CVE-2021-34520
📋 TL;DR
This vulnerability allows remote attackers to execute arbitrary code on Microsoft SharePoint Server by deserializing untrusted data. It affects organizations running vulnerable SharePoint Server versions, potentially enabling attackers to take control of affected systems.
💻 Affected Systems
- Microsoft SharePoint Server
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Complete compromise of SharePoint Server leading to data theft, lateral movement within the network, and deployment of ransomware or other malware.
Likely Case
Unauthorized access to sensitive SharePoint data, installation of backdoors, and potential privilege escalation within the SharePoint environment.
If Mitigated
Limited impact with proper network segmentation, application controls, and monitoring in place, though the vulnerability still presents a significant risk.
🎯 Exploit Status
Exploitation requires authentication to SharePoint, but standard user credentials may be sufficient. The vulnerability involves deserialization of untrusted data.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: July 2021 security updates
Vendor Advisory: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-34520
Restart Required: Yes
Instructions:
1. Apply the July 2021 security update for SharePoint Server from Microsoft Update. 2. Restart the SharePoint Server. 3. Verify the update was applied successfully.
🔧 Temporary Workarounds
Disable SharePoint Designer
windowsRestrict use of SharePoint Designer which may be involved in exploitation vectors
🧯 If You Can't Patch
- Implement strict network segmentation to isolate SharePoint servers from critical assets
- Enforce strong authentication requirements and monitor for unusual authentication patterns
🔍 How to Verify
Check if Vulnerable:
Check SharePoint Server version and compare against patched versions. Unpatched versions from before July 2021 are vulnerable.Check Version:
Get-SPFarm | Select BuildVersionVerify Fix Applied:
Verify that the July 2021 security update is installed via Windows Update history or SharePoint version information.📡 Detection & Monitoring
Log Indicators:
- Unusual authentication patterns to SharePoint
- Unexpected process execution on SharePoint servers
- Suspicious deserialization events in application logs
Network Indicators:
- Unusual outbound connections from SharePoint servers
- Suspicious PowerShell or command execution traffic
SIEM Query:
source="sharepoint" AND (event_id=6398 OR event_id=6399) AND process_execution="powershell.exe"