CVE-2021-27076
📋 TL;DR
CVE-2021-27076 is a remote code execution vulnerability in Microsoft SharePoint Server that allows authenticated attackers to execute arbitrary code on affected systems. This affects organizations running vulnerable SharePoint Server versions, potentially enabling attackers to gain control of SharePoint servers and access sensitive data.
💻 Affected Systems
- Microsoft SharePoint Server
- Microsoft SharePoint Foundation
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Complete compromise of SharePoint server leading to data exfiltration, lateral movement within network, and persistent backdoor installation.
Likely Case
Unauthorized access to sensitive SharePoint data, privilege escalation, and potential ransomware deployment.
If Mitigated
Limited impact with proper network segmentation, authentication controls, and monitoring in place.
🎯 Exploit Status
Exploitation requires authenticated access to SharePoint; proof-of-concept code has been publicly released.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: March 2021 security updates
Vendor Advisory: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-27076
Restart Required: Yes
Instructions:
1. Apply March 2021 security updates for SharePoint Server 2013/2016/2019. 2. Restart SharePoint services. 3. Verify patch installation through Windows Update history.
🔧 Temporary Workarounds
Restrict SharePoint Access
allLimit SharePoint access to trusted users only and implement strong 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 multi-factor authentication for all SharePoint users.
- Deploy network monitoring and intrusion detection systems focused on SharePoint traffic patterns.
🔍 How to Verify
Check if Vulnerable:
Check SharePoint version and verify if March 2021 security updates are installed.Check Version:
Get-SPFarm | Select BuildVersion (PowerShell) or check Central Administration > Upgrade and Migration > Check product and patch installation statusVerify Fix Applied:
Verify installation of KB5000871 (SharePoint 2013), KB5000872 (SharePoint 2016), or KB5000873 (SharePoint 2019).📡 Detection & Monitoring
Log Indicators:
- Unusual PowerShell execution from SharePoint processes
- Suspicious file uploads to SharePoint
- Authentication anomalies
Network Indicators:
- Abnormal outbound connections from SharePoint servers
- Unexpected PowerShell remoting traffic
SIEM Query:
source="sharepoint" AND (process="powershell" OR cmdline="*Invoke-Expression*" OR cmdline="*IEX*")