrative privileges to only necessary personnel. Network segmentation and monitoring of administrative access activities can help detect potential exploitation attempts. The vulnerability's characteristics align with ATT&CK technique T1078 for valid accounts and T1548.001 for abuse of process isolation, suggesting that defensive measures should include monitoring for unusual command execution patterns and implementing principle of least privilege controls. Additionally, organizations should consider implementing network access controls to prevent local access to administrative interfaces and establish robust audit trails for all administrative activities to detect potential exploitation attempts.