CVE-2022-21851: Windows 10 Remote Desktop RCE Vulnerability

CVE-2022-21851 Overview

CVE-2022-21851 is a Remote Code Execution vulnerability affecting the Microsoft Remote Desktop Client (RDP Client) across virtually all supported versions of Windows. This vulnerability allows an attacker to execute arbitrary code on a victim's system when the user connects to a malicious RDP server controlled by the attacker. The attack requires user interaction—specifically, the victim must initiate a Remote Desktop connection to the attacker's server.

Critical Impact

Successful exploitation allows attackers to achieve full system compromise through remote code execution when victims connect to malicious RDP servers, potentially leading to complete loss of confidentiality, integrity, and availability.

Affected Products

• Microsoft Windows 10 (all versions including 1607, 1809, 1909, 20H2, 21H1, 21H2)
• Microsoft Windows 11 (x64 and ARM64)
• Microsoft Windows 7 SP1
• Microsoft Windows 8.1 (Enterprise and RT editions)
• Microsoft Windows Server 2008 R2 SP1/SP2
• Microsoft Windows Server 2012 and 2012 R2
• Microsoft Windows Server 2016
• Microsoft Windows Server 2019
• Microsoft Windows Server 2022 and 20H2

Discovery Timeline

• January 11, 2022 - CVE-2022-21851 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2022-21851

Vulnerability Analysis

This vulnerability resides in the Windows Remote Desktop Client (mstsc.exe) and affects how the client processes certain data received from an RDP server during a connection handshake or session. The vulnerability is classified as a client-side Remote Code Execution (RCE) flaw, meaning the attack is triggered when a user connects to a malicious server rather than when an attacker connects to a vulnerable service.

The attack scenario involves social engineering, where an attacker convinces a victim to connect to a rogue RDP server. Once the connection is established, the malicious server can send specially crafted responses that exploit the vulnerability in the RDP client, leading to arbitrary code execution in the context of the user running the Remote Desktop Client.

Root Cause

The root cause of CVE-2022-21851 has not been publicly disclosed in detail by Microsoft (classified as NVD-CWE-noinfo). However, based on the vulnerability class and attack pattern, it likely involves improper validation or handling of server-provided data during RDP protocol negotiation or session establishment. This could include buffer handling issues, type confusion, or improper parsing of protocol-specific structures sent by the server to the client.

Attack Vector

The attack vector is network-based and requires user interaction. An attacker must:

1. Set up a malicious RDP server with exploit payloads
2. Convince the victim to initiate an RDP connection to the attacker-controlled server (via phishing, DNS hijacking, or social engineering)
3. The malicious server sends crafted data that triggers the vulnerability in the victim's RDP client
4. Arbitrary code executes on the victim's machine with the privileges of the user running the RDP client

The vulnerability can be particularly dangerous in environments where users frequently connect to external or untrusted RDP servers, or where attackers can intercept or redirect legitimate RDP connection attempts.

Detection Methods for CVE-2022-21851

Indicators of Compromise

• Unusual RDP client (mstsc.exe) crashes or unexpected behavior during connection attempts
• Network connections to unknown or suspicious RDP servers on port 3389
• Unexpected child processes spawned by mstsc.exe that are not typical RDP-related processes
• Memory access violations or application errors logged for Remote Desktop Client

Detection Strategies

• Monitor for mstsc.exe process creating suspicious child processes or loading unexpected DLLs
• Implement network monitoring to detect RDP connections to external or non-whitelisted IP addresses
• Deploy endpoint detection rules for anomalous behavior following RDP client execution
• Use SentinelOne's behavioral AI to detect post-exploitation activities following RDP client compromise

Monitoring Recommendations

• Enable Windows Event logging for Remote Desktop Client connections and errors
• Monitor network traffic for outbound RDP connections to non-corporate destinations
• Implement DNS monitoring to detect potential redirection attacks targeting RDP infrastructure
• Configure alerts for Application Event Log entries related to mstsc.exe faults or crashes

How to Mitigate CVE-2022-21851

Immediate Actions Required

• Apply the January 2022 Microsoft security updates immediately to all affected systems
• Educate users about the risks of connecting to untrusted RDP servers
• Implement network policies to restrict outbound RDP connections to approved destinations only
• Consider using Remote Desktop Gateway to control and audit RDP connection destinations

Patch Information

Microsoft addressed this vulnerability in the January 2022 Patch Tuesday security updates. Organizations should apply the relevant cumulative updates for their Windows versions. Detailed patch information and download links are available from the Microsoft Security Update Guide for CVE-2022-21851. Organizations using Windows Update for Business or WSUS should ensure the January 2022 updates are approved and deployed.

Workarounds

• Restrict RDP client usage to only connect to trusted, internal servers until patches are applied
• Use firewall rules to block outbound connections on port 3389 to external networks
• Deploy application whitelisting to prevent execution of malicious payloads that could be delivered via this vulnerability
• Implement Network Level Authentication (NLA) requirements on all RDP servers to add an authentication layer before full session establishment

# Block outbound RDP connections to external networks via Windows Firewall
netsh advfirewall firewall add rule name="Block External RDP" dir=out action=block protocol=tcp remoteport=3389 remoteip=0.0.0.0/0
# Note: Adjust the remoteip parameter to exclude internal network ranges as needed