CVE-2021-26867
📋 TL;DR
This vulnerability allows an authenticated attacker on a guest virtual machine to execute arbitrary code on the Hyper-V host. It affects Windows systems running Hyper-V with specific configurations, potentially leading to host compromise from a guest VM.
💻 Affected Systems
- Windows Hyper-V
📦 What is this software?
Windows 10 by Microsoft
Windows 10 by Microsoft
Windows 10 by Microsoft
⚠️ Risk & Real-World Impact
Worst Case
Complete compromise of the Hyper-V host, allowing attacker to control all guest VMs, access host data, and pivot to other systems.
Likely Case
Attacker gains elevated privileges on the host, potentially installing malware, stealing data, or disrupting virtualization services.
If Mitigated
Limited impact due to network segmentation, strict access controls, and monitoring preventing successful exploitation.
🎯 Exploit Status
Requires authenticated access to a guest VM; Microsoft has not disclosed technical details to prevent exploitation.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: March 2021 security updates (KB5000802 for Windows 10, KB5000803 for Windows Server)
Vendor Advisory: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-26867
Restart Required: Yes
Instructions:
1. Apply March 2021 security updates via Windows Update. 2. For enterprise: Deploy through WSUS or SCCM. 3. Restart affected systems after patching.
🔧 Temporary Workarounds
Disable Hyper-V if not needed
windowsRemove Hyper-V role from systems where virtualization is not required
Disable-WindowsOptionalFeature -Online -FeatureName Microsoft-Hyper-V-All
Network segmentation
allIsolate Hyper-V management networks from regular user networks
🧯 If You Can't Patch
- Implement strict access controls to guest VMs and monitor for suspicious activity
- Segment Hyper-V hosts on isolated management networks with firewall rules limiting connections
🔍 How to Verify
Check if Vulnerable:
Check if system has Hyper-V enabled and is running affected Windows versions without March 2021 patchesCheck Version:
systeminfo | findstr /B /C:"OS Name" /C:"OS Version"Verify Fix Applied:
Verify KB5000802 (Windows 10) or KB5000803 (Windows Server) is installed via 'Get-HotFix -Id KB5000802' or checking Windows Update history📡 Detection & Monitoring
Log Indicators:
- Unusual Hyper-V service activity
- Failed authentication attempts to Hyper-V components
- Suspicious process creation on host from VM context
Network Indicators:
- Anomalous network traffic between VMs and host management interfaces
- Unexpected RPC/DCOM connections to Hyper-V services
SIEM Query:
EventID=4688 AND ProcessName LIKE '%vmwp%' AND CommandLine CONTAINS suspicious_pattern