CVE-2021-34535
📋 TL;DR
This vulnerability allows an attacker to execute arbitrary code on a victim's system by tricking them into connecting to a malicious RDP server. It affects Windows Remote Desktop Client users who connect to untrusted servers. The attacker could gain the same user rights as the logged-on user.
💻 Affected Systems
- Windows Remote Desktop Client
📦 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 with attacker gaining SYSTEM privileges, installing malware, stealing credentials, and pivoting to other systems.
Likely Case
Attacker gains user-level access to execute code, steal data, and install ransomware or backdoors on the compromised system.
If Mitigated
Limited impact due to network segmentation, least privilege accounts, and proper patch management preventing exploitation.
🎯 Exploit Status
Requires user interaction to connect to malicious RDP server. No public exploit code available as of last analysis.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: July 2021 security updates (KB5004237, KB5004245, etc.)
Vendor Advisory: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-34535
Restart Required: Yes
Instructions:
1. Apply July 2021 Windows security updates via Windows Update. 2. For enterprise: Deploy patches through WSUS or SCCM. 3. Restart systems after patch installation.
🔧 Temporary Workarounds
Disable RDP client connections
windowsPrevent use of Remote Desktop Client to connect to external servers
Group Policy: Computer Configuration > Administrative Templates > Windows Components > Remote Desktop Services > Remote Desktop Connection Client > "Do not allow passwords to be saved" = Enabled
Registry: HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows NT\Terminal Services\fPromptForPassword = 1
Network restriction
windowsBlock outbound RDP connections to untrusted networks
Windows Firewall: New outbound rule blocking TCP port 3389 to external networks
netsh advfirewall firewall add rule name="Block External RDP" dir=out action=block protocol=TCP remoteport=3389
🧯 If You Can't Patch
- Implement network segmentation to restrict RDP traffic to trusted servers only
- Use application whitelisting to prevent execution of unauthorized code
- Educate users about risks of connecting to unknown RDP servers
- Monitor for unusual RDP client connections in network logs
🔍 How to Verify
Check if Vulnerable:
Check Windows version and if July 2021 security updates are installed via winver or systeminfoCheck Version:
systeminfo | findstr /B /C:"OS Name" /C:"OS Version"Verify Fix Applied:
Verify KB5004237 (Windows 10 2004/20H2/21H1) or KB5004245 (Windows 10 1909) is installed📡 Detection & Monitoring
Log Indicators:
- Windows Event ID 1149 (RDP client connection failures)
- Unexpected RDP client connections to external IPs
- Process creation from rdclient.exe with unusual parameters
Network Indicators:
- Outbound RDP connections to unknown external IPs
- RDP traffic to non-standard ports
- Unusual volume of RDP client connections
SIEM Query:
source="Windows Security" EventID=4688 ProcessName="*rdclient*" | stats count by ProcessCommandLine