CVE-2020-0611: Windows Remote Desktop Client RCE Flaw

CVE-2020-0611 Overview

A remote code execution vulnerability exists in the Windows Remote Desktop Client when a user connects to a malicious server. This client-side vulnerability allows an attacker who controls a malicious RDP server to execute arbitrary code on the connecting client machine with the same privileges as the user running the Remote Desktop Client application.

Critical Impact

An attacker who successfully exploits this vulnerability could execute arbitrary code on the client system, potentially leading to complete system compromise, data theft, or lateral movement within an organization's network.

Affected Products

• Microsoft Windows 10 (all versions through 1909)
• Microsoft Windows 7 SP1
• Microsoft Windows 8.1
• Microsoft Windows RT 8.1
• Microsoft Windows Server 2008 R2 SP1
• Microsoft Windows Server 2012 and 2012 R2
• Microsoft Windows Server 2016 (all versions through 1909)
• Microsoft Windows Server 2019

Discovery Timeline

• 2020-01-14 - CVE-2020-0611 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-0611

Vulnerability Analysis

This vulnerability represents a client-side remote code execution flaw in the Windows Remote Desktop Protocol (RDP) client (mstsc.exe). Unlike server-side RDP vulnerabilities that target systems accepting incoming connections, CVE-2020-0611 specifically targets the RDP client application when it initiates an outbound connection to a malicious server.

The attack scenario requires social engineering or network-based redirection techniques to convince a victim to connect their RDP client to an attacker-controlled server. Once the connection is established, the malicious server can send specially crafted responses that exploit the vulnerability in the client's parsing or handling logic.

The vulnerability affects the core RDP client component across a wide range of Windows operating systems, from Windows 7 through the latest Windows 10 and Windows Server 2019 releases at the time of disclosure.

Root Cause

The vulnerability stems from improper handling of server-supplied data within the Remote Desktop Client. When the client connects to a malicious server, the server can send specially crafted responses that cause the client to process data in an unsafe manner, leading to memory corruption or other exploitable conditions that enable code execution.

Attack Vector

The attack requires network access and user interaction to be successful:

1. Server Setup: An attacker establishes a malicious RDP server configured to exploit CVE-2020-0611
2. Victim Targeting: The attacker uses social engineering, phishing, or network redirection techniques to trick a victim into connecting to the malicious server
3. Connection Initiation: When the victim's RDP client connects to the attacker's server, malicious data is transmitted
4. Exploitation: The malicious server response triggers the vulnerability in the client, executing arbitrary code with the victim's privileges

This attack pattern is particularly effective in scenarios where users frequently connect to various RDP servers, such as IT administrators, developers, or remote support personnel. Attackers may create convincing pretexts like "connect to this server to troubleshoot an issue" or compromise DNS/network infrastructure to redirect legitimate RDP connections.

Detection Methods for CVE-2020-0611

Indicators of Compromise

• Unusual RDP client connections to external or unknown IP addresses
• Unexpected process creation or child processes spawned by mstsc.exe
• Memory anomalies or crashes in the Remote Desktop Client application
• Network connections from mstsc.exe to non-standard ports or suspicious domains

Detection Strategies

• Monitor for mstsc.exe establishing connections to IP addresses outside of known/trusted RDP server ranges
• Implement network monitoring to detect RDP connections to external or unvetted servers
• Deploy endpoint detection rules to identify anomalous behavior from the RDP client process
• Use SentinelOne's behavioral AI to detect exploitation attempts and post-exploitation activity

Monitoring Recommendations

• Enable comprehensive logging for Remote Desktop Client activities and connection attempts
• Implement network segmentation and firewall rules to restrict outbound RDP connections to approved servers only
• Configure SIEM alerts for RDP client connections to new or unknown destinations
• Monitor for privilege escalation attempts following RDP client activity

How to Mitigate CVE-2020-0611

Immediate Actions Required

• Apply the Microsoft security update released in January 2020 to all affected systems immediately
• Restrict RDP client usage to trusted servers only through network policies
• Educate users about the risks of connecting to untrusted RDP servers
• Implement Network Level Authentication (NLA) where possible to reduce attack surface

Patch Information

Microsoft released security updates addressing this vulnerability as part of the January 2020 Patch Tuesday release. The patch corrects how the Windows Remote Desktop Client handles connection data received from servers. Organizations should apply these updates through Windows Update, WSUS, or manual deployment.

For detailed patch information and download links, refer to the Microsoft Security Advisory for CVE-2020-0611.

Workarounds

• Implement strict allow-listing of approved RDP server destinations at the network firewall level
• Use VPN connections before establishing RDP sessions to limit exposure to untrusted networks
• Deploy application control policies to monitor and alert on RDP client usage to unknown destinations
• Consider using Remote Desktop Gateway servers to proxy and control all RDP connections within the organization

# Windows Firewall rule to restrict outbound RDP connections
# Allow RDP only to specific trusted server IP addresses
netsh advfirewall firewall add rule name="Restrict RDP Outbound" dir=out action=block protocol=tcp remoteport=3389
netsh advfirewall firewall add rule name="Allow RDP to Trusted Servers" dir=out action=allow protocol=tcp remoteport=3389 remoteip=10.0.0.0/8,192.168.0.0/16