CVE-2021-43215
📋 TL;DR
CVE-2021-43215 is a critical memory corruption vulnerability in Microsoft's iSNS Server that allows remote attackers to execute arbitrary code on affected systems. This vulnerability affects Windows Server installations with the iSNS Server feature enabled. Attackers can exploit this without authentication to gain full control of vulnerable servers.
💻 Affected Systems
- Microsoft 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
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
Windows 10 by Microsoft
Windows 10 by Microsoft
Windows 10 by Microsoft
Windows 10 by Microsoft
Windows 10 by Microsoft
Windows 11 by Microsoft
Windows 11 by Microsoft
Windows 7 by Microsoft
Windows 7 by Microsoft
Windows 8.1 by Microsoft
Windows 8.1 by Microsoft
Windows Rt 8.1 by Microsoft
Windows Server by Microsoft
Windows Server by Microsoft
⚠️ Risk & Real-World Impact
Worst Case
Remote unauthenticated attacker gains SYSTEM-level privileges and full control of the server, enabling data theft, ransomware deployment, or lateral movement within the network.
Likely Case
Remote code execution leading to complete server compromise, installation of backdoors, credential harvesting, and use as pivot point for further attacks.
If Mitigated
If iSNS Server is disabled or network access is restricted, impact is limited to denial of service or local privilege escalation scenarios.
🎯 Exploit Status
Public exploit code exists and the vulnerability is relatively straightforward to exploit due to memory corruption in protocol handling.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: December 2021 security updates
Vendor Advisory: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-43215
Restart Required: Yes
Instructions:
1. Apply December 2021 Windows Server security updates via Windows Update. 2. Alternatively, download and install the standalone security update from Microsoft Update Catalog. 3. Restart the server after installation.
🔧 Temporary Workarounds
Disable iSNS Server Service
windowsStop and disable the iSNS Server service if not required for your environment
sc stop isnsserver
sc config isnsserver start= disabled
Block Network Access
windowsRestrict network access to iSNS Server port (3205/tcp) using Windows Firewall
netsh advfirewall firewall add rule name="Block iSNS" dir=in action=block protocol=TCP localport=3205
🧯 If You Can't Patch
- Disable iSNS Server feature completely via Server Manager or PowerShell
- Implement strict network segmentation and firewall rules to block all access to port 3205/tcp
🔍 How to Verify
Check if Vulnerable:
Check if iSNS Server feature is installed: Get-WindowsFeature -Name iSNSServer | Select-Object InstalledCheck Version:
systeminfo | findstr /B /C:"OS Name" /C:"OS Version"Verify Fix Applied:
Verify December 2021 security update is installed: Get-HotFix -Id KB5008207 (Server 2019) or KB5008206 (Server 2022)📡 Detection & Monitoring
Log Indicators:
- Event ID 4625 failed logons to iSNS service
- Unexpected process creation from isnsserver.exe
- Crash dumps from isnsserver.exe
Network Indicators:
- Unusual traffic to port 3205/tcp
- Malformed iSNS protocol packets
- Connection attempts from unexpected sources
SIEM Query:
source="windows" AND (event_id=4625 AND service_name="iSNSServer") OR (process_name="isnsserver.exe" AND parent_process!="services.exe")