CVE-2023-20241: Cisco AnyConnect Secure Client DoS Flaw

CVE-2023-20241 Overview

CVE-2023-20241 is an out-of-bounds memory read vulnerability affecting Cisco Secure Client Software (formerly known as AnyConnect Secure Mobility Client). This vulnerability enables an authenticated, local attacker to cause a denial of service (DoS) condition by crashing the VPN Agent service on multi-user systems.

The vulnerability stems from improper memory handling within the Cisco Secure Client Software. An attacker with valid local credentials can exploit this flaw by logging into an affected device while another user is accessing the Cisco Secure Client, then sending specially crafted packets to a port on the local host to trigger the out-of-bounds memory read condition.

Critical Impact

Successful exploitation crashes the VPN Agent service, denying VPN connectivity to all users on the affected multi-user system.

Affected Products

• Cisco AnyConnect Secure Mobility Client versions 4.9.00086 through 4.9.06037
• Cisco Secure Client versions 4.10.00093 through 4.10.07073
• Cisco Secure Client versions 5.0.00238 through 5.0.03076

Discovery Timeline

• 2023-11-22 - CVE-2023-20241 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2023-20241

Vulnerability Analysis

This vulnerability is classified as CWE-125 (Out-of-Bounds Read), a memory safety issue where the application reads data past the end of an intended buffer. In the context of Cisco Secure Client, this out-of-bounds read occurs when processing certain network packets, allowing an attacker to trigger memory access violations that crash the VPN Agent service.

The attack requires the adversary to have valid local credentials on a multi-user system, limiting exploitation to authenticated users. However, once these conditions are met, the attacker can reliably crash the VPN service, disrupting connectivity for all users on the affected system.

Root Cause

The root cause of CVE-2023-20241 lies in insufficient bounds checking within the Cisco Secure Client Software's packet processing routines. When the software receives crafted packets on the local host, it fails to properly validate buffer boundaries before performing memory read operations. This allows an attacker to trigger reads beyond allocated memory regions, causing the VPN Agent service to crash due to memory access violations.

Attack Vector

The attack vector for CVE-2023-20241 requires local access with valid credentials on a multi-user system. The exploitation sequence involves:

1. The attacker authenticates to the target system using valid local credentials
2. The attacker waits for or identifies another user actively using Cisco Secure Client on the same system
3. The attacker sends specially crafted packets to a specific port on the local host
4. The malformed packets trigger an out-of-bounds memory read in the VPN Agent service
5. The memory access violation causes the VPN Agent service to crash, affecting all users

This attack does not require user interaction beyond the initial authentication and targets the local network stack rather than remote network interfaces.

Detection Methods for CVE-2023-20241

Indicators of Compromise

• Unexpected VPN Agent service crashes or restarts on multi-user systems
• Event logs showing memory access violations or unhandled exceptions in Cisco Secure Client processes
• Unusual local network traffic patterns on ports used by Cisco Secure Client
• Multiple users reporting simultaneous VPN disconnections on shared systems

Detection Strategies

• Monitor Windows Event Logs for application crashes related to vpnagent.exe or Cisco Secure Client components
• Implement endpoint detection rules for abnormal local port activity targeting Cisco Secure Client service ports
• Configure alerts for repeated VPN Agent service restarts within short time intervals
• Deploy SentinelOne's behavioral AI to detect anomalous process behavior associated with DoS attacks

Monitoring Recommendations

• Enable verbose logging for Cisco Secure Client to capture detailed service state information
• Configure SIEM alerts for correlated VPN Agent crash events across multiple systems
• Monitor local inter-process communication (IPC) traffic for malformed packet patterns
• Review authentication logs for unusual local login patterns on multi-user VPN endpoints

How to Mitigate CVE-2023-20241

Immediate Actions Required

• Update Cisco Secure Client to the latest patched version as recommended by Cisco
• Review multi-user systems running affected versions and prioritize patching based on user density
• Restrict local login access to trusted users on systems running vulnerable VPN client versions
• Enable enhanced monitoring on affected systems until patches can be applied

Patch Information

Cisco has released security updates to address CVE-2023-20241. Organizations should consult the Cisco Security Advisory for detailed patch information and upgrade guidance. The advisory provides specific version numbers for patched releases across the affected product lines.

Administrators should upgrade:

• Cisco AnyConnect Secure Mobility Client 4.9.x to a patched release
• Cisco Secure Client 4.10.x to a patched release
• Cisco Secure Client 5.0.x to a patched release

Workarounds

• Limit concurrent user sessions on systems running Cisco Secure Client where practical
• Implement strict access controls to reduce the number of authenticated local users on shared systems
• Consider deploying VPN clients on single-user workstations rather than multi-user environments
• Monitor for service disruptions and implement automatic restart mechanisms as a temporary measure

# Check installed Cisco Secure Client version on Windows
wmic product where "name like '%Cisco Secure Client%'" get name,version

# Check for VPN Agent service status
sc query vpnagent

# Monitor VPN Agent service for crashes (PowerShell)
Get-EventLog -LogName Application -Source "Application Error" | Where-Object {$_.Message -like "*vpnagent*"}