CVE-2021-1655
📋 TL;DR
This vulnerability allows an authenticated attacker to execute arbitrary code with SYSTEM privileges on Windows systems by exploiting the Client Side Caching (CSC) service. It affects Windows 10, Windows Server 2016, and later versions. Attackers must have local access to the target system to exploit this flaw.
💻 Affected Systems
- Windows 10
- Windows Server 2016
- Windows Server 2019
- Windows Server 2022
📦 What is this software?
Windows 10 by Microsoft
Windows 10 by Microsoft
Windows 10 by Microsoft
Windows 10 by Microsoft
Windows 10 by Microsoft
Windows 10 by Microsoft
Windows 10 by Microsoft
Windows 7 by Microsoft
Windows 8.1 by Microsoft
Windows Rt 8.1 by Microsoft
⚠️ Risk & Real-World Impact
Worst Case
An attacker gains full SYSTEM privileges on the compromised machine, enabling complete system takeover, installation of malware, credential theft, and lateral movement across the network.
Likely Case
Privilege escalation from a standard user account to SYSTEM, allowing attackers to bypass security controls, install persistent backdoors, and access sensitive system resources.
If Mitigated
With proper patching and least privilege principles, the impact is limited to isolated systems where attackers cannot achieve persistence or lateral movement.
🎯 Exploit Status
Exploitation requires authenticated user access. Public proof-of-concept code exists, making this vulnerability attractive for attackers who have already compromised user credentials.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: January 2021 security updates (KB4598242 for Windows 10 2004/20H2, KB4598230 for 1909, etc.)
Vendor Advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-1655
Restart Required: Yes
Instructions:
1. Apply the January 2021 Windows security updates from Windows Update. 2. For enterprise environments, deploy updates via WSUS, SCCM, or Intune. 3. Restart systems after patch installation.
🔧 Temporary Workarounds
Disable CSC Service
windowsDisables the Client Side Caching service to prevent exploitation, but will break offline file synchronization features.
sc config csc start= disabled
sc stop csc
Restrict Service Permissions
windowsModify CSC service permissions to prevent non-administrative users from interacting with the service.
sc sdset csc D:(A;;CCLCSWRPWPDTLOCRRC;;;SY)(A;;CCDCLCSWRPWPDTLOCRSDRCWDWO;;;BA)(A;;CCLCSWLOCRRC;;;IU)(A;;CCLCSWLOCRRC;;;SU)
🧯 If You Can't Patch
- Implement strict least privilege principles to limit user access and reduce attack surface
- Monitor for suspicious CSC service activity and privilege escalation attempts
🔍 How to Verify
Check if Vulnerable:
Check if the January 2021 security updates are installed via 'wmic qfe list' or 'Get-Hotfix -Id KB4598242, KB4598230' in PowerShellCheck Version:
systeminfo | findstr /B /C:"OS Name" /C:"OS Version"Verify Fix Applied:
Verify the CSC service version is updated and no longer vulnerable by checking file versions of cscsvc.dll📡 Detection & Monitoring
Log Indicators:
- Event ID 4688 with CSC service spawning processes
- Unexpected CSC service activity in Windows Event Logs
- Privilege escalation attempts from standard users to SYSTEM
Network Indicators:
- Unusual SMB/CIFS traffic patterns from CSC service
- Lateral movement attempts following privilege escalation
SIEM Query:
EventID=4688 AND (NewProcessName="*csc*" OR ParentProcessName="*csc*") AND SubjectUserName!="SYSTEM"