CVE-2026-20927
📋 TL;DR
A race condition vulnerability in Windows SMB Server allows authenticated attackers to cause denial of service by exploiting improper synchronization of shared resources. This affects Windows systems running vulnerable SMB server implementations, potentially disrupting file sharing and network services.
💻 Affected Systems
- Windows SMB Server
📦 What is this software?
Windows 10 1607 by Microsoft
Windows 10 1607 by Microsoft
Windows 10 1809 by Microsoft
Windows 10 1809 by Microsoft
Windows 10 21h2 by Microsoft
Windows 10 22h2 by Microsoft
Windows 11 23h2 by Microsoft
Windows 11 24h2 by Microsoft
Windows 11 25h2 by Microsoft
⚠️ Risk & Real-World Impact
Worst Case
Complete disruption of SMB services across the network, preventing file sharing, printer access, and other SMB-dependent operations until service restart.
Likely Case
Temporary service degradation or crashes affecting specific SMB connections, requiring service restarts on affected systems.
If Mitigated
Minimal impact with proper network segmentation and access controls limiting authenticated attacker access to SMB services.
🎯 Exploit Status
Exploitation requires timing precision and authenticated access to SMB services. Race conditions can be challenging to reliably exploit.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: To be specified in Microsoft Security Update
Vendor Advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-20927
Restart Required: Yes
Instructions:
1. Check Microsoft Security Update Guide for CVE-2026-20927. 2. Apply the appropriate security update for your Windows version. 3. Restart affected systems to complete installation.
🔧 Temporary Workarounds
Restrict SMB Access
windowsLimit SMB access to only necessary users and systems using firewall rules and access controls.
netsh advfirewall firewall add rule name="Block SMB" dir=in action=block protocol=TCP localport=445,139
Disable SMBv1
windowsDisable legacy SMBv1 protocol which may be more vulnerable and provides minimal functionality.
Disable-WindowsOptionalFeature -Online -FeatureName SMB1Protocol
🧯 If You Can't Patch
- Implement strict network segmentation to isolate SMB servers from untrusted networks
- Enforce principle of least privilege for SMB access and monitor for unusual authentication patterns
🔍 How to Verify
Check if Vulnerable:
Check Windows Update history for CVE-2026-20927 patch or use: Get-HotFix | Where-Object {$_.HotFixID -match "KB"}Check Version:
systeminfo | findstr /B /C:"OS Name" /C:"OS Version"Verify Fix Applied:
Verify patch installation via Windows Update history or PowerShell: Get-HotFix -Id KB######📡 Detection & Monitoring
Log Indicators:
- Multiple SMB connection failures
- SMB service crashes in Event Viewer
- Unexpected SMB service restarts
Network Indicators:
- Unusual SMB traffic patterns
- Multiple concurrent SMB connections from single source
- SMB protocol anomalies
SIEM Query:
source="windows" AND (event_id=7031 OR event_id=7034) AND service_name="LanmanServer"