CVE-2021-40487
📋 TL;DR
CVE-2021-40487 is a remote code execution vulnerability in Microsoft SharePoint Server that allows authenticated attackers to execute arbitrary code on affected systems. This vulnerability affects SharePoint Server installations where attackers can upload specially crafted files. Organizations using vulnerable SharePoint Server versions are at risk.
💻 Affected Systems
- Microsoft SharePoint Server
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Complete compromise of SharePoint Server with attacker gaining SYSTEM privileges, data exfiltration, lateral movement to other systems, and persistent backdoor installation.
Likely Case
Attacker gains control of SharePoint Server instance, accesses sensitive documents and user data, and potentially moves to other systems in the network.
If Mitigated
Limited impact with proper network segmentation, application whitelisting, and monitoring detecting anomalous file uploads.
🎯 Exploit Status
Exploitation requires authenticated access and involves uploading specially crafted files. Proof-of-concept code has been published.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: October 2021 Security Updates
Vendor Advisory: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-40487
Restart Required: Yes
Instructions:
1. Apply October 2021 Security Updates for SharePoint Server. 2. Restart SharePoint services. 3. Verify patch installation via Windows Update history or version check.
🔧 Temporary Workarounds
Restrict file upload types
windowsConfigure SharePoint to block potentially dangerous file types from being uploaded
Implement strict authentication controls
allEnforce multi-factor authentication and limit user permissions to minimum required
🧯 If You Can't Patch
- Implement network segmentation to isolate SharePoint servers from critical systems
- Deploy application control solutions to prevent execution of unauthorized code
🔍 How to Verify
Check if Vulnerable:
Check SharePoint Server version and compare against patched versions. Vulnerable if running SharePoint Server 2019, 2016, or 2013 SP1 without October 2021 updates.Check Version:
Get-SPFarm | Select BuildVersionVerify Fix Applied:
Verify October 2021 security updates are installed via Windows Update history or PowerShell command: Get-HotFix -Id KB5001924 (SharePoint 2019), KB5001925 (SharePoint 2016), KB5001926 (SharePoint 2013)📡 Detection & Monitoring
Log Indicators:
- Unusual file uploads to SharePoint, particularly with suspicious extensions or content
- Unexpected process creation from SharePoint worker processes
- Authentication attempts from unusual locations or times
Network Indicators:
- Outbound connections from SharePoint servers to suspicious external IPs
- Unusual spikes in SharePoint traffic patterns
SIEM Query:
source="SharePoint" AND (event_id=6398 OR event_id=6399) AND file_extension IN ("aspx", "ashx", "asmx")