CVE-2024-20492: Cisco Telepresence VCS Privilege Escalation

CVE-2024-20492 Overview

A command injection vulnerability exists in the restricted shell of Cisco Expressway Series that could allow an authenticated, local attacker to perform command injection attacks on the underlying operating system and elevate privileges to root. This vulnerability requires the attacker to have Administrator-level credentials with read-write privileges on an affected device.

The vulnerability stems from insufficient validation of user-supplied input within the CLI interface. An attacker could exploit this vulnerability by submitting a series of crafted CLI commands, allowing them to escape the restricted shell and gain root privileges on the underlying operating system of the affected device.

Critical Impact

Authenticated attackers with administrator credentials can escape the restricted shell environment and gain full root access to the underlying operating system, potentially compromising the entire Cisco Expressway deployment.

Affected Products

• Cisco Expressway Control (Expressway-C) devices
• Cisco Expressway Edge (Expressway-E) devices
• Cisco TelePresence Video Communication Server (Expressway versions x8.1 through x15.0.3)

Discovery Timeline

• October 2, 2024 - CVE-2024-20492 published to NVD
• October 8, 2024 - Last updated in NVD database

Technical Details for CVE-2024-20492

Vulnerability Analysis

This command injection vulnerability (CWE-77) affects the restricted shell interface of Cisco Expressway Series devices. The restricted shell is designed to limit administrative users to a predefined set of commands, preventing direct access to the underlying operating system. However, due to insufficient input validation, specially crafted commands can break out of these restrictions.

The vulnerability requires local access and administrator-level credentials with read-write privileges, which limits the attack surface. However, once exploited, the attacker gains complete root access to the underlying operating system, allowing them to modify system configurations, install persistent backdoors, intercept communications, or pivot to other network resources.

Root Cause

The vulnerability is caused by insufficient validation of user-supplied input in the CLI command parser. When processing certain commands, the restricted shell fails to properly sanitize special characters or command sequences, allowing an attacker to inject arbitrary operating system commands that execute outside the restricted shell context.

Attack Vector

The attack vector is local, requiring the attacker to have authenticated administrator-level access to the device CLI. The exploitation process involves submitting a series of carefully crafted CLI commands that exploit the input validation weakness to escape the restricted shell environment. Once the attacker escapes the restricted shell, they can execute arbitrary commands with root privileges on the underlying Linux-based operating system.

The attack chain typically follows this pattern: an attacker first obtains or compromises administrator credentials, then connects to the device CLI either through console access, SSH, or other management interfaces. By submitting specially crafted input that includes shell metacharacters or command sequences, the attacker can break out of the restricted environment and gain full system access.

Detection Methods for CVE-2024-20492

Indicators of Compromise

• Unusual command execution patterns in CLI audit logs from administrator accounts
• Unexpected processes running with root privileges on Expressway devices
• Modifications to system files or configurations outside of normal administrative operations
• Evidence of shell escape attempts in device logs

Detection Strategies

• Monitor CLI session logs for unusual command sequences or shell metacharacter usage
• Implement behavioral analysis to detect anomalous administrative activity patterns
• Review authentication logs for administrator account access from unexpected sources
• Deploy endpoint detection and response (EDR) capabilities where supported to monitor process execution

Monitoring Recommendations

• Enable comprehensive logging on all Cisco Expressway devices and forward logs to a centralized SIEM
• Configure alerts for privileged command execution and authentication events
• Implement network segmentation to limit lateral movement from compromised Expressway devices
• Regularly audit administrator accounts and enforce multi-factor authentication

How to Mitigate CVE-2024-20492

Immediate Actions Required

• Apply the latest security patches from Cisco as referenced in the Cisco Security Advisory
• Review and audit all administrator accounts with read-write privileges
• Restrict CLI access to only essential personnel and from trusted management networks
• Monitor administrator sessions for suspicious activity

Patch Information

Cisco has released a security advisory addressing this vulnerability. Organizations should consult the Cisco Security Advisory cisco-sa-expw-escalation-3bkz77bD for specific fixed software versions and upgrade guidance. The vulnerability affects a wide range of versions from x8.1 through x15.0.3, so organizations should identify their current version and plan an upgrade to a fixed release.

Workarounds

• Limit administrator access to only trusted personnel who require read-write privileges
• Implement strict access control lists (ACLs) to restrict management interface access to specific IP addresses
• Use jump hosts or bastion servers for all administrative access to Expressway devices
• Consider implementing session recording for all administrative CLI sessions to aid in forensic analysis

# Example: Restrict SSH access to management interfaces (apply via device configuration)
# Consult Cisco documentation for specific configuration commands
# Limit management access to trusted IP ranges only
# Enable enhanced logging for CLI sessions