CVE-2020-16944
📋 TL;DR
CVE-2020-16944 is a cross-site scripting (XSS) vulnerability in Microsoft SharePoint Server where improper input sanitization allows authenticated attackers to execute malicious scripts in users' browsers. This affects SharePoint Server installations that haven't applied the security update. Attackers can steal sensitive information, perform actions as the victim, or modify SharePoint content.
💻 Affected Systems
- Microsoft SharePoint Server
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Attacker gains full control of victim's SharePoint account, steals sensitive data, modifies permissions, deletes content, and uses the compromised account to attack other systems.
Likely Case
Attacker steals session cookies and authentication tokens, accesses unauthorized content, and performs limited actions within the victim's SharePoint permissions.
If Mitigated
With proper input validation and output encoding, the attack fails to execute malicious scripts, limiting impact to failed exploitation attempts.
🎯 Exploit Status
Exploitation requires authenticated attacker and user interaction via social engineering.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: October 2020 security updates
Vendor Advisory: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2020-16944
Restart Required: Yes
Instructions:
1. Download and install October 2020 security update for SharePoint Server from Microsoft Update Catalog. 2. Apply update to all SharePoint servers. 3. Restart SharePoint services. 4. Test functionality.
🔧 Temporary Workarounds
Enable Content Security Policy (CSP)
windowsImplement CSP headers to restrict script execution from unauthorized sources.
Add CSP headers to SharePoint web.config: <add name="Content-Security-Policy" value="default-src 'self'" />
Input Validation Rules
windowsImplement additional input validation for SharePoint web applications.
Configure SharePoint input validation via PowerShell: Set-SPWebApplication -Identity <WebApp> -RequestValidationMode 2.0
🧯 If You Can't Patch
- Implement web application firewall (WAF) rules to block XSS payloads in SharePoint requests.
- Restrict user permissions to minimum required and implement strict access controls.
🔍 How to Verify
Check if Vulnerable:
Check SharePoint Server version and compare with patched versions. Unpatched versions before October 2020 updates are vulnerable.Check Version:
Get-SPFarm | Select BuildVersionVerify Fix Applied:
Verify SharePoint Server has October 2020 security updates installed and test XSS payloads are properly sanitized.📡 Detection & Monitoring
Log Indicators:
- Unusual POST requests with script tags or JavaScript in parameters
- Multiple failed XSS attempts in IIS logs
Network Indicators:
- HTTP requests containing <script> tags or JavaScript payloads to SharePoint endpoints
SIEM Query:
source="IIS" AND ("<script>" OR "javascript:" OR "onerror=" OR "onload=") AND dest_port=443