CVE-2021-37595
📋 TL;DR
This vulnerability in FreeRDP's Windows client allows remote code execution through improper input validation in clipboard file content requests. Attackers can exploit missing checks in the FILECONTENTS_RANGE PDU handler to execute arbitrary code on vulnerable systems. All Windows users running FreeRDP versions before 2.4.0 are affected.
💻 Affected Systems
- FreeRDP
📦 What is this software?
Freerdp by Freerdp
⚠️ Risk & Real-World Impact
Worst Case
Full system compromise with remote code execution leading to complete control of the affected system, data exfiltration, and lateral movement within the network.
Likely Case
Remote code execution allowing attackers to install malware, create backdoors, or steal sensitive information from the compromised system.
If Mitigated
Denial of service or limited information disclosure if exploit attempts are blocked by network controls or security software.
🎯 Exploit Status
The vulnerability requires the attacker to send a malicious FILECONTENTS_RANGE PDU, which can be done without authentication if the client connects to a malicious server.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 2.4.0 and later
Vendor Advisory: https://github.com/FreeRDP/FreeRDP/security/advisories
Restart Required: Yes
Instructions:
1. Download FreeRDP 2.4.0 or later from the official repository. 2. Uninstall the vulnerable version. 3. Install the patched version. 4. Restart the system to ensure all components are updated.
🔧 Temporary Workarounds
Disable Clipboard Redirection
allDisable clipboard redirection feature which is required for exploitation
freerdp /clipboard:off
xfreerdp /clipboard:off
Network Segmentation
allRestrict RDP connections to trusted networks only
🧯 If You Can't Patch
- Implement strict network controls to limit RDP connections to trusted sources only
- Deploy endpoint protection that can detect and block RDP-based exploitation attempts
🔍 How to Verify
Check if Vulnerable:
Check FreeRDP version with: freerdp --version or wfreerdp.exe --version. If version is below 2.4.0, the system is vulnerable.Check Version:
freerdp --versionVerify Fix Applied:
After updating, verify version is 2.4.0 or higher using the same command. Test clipboard functionality to ensure it still works properly.📡 Detection & Monitoring
Log Indicators:
- Unusual RDP connection patterns
- Multiple failed clipboard operations
- Process creation from RDP client processes
Network Indicators:
- Unusual FILECONTENTS_RANGE PDUs in RDP traffic
- RDP connections to/from untrusted sources
SIEM Query:
source="rdp_logs" AND (event_type="clipboard_operation" AND result="failed") OR (process_name="mstsc.exe" OR "freerdp" AND parent_process="explorer.exe")