CVE-2021-31181
📋 TL;DR
CVE-2021-31181 is a remote code execution vulnerability in Microsoft SharePoint Server that allows attackers to execute arbitrary code on affected systems by exploiting improper control of generation of code ('Code Injection'). This affects organizations running vulnerable SharePoint Server versions, potentially enabling complete system compromise.
💻 Affected Systems
- Microsoft SharePoint Server
- Microsoft SharePoint Foundation
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Complete compromise of the SharePoint server, allowing attackers to execute arbitrary code with SYSTEM privileges, steal sensitive data, install malware, and pivot to other internal systems.
Likely Case
Attackers gain initial foothold on the SharePoint server, potentially accessing sensitive documents and user data, and establishing persistence for further attacks.
If Mitigated
With proper network segmentation, least privilege, and monitoring, impact is limited to the SharePoint application tier with minimal lateral movement potential.
🎯 Exploit Status
Exploitation requires authentication to SharePoint, but multiple public proof-of-concept examples exist demonstrating the vulnerability.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: May 2021 Security Updates
Vendor Advisory: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-31181
Restart Required: Yes
Instructions:
1. Apply the May 2021 security updates for SharePoint Server from Microsoft Update. 2. Restart the SharePoint server. 3. Verify the update was successfully installed.
🔧 Temporary Workarounds
Disable custom controls
windowsRestrict or disable unsafe custom controls in SharePoint to prevent exploitation vectors
🧯 If You Can't Patch
- Implement strict network segmentation to isolate SharePoint servers from critical systems
- Enforce strong authentication requirements and monitor for suspicious SharePoint activity
🔍 How to Verify
Check if Vulnerable:
Check SharePoint Server version and compare against patched versions. Unpatched versions prior to May 2021 updates are vulnerable.Check Version:
Get-SPFarm | Select BuildVersionVerify Fix Applied:
Verify that the May 2021 security updates for SharePoint are installed via Windows Update history or PowerShell Get-HotFix📡 Detection & Monitoring
Log Indicators:
- Unusual SharePoint control execution events
- Suspicious PowerShell or command execution from SharePoint processes
- Unexpected ViewState manipulation attempts
Network Indicators:
- Unusual outbound connections from SharePoint servers
- Suspicious HTTP requests to SharePoint with crafted parameters
SIEM Query:
source="SharePoint" AND (event_id=6398 OR event_id=6399) AND (process_execution OR command_injection)🔗 References
- http://packetstormsecurity.com/files/163208/Microsoft-SharePoint-Unsafe-Control-And-ViewState-Remote-Code-Execution.html
- https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-31181
- https://www.zerodayinitiative.com/advisories/ZDI-21-573/
- http://packetstormsecurity.com/files/163208/Microsoft-SharePoint-Unsafe-Control-And-ViewState-Remote-Code-Execution.html
- https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-31181
- https://www.zerodayinitiative.com/advisories/ZDI-21-573/
- https://packetstorm.news/files/id/163208
