CVE-2021-40124: Cisco AnyConnect Privilege Escalation Flaw

CVE-2021-40124 Overview

A vulnerability in the Network Access Manager (NAM) module of Cisco AnyConnect Secure Mobility Client for Windows could allow an authenticated, local attacker to escalate privileges on an affected device. This vulnerability is due to incorrect privilege assignment to scripts executed before user logon. An attacker could exploit this vulnerability by configuring a script to be executed before logon. A successful exploit could allow the attacker to execute arbitrary code with SYSTEM privileges.

Critical Impact

Local attackers with authenticated access can escalate privileges to SYSTEM level, enabling complete compromise of the affected Windows system through malicious pre-logon script execution.

Affected Products

• Cisco AnyConnect Secure Mobility Client for Windows
• Cisco AnyConnect Secure Mobility Client with Network Access Manager (NAM) module enabled

Discovery Timeline

• 2021-11-04 - CVE CVE-2021-40124 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-40124

Vulnerability Analysis

This privilege escalation vulnerability exists within the Network Access Manager (NAM) module of Cisco AnyConnect Secure Mobility Client for Windows. The core issue stems from incorrect privilege assignment (CWE-266) and improper privilege management (CWE-269) affecting scripts that execute during the pre-logon phase of the Windows authentication process.

The NAM module is designed to provide comprehensive network access management capabilities, including 802.1X authentication and network profile management. As part of its functionality, NAM supports the execution of custom scripts before user logon to perform network configuration tasks. However, the vulnerability arises because these pre-logon scripts inherit elevated SYSTEM privileges regardless of the security context in which they should operate.

Root Cause

The vulnerability is caused by incorrect privilege assignment to scripts executed before user logon. The NAM module fails to properly restrict the privilege level at which pre-logon scripts execute, allowing them to run with SYSTEM-level permissions. This design flaw enables authenticated local attackers to leverage the pre-logon script functionality as a privilege escalation vector by configuring malicious scripts that will execute with the highest Windows privilege level.

Attack Vector

The attack vector for this vulnerability is local, requiring the attacker to have authenticated access to the target system. The exploitation path involves:

1. An authenticated local user gains access to the AnyConnect NAM configuration interface
2. The attacker configures a malicious script to be executed during the pre-logon phase
3. During the next system startup or user logon event, the NAM module executes the configured script
4. Due to the incorrect privilege assignment, the script executes with SYSTEM privileges
5. The attacker's payload runs with full administrative control over the system

The vulnerability requires no user interaction beyond the initial configuration of the malicious script. Once configured, the privilege escalation occurs automatically during the pre-logon phase, making detection challenging as the malicious activity occurs before standard user-level monitoring is active.

Detection Methods for CVE-2021-40124

Indicators of Compromise

• Unexpected or unauthorized modifications to AnyConnect NAM pre-logon script configurations
• New or modified scripts appearing in AnyConnect NAM configuration directories
• Unusual process execution activity during system startup with SYSTEM privileges originating from NAM components
• Windows Event Log entries showing script execution from NAM module with elevated privileges

Detection Strategies

• Monitor AnyConnect NAM configuration files and directories for unauthorized changes
• Implement file integrity monitoring (FIM) on AnyConnect installation directories
• Review Windows Security Event Logs for privilege escalation attempts associated with vpnagent.exe or NAM-related processes
• Deploy endpoint detection solutions to identify suspicious script execution during pre-logon phase

Monitoring Recommendations

• Enable verbose logging for Cisco AnyConnect client operations
• Configure alerting for modifications to NAM profile configurations
• Monitor for unusual SYSTEM-level process creation during Windows startup sequences
• Implement behavioral analysis for pre-logon script execution patterns

How to Mitigate CVE-2021-40124

Immediate Actions Required

• Review all pre-logon scripts configured in the NAM module for unauthorized or suspicious entries
• Restrict administrative access to AnyConnect NAM configuration to trusted users only
• Apply the latest security patch from Cisco as detailed in their security advisory
• Consider temporarily disabling pre-logon script functionality if not required for operations

Patch Information

Cisco has released a security update to address this vulnerability. Administrators should consult the Cisco Security Advisory cisco-sa-anyconnect-nam-priv-yCsRNUGT for specific version information and download the appropriate patched version of Cisco AnyConnect Secure Mobility Client for their environment.

Workarounds

• Disable the Network Access Manager (NAM) module if 802.1X authentication is not required
• Remove or disable pre-logon script execution capabilities through NAM policy configuration
• Implement strict access controls limiting which users can modify NAM configurations
• Deploy application whitelisting to prevent unauthorized script execution during pre-logon phase

# Configuration example
# Verify AnyConnect version and check for vulnerable NAM module
# Run from elevated command prompt
"C:\Program Files (x86)\Cisco\Cisco AnyConnect Secure Mobility Client\vpnagent.exe" -v

# Review NAM configuration for pre-logon scripts
# Configuration files typically located in:
# C:\ProgramData\Cisco\Cisco AnyConnect Secure Mobility Client\Network Access Manager\