CVE-2021-31977
📋 TL;DR
This vulnerability in Windows Hyper-V allows an authenticated attacker on a guest virtual machine to send specially crafted requests to the host, causing a denial of service (host crash). It affects Windows Server systems running Hyper-V virtualization.
💻 Affected Systems
- Windows Server
📦 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
⚠️ Risk & Real-World Impact
Worst Case
Complete host system crash requiring physical reboot, disrupting all virtual machines running on that host.
Likely Case
Hyper-V host becomes unresponsive, requiring manual restart and causing downtime for all hosted VMs.
If Mitigated
With proper network segmentation and access controls, impact is limited to isolated virtualization environments.
🎯 Exploit Status
Requires authenticated access to a guest VM and knowledge of Hyper-V internals.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: June 2021 security updates (KB5003637 for Server 2019, KB5003635 for Server 2022)
Vendor Advisory: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-31977
Restart Required: Yes
Instructions:
1. Apply June 2021 Windows Server security updates via Windows Update. 2. Restart the Hyper-V host server. 3. Verify patch installation with 'systeminfo' command.
🔧 Temporary Workarounds
Disable Hyper-V
windowsRemove Hyper-V role if not required, eliminating attack surface
Disable-WindowsOptionalFeature -Online -FeatureName Microsoft-Hyper-V-All
Network segmentation
allIsolate Hyper-V management network from guest VM networks
🧯 If You Can't Patch
- Implement strict access controls to guest VMs and monitor for suspicious activity
- Segment virtualization infrastructure and implement network monitoring for Hyper-V traffic anomalies
🔍 How to Verify
Check if Vulnerable:
Check if Hyper-V role is enabled and Windows Server version is affected (2019, 2022, 2004, 20H2)Check Version:
systeminfo | findstr /B /C:"OS Name" /C:"OS Version"Verify Fix Applied:
Verify June 2021 security updates are installed via 'systeminfo' or 'Get-HotFix -Id KB5003637'📡 Detection & Monitoring
Log Indicators:
- Event ID 41 - Kernel-Power (unexpected shutdown)
- Hyper-V event logs showing VM connection issues
Network Indicators:
- Unusual Hyper-V management protocol traffic from guest VMs
SIEM Query:
EventID=41 AND Source="Microsoft-Windows-Kernel-Power" AND ComputerName contains "HV"