CVE-2021-1567
📋 TL;DR
This vulnerability allows an authenticated local attacker to perform DLL hijacking through a race condition in Cisco AnyConnect's signature verification process. Successful exploitation enables arbitrary code execution with SYSTEM privileges. Affected systems are Windows devices running Cisco AnyConnect Secure Mobility Client with the VPN Posture (HostScan) Module installed.
💻 Affected Systems
- Cisco AnyConnect Secure Mobility Client
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Attacker gains SYSTEM privileges and full control of the Windows system, enabling data theft, persistence, lateral movement, and complete system compromise.
Likely Case
Privilege escalation from standard user to SYSTEM, allowing installation of malware, credential harvesting, or disabling security controls.
If Mitigated
Limited impact due to patching, proper access controls, and monitoring preventing successful exploitation.
🎯 Exploit Status
Exploitation requires valid Windows credentials, local access, and sending crafted IPC messages to trigger the race condition.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 4.10.00093 and later
Vendor Advisory: https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-anyconnect-pos-dll-ff8j6dFv
Restart Required: Yes
Instructions:
1. Download AnyConnect version 4.10.00093 or later from Cisco's website. 2. Uninstall the current AnyConnect client. 3. Install the updated version. 4. Restart the system to ensure all components are properly loaded.
🔧 Temporary Workarounds
Remove HostScan Module
windowsUninstall the VPN Posture (HostScan) Module if not required for compliance or security policies.
Control Panel > Programs > Uninstall a program > Select 'Cisco AnyConnect VPN Posture Module' > Uninstall
Restrict Local User Privileges
windowsImplement least privilege by ensuring users only have necessary permissions and cannot execute arbitrary code.
🧯 If You Can't Patch
- Remove the HostScan module from AnyConnect if posture assessment is not required
- Implement application whitelisting to prevent execution of unauthorized DLLs
🔍 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 4.10.00093 or higher and HostScan module is either updated or removed📡 Detection & Monitoring
Log Indicators:
- Unusual process creation from AnyConnect components
- Failed DLL signature verification events
- IPC communication anomalies with AnyConnect processes
Network Indicators:
- None - this is a local privilege escalation vulnerability
SIEM Query:
Process creation where parent process contains 'anyconnect' or 'hostscan' and child process is unexpected or suspicious