CVE-2021-1366
📋 TL;DR
This vulnerability allows an authenticated local attacker on Windows systems with Cisco AnyConnect Secure Mobility Client and VPN Posture (HostScan) Module installed to perform DLL hijacking via crafted IPC messages. Successful exploitation could lead to arbitrary code execution with SYSTEM privileges. Only affects Windows installations with the HostScan module.
💻 Affected Systems
- Cisco AnyConnect Secure Mobility Client for Windows
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Attacker gains SYSTEM privileges and full control of the affected Windows machine, enabling persistence, lateral movement, and data exfiltration.
Likely Case
Privilege escalation from standard user to SYSTEM, allowing installation of malware, credential theft, and bypassing security controls.
If Mitigated
With proper access controls and monitoring, impact limited to isolated system compromise with rapid detection and containment.
🎯 Exploit Status
Requires local authenticated access and ability to send crafted IPC messages to the AnyConnect process.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 4.9.06037 and later
Vendor Advisory: https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-anyconnect-dll-hijac-JrcTOQMC
Restart Required: Yes
Instructions:
1. Download AnyConnect version 4.9.06037 or later from Cisco. 2. Uninstall previous version. 3. Install updated version. 4. Restart system.
🔧 Temporary Workarounds
Remove HostScan Module
windowsUninstall the VPN Posture (HostScan) Module if not required for compliance
Control Panel > Programs > Uninstall a program > Select 'Cisco AnyConnect VPN Posture Module' > Uninstall
Restrict Local User Privileges
windowsImplement least privilege access controls to limit local authenticated attackers
🧯 If You Can't Patch
- Implement application whitelisting to prevent unauthorized DLL execution
- Enable Windows Defender Application Control or similar to restrict DLL loading
🔍 How to Verify
Check if Vulnerable:
Check AnyConnect version via GUI (Help > About) or command line: 'C:\Program Files (x86)\Cisco\Cisco AnyConnect Secure Mobility Client\vpnui.exe' /versionCheck Version:
"C:\Program Files (x86)\Cisco\Cisco AnyConnect Secure Mobility Client\vpnui.exe" /versionVerify Fix Applied:
Confirm version is 4.9.06037 or higher and HostScan module is either updated or removed📡 Detection & Monitoring
Log Indicators:
- Unusual process creation from AnyConnect binaries
- Failed DLL loading attempts in application logs
- IPC communication anomalies
Network Indicators:
- None - local exploitation only
SIEM Query:
Process Creation where Image contains 'anyconnect' and CommandLine contains unusual DLL paths