CVE-2023-20240: Cisco AnyConnect Secure Mobility DoS Flaw

CVE-2023-20240 Overview

CVE-2023-20240 is an out-of-bounds memory read vulnerability affecting Cisco Secure Client Software (formerly AnyConnect Secure Mobility Client). This vulnerability allows an authenticated, local attacker to cause a denial of service (DoS) condition on affected systems. The flaw exists within the VPN Agent service and can be exploited when multiple users access the Cisco Secure Client simultaneously on a multi-user system.

The vulnerability stems from improper handling of memory boundaries within the Cisco Secure Client Software. An attacker with valid local credentials can exploit this condition by sending specially crafted packets to a port on the local host while another user is accessing the Cisco Secure Client, causing the VPN Agent service to crash and become unavailable to all users on the system.

Critical Impact

Successful exploitation crashes the VPN Agent service, disrupting VPN connectivity for all users on the affected multi-user system and potentially impacting enterprise remote access operations.

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-2023-20240 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2023-20240

Vulnerability Analysis

This vulnerability is classified as CWE-125 (Out-of-Bounds Read), a memory corruption vulnerability where the software reads data past the end of the intended buffer. In the context of Cisco Secure Client, this out-of-bounds memory read occurs when the software improperly handles crafted packets sent to a local port during concurrent user sessions.

The attack requires local access and valid user credentials on a multi-user system, such as a shared workstation or terminal server environment. The attacker must time their exploitation attempt to coincide with another user's active Cisco Secure Client session. While the vulnerability does not allow for data exfiltration or code execution, the availability impact is significant as it denies VPN service to all users on the affected system.

Root Cause

The root cause of CVE-2023-20240 lies in insufficient bounds checking within the Cisco Secure Client's packet processing routines. When the VPN Agent service receives specially crafted packets on a local port, it attempts to read memory beyond the allocated buffer boundaries. This out-of-bounds read operation triggers an access violation that crashes the VPN Agent service.

The vulnerability specifically manifests in multi-user environments where the timing of concurrent client connections creates a window for exploitation. The lack of proper input validation on local network traffic allows malformed packets to reach vulnerable code paths within the VPN Agent.

Attack Vector

The attack vector for CVE-2023-20240 requires local access and follows this exploitation pattern:

1. The attacker obtains valid credentials on a multi-user system running vulnerable Cisco Secure Client software
2. The attacker monitors for another user to initiate a Cisco Secure Client VPN session
3. While the legitimate session is active, the attacker logs into the same system
4. The attacker sends specially crafted packets to a local port used by the Cisco Secure Client
5. The malformed packets trigger an out-of-bounds memory read in the VPN Agent service
6. The VPN Agent service crashes, terminating VPN connectivity for all users on the system

The exploitation mechanism targets the local network interface rather than remote attack surfaces. The attacker leverages the concurrent session requirement to trigger the vulnerable code path, making this a timing-dependent attack that requires coordination with legitimate user activity.

Detection Methods for CVE-2023-20240

Indicators of Compromise

• Unexpected VPN Agent service crashes or restarts on multi-user systems
• Multiple Windows Event Log entries indicating vpnagent.exe process termination with access violation errors
• Anomalous local network traffic to Cisco Secure Client listening ports during active VPN sessions
• User complaints of simultaneous VPN disconnections across shared workstations

Detection Strategies

• Monitor for repeated VPN Agent service crashes using Windows Event Log analysis (Event ID 1000 for application crashes)
• Implement endpoint detection rules to identify unusual local port activity targeting Cisco Secure Client services
• Deploy SentinelOne Singularity to detect and alert on process crash patterns associated with memory access violations
• Configure SIEM correlation rules to identify patterns of VPN service disruptions across multi-user environments

Monitoring Recommendations

• Enable verbose logging for Cisco Secure Client to capture packet processing errors
• Configure Windows application crash dump collection for vpnagent.exe to support forensic analysis
• Monitor local network connections to Cisco Secure Client ports (typically TCP/UDP ports in the ephemeral range)
• Establish baseline metrics for VPN Agent service stability to detect anomalous crash frequency

How to Mitigate CVE-2023-20240

Immediate Actions Required

• Upgrade Cisco Secure Client to the latest patched version as specified in the Cisco Security Advisory
• Prioritize patching on multi-user systems such as terminal servers, VDI environments, and shared workstations
• Review and restrict local user access on systems running vulnerable Cisco Secure Client versions
• Implement network segmentation to limit local network access between user sessions on shared systems

Patch Information

Cisco has released security updates to address this vulnerability. Organizations should consult the Cisco Security Advisory for specific version information and download links. The advisory provides detailed guidance on determining affected versions and obtaining the appropriate patches for both Cisco AnyConnect Secure Mobility Client and Cisco Secure Client deployments.

Affected version ranges include:

• Cisco AnyConnect Secure Mobility Client: 4.9.x versions through 4.9.06037
• Cisco Secure Client: 4.10.x versions through 4.10.07073
• Cisco Secure Client: 5.0.x versions through 5.0.03076

Workarounds

• Limit physical and remote access to multi-user systems running Cisco Secure Client where patching is not immediately possible
• Implement application whitelisting to restrict unauthorized processes from sending local network traffic
• Configure local firewall rules to restrict access to Cisco Secure Client service ports from non-privileged user sessions
• Consider migrating single-user workloads away from multi-user systems until patches can be applied

# Example: Windows Firewall rule to restrict local port access
# Note: Specific ports should be identified based on your deployment
netsh advfirewall firewall add rule name="Block Cisco SC Local Access" dir=in action=block protocol=tcp localport=<port> remoteip=localsubnet