CVE-2021-1237
📋 TL;DR
This vulnerability allows authenticated local attackers on Windows systems with Cisco AnyConnect Secure Mobility Client to perform DLL injection attacks. By placing a malicious configuration file in a specific path, attackers can load a malicious DLL when the application starts, potentially executing arbitrary code with SYSTEM privileges. Affected users are those running vulnerable versions of Cisco AnyConnect on Windows.
💻 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 affected machine, enabling persistence, data theft, lateral movement, and complete system compromise.
Likely Case
Privilege escalation from a standard user account to SYSTEM, allowing installation of malware, credential harvesting, or disabling security controls.
If Mitigated
With proper access controls and monitoring, exploitation would be detected and contained before significant damage occurs.
🎯 Exploit Status
Exploitation requires local authenticated access and ability to write files to specific system paths. No public exploit code is known.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 4.10.00093 and later
Vendor Advisory: https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-anyconnect-dll-injec-pQnryXLf
Restart Required: Yes
Instructions:
1. Download AnyConnect version 4.10.00093 or later from Cisco. 2. Uninstall previous AnyConnect version. 3. Install the updated version. 4. Restart the system.
🔧 Temporary Workarounds
Restrict file write permissions
windowsApply strict access controls to prevent unauthorized users from writing files to the vulnerable path used for DLL injection.
Use Windows ACLs to restrict write access to the specific path mentioned in the advisory
🧯 If You Can't Patch
- Implement strict least privilege access controls to limit which users can log into systems with AnyConnect installed
- Monitor for suspicious file creation in the vulnerable path and DLL loading events from AnyConnect processes
🔍 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" /?Check Version:
"C:\Program Files (x86)\Cisco\Cisco AnyConnect Secure Mobility Client\vpnui.exe" /?Verify Fix Applied:
Confirm version is 4.10.00093 or higher using the same method as checking vulnerability📡 Detection & Monitoring
Log Indicators:
- Windows Event Logs showing file creation in vulnerable path
- Process creation events for AnyConnect loading unexpected DLLs
- Security logs showing privilege escalation attempts
Network Indicators:
- Unusual outbound connections from systems with AnyConnect after application startup
SIEM Query:
source="windows" (event_id=4688 OR event_id=4663) process_name="*anyconnect*" OR file_path="*vulnerable_path*"