CVE-2021-1657
📋 TL;DR
CVE-2021-1657 is a remote code execution vulnerability in the Windows Fax Compose Form. An attacker could exploit this by tricking a user into opening a specially crafted file, potentially allowing arbitrary code execution with the user's privileges. This affects Windows systems with fax services enabled.
💻 Affected Systems
- Microsoft Windows
📦 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
Full system compromise via remote code execution leading to data theft, ransomware deployment, or persistent backdoor installation.
Likely Case
Limited compromise of individual user accounts through malicious document execution, potentially leading to lateral movement within the network.
If Mitigated
No impact if systems are patched or fax services are disabled; limited to user-level access if proper application whitelisting is in place.
🎯 Exploit Status
Exploitation requires user interaction (opening malicious file) and fax services to be enabled. No public exploit code available as of analysis.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: January 2021 security updates (KB4598242 for Windows 10 20H2, KB4598230 for Windows Server 2019, etc.)
Vendor Advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-1657
Restart Required: Yes
Instructions:
1. Apply January 2021 Windows security updates via Windows Update. 2. For enterprise: Deploy through WSUS or Microsoft Update Catalog. 3. Restart systems after patch installation.
🔧 Temporary Workarounds
Disable Windows Fax and Scan Service
windowsDisables the vulnerable fax service component
sc config Fax stop= disabled
sc stop Fax
Remove Fax Feature
windowsUninstalls the Windows Fax and Scan feature entirely
Dism /online /Disable-Feature /FeatureName:FaxServicesClientPackage
🧯 If You Can't Patch
- Disable Windows Fax and Scan service via Group Policy or manual configuration
- Implement application control policies to block execution of fax-related binaries
- Educate users about risks of opening unknown files and implement email filtering
🔍 How to Verify
Check if Vulnerable:
Check if Fax service is running: 'sc query Fax' and verify Windows version is unpatchedCheck Version:
systeminfo | findstr /B /C:"OS Name" /C:"OS Version"Verify Fix Applied:
Verify January 2021 security updates are installed via 'systeminfo' or Windows Update history📡 Detection & Monitoring
Log Indicators:
- Event ID 1000 application crashes from fxssvc.exe
- Unexpected fax service process execution
- Security logs showing suspicious file execution
Network Indicators:
- Unusual outbound connections from fax service processes
- SMB/network scanning from fax-related processes
SIEM Query:
Process creation where parent_process contains 'explorer.exe' and process_name contains 'fxssvc' or 'fxscover'