CVE-2021-26420
📋 TL;DR
CVE-2021-26420 is a remote code execution vulnerability in Microsoft SharePoint Server that allows authenticated attackers to execute arbitrary code on affected servers. This affects organizations running vulnerable SharePoint Server versions, potentially compromising sensitive data and server integrity.
💻 Affected Systems
- Microsoft SharePoint Server
- Microsoft SharePoint Foundation
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Complete server compromise leading to data theft, lateral movement within the network, and persistent backdoor installation.
Likely Case
Unauthorized access to sensitive SharePoint data, privilege escalation, and limited code execution within SharePoint context.
If Mitigated
Attack blocked at perimeter or through authentication controls, with minimal impact due to proper segmentation.
🎯 Exploit Status
Requires authenticated access; exploit details not publicly available but likely weaponized by threat actors.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: March 2021 security updates
Vendor Advisory: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-26420
Restart Required: Yes
Instructions:
1. Apply March 2021 security updates for SharePoint Server. 2. Restart SharePoint services. 3. Verify patch installation via Windows Update history.
🔧 Temporary Workarounds
Restrict SharePoint access
allLimit SharePoint access to trusted users only and implement strict authentication controls.
Network segmentation
allIsolate SharePoint servers from critical network segments and implement firewall rules.
🧯 If You Can't Patch
- Implement strict access controls and monitor for suspicious SharePoint activity.
- Deploy web application firewall (WAF) rules to block suspicious SharePoint requests.
🔍 How to Verify
Check if Vulnerable:
Check SharePoint Server version and verify if March 2021 security updates are installed.Check Version:
Get-SPFarm | Select BuildVersionVerify Fix Applied:
Verify March 2021 security updates are installed via Windows Update history or PowerShell: Get-HotFix -Id KB5000871📡 Detection & Monitoring
Log Indicators:
- Unusual SharePoint activity patterns
- Suspicious file uploads or server-side code execution attempts
- Authentication anomalies
Network Indicators:
- Abnormal SharePoint web service requests
- Unexpected outbound connections from SharePoint servers
SIEM Query:
source="sharepoint" AND (event_id=6398 OR event_id=6399) AND (process_execution OR file_upload)