CVE-2020-3204: Cisco IOS Software RCE Vulnerability

CVE-2020-3204 Overview

A vulnerability exists in the Tool Command Language (Tcl) interpreter of Cisco IOS Software and Cisco IOS XE Software that could allow an authenticated, local attacker with privileged EXEC credentials to execute arbitrary code on the underlying operating system with root privileges. The vulnerability stems from insufficient input validation of data passed to the Tcl interpreter, enabling attackers to load malicious Tcl code on affected devices. Successful exploitation could result in memory corruption or arbitrary code execution with root privileges on the underlying OS.

Critical Impact

Authenticated attackers with privileged EXEC access can achieve root-level code execution on affected Cisco IOS and IOS XE devices, potentially leading to complete device compromise.

Affected Products

• Cisco IOS Software (multiple versions including 12.2, 12.4, 15.x series)
• Cisco IOS XE Software (versions 3.2 through 16.12)
• Cisco network infrastructure devices running vulnerable IOS/IOS XE versions

Discovery Timeline

• June 3, 2020 - CVE-2020-3204 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2020-3204

Vulnerability Analysis

This vulnerability (CWE-20: Improper Input Validation) affects the Tcl interpreter embedded within Cisco IOS and IOS XE operating systems. The Tcl interpreter is used to execute scripts and automate various tasks on Cisco network devices. The flaw exists because the interpreter fails to properly validate input data before processing, allowing an authenticated attacker with privileged EXEC credentials to inject malicious Tcl code.

When exploited, the vulnerability can lead to two primary outcomes: memory corruption within the device's operating system, or direct execution of attacker-supplied code with root-level privileges. Since network infrastructure devices typically occupy critical positions within enterprise networks, compromising these devices could enable lateral movement, traffic interception, or network-wide disruption.

Root Cause

The root cause of CVE-2020-3204 is insufficient input validation within the Tcl interpreter component of Cisco IOS and IOS XE Software. The interpreter does not adequately sanitize or validate data passed to it during script execution, creating an opportunity for attackers to inject and execute malicious code. This improper input validation allows specially crafted Tcl scripts to bypass intended security boundaries and execute commands with elevated privileges on the underlying operating system.

Attack Vector

The attack vector for this vulnerability requires local access to the device with privileged EXEC credentials. An attacker must first authenticate to the affected Cisco device with appropriate access levels (typically privilege level 15 or equivalent). Once authenticated, the attacker can load malicious Tcl code through the device's command-line interface.

The exploitation flow involves crafting a malicious Tcl script designed to exploit the input validation weakness, uploading or entering this script on the target device, and executing it through the Tcl interpreter. The malicious code then runs with root privileges on the underlying operating system, providing the attacker with complete control over the device.

Detection Methods for CVE-2020-3204

Indicators of Compromise

• Unexpected Tcl script execution or tclsh command usage in device logs
• Unusual processes or memory consumption patterns on affected devices
• Suspicious configuration changes or new user accounts created on network devices
• Evidence of privilege escalation attempts from EXEC mode

Detection Strategies

• Monitor authentication logs for privileged EXEC access patterns and anomalous login behavior
• Implement syslog monitoring for Tcl interpreter activity and script execution events
• Deploy network behavior analysis to detect unusual traffic patterns from infrastructure devices
• Review command accounting logs for tclsh and related Tcl commands

Monitoring Recommendations

• Enable AAA accounting to log all command-line activity on affected devices
• Configure centralized syslog collection to capture security events from IOS/IOS XE devices
• Implement SNMP traps for configuration changes and unexpected system events
• Establish baseline device behavior and alert on deviations

How to Mitigate CVE-2020-3204

Immediate Actions Required

• Review and restrict privileged EXEC access to only essential personnel
• Audit user accounts with access to affected devices and remove unnecessary privileges
• Implement network segmentation to limit access to management interfaces
• Enable logging and monitoring for Tcl interpreter usage on affected devices

Patch Information

Cisco has released security patches to address this vulnerability. Administrators should consult the Cisco Security Advisory to identify the appropriate fixed software version for their deployment. Organizations should prioritize patching devices based on their criticality and exposure level.

The advisory provides detailed information on fixed releases for both Cisco IOS and Cisco IOS XE Software versions. Customers with active service contracts can obtain fixed software through the Cisco Software Center.

Workarounds

• Disable or restrict access to the Tcl interpreter where operationally feasible
• Implement strict role-based access control to limit privileged EXEC access
• Use infrastructure access control lists (ACLs) to restrict management plane access
• Consider implementing terminal access controller access-control system (TACACS+) for enhanced authentication and authorization

# Example: Restrict VTY line access to specific management network
access-list 10 permit 10.10.10.0 0.0.0.255
line vty 0 15
 access-class 10 in
 privilege level 15
 login authentication TACACS_AUTH