CVE-2021-31964
📋 TL;DR
CVE-2021-31964 is a spoofing vulnerability in Microsoft SharePoint Server that allows an attacker to trick users into clicking malicious links that appear to originate from trusted SharePoint sites. This affects organizations running vulnerable SharePoint Server versions, potentially enabling phishing attacks against authenticated users.
💻 Affected Systems
- Microsoft SharePoint Server
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Successful exploitation could lead to credential theft, session hijacking, or malware deployment through convincing phishing campaigns that bypass typical user skepticism by appearing as legitimate SharePoint links.
Likely Case
Attackers craft convincing phishing emails with SharePoint links that redirect to malicious sites, leading to credential harvesting or malware installation when users click them.
If Mitigated
With proper user awareness training and email filtering, impact is limited to occasional successful phishing attempts that can be contained through incident response.
🎯 Exploit Status
Exploitation requires user interaction (clicking a link) but is technically simple once the vulnerability is understood.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: June 2021 security updates
Vendor Advisory: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-31964
Restart Required: Yes
Instructions:
1. Download the June 2021 security update for your SharePoint version from Microsoft Update Catalog. 2. Apply the update following Microsoft's SharePoint patching procedures. 3. Restart SharePoint services as required.
🔧 Temporary Workarounds
URL validation configuration
windowsConfigure SharePoint to validate and sanitize URLs more strictly
🧯 If You Can't Patch
- Implement strict email filtering to detect and block SharePoint link spoofing attempts
- Enhance user awareness training specifically about SharePoint link verification and phishing risks
🔍 How to Verify
Check if Vulnerable:
Check SharePoint Server version and compare against patched versions (post-June 2021 updates)Check Version:
Get-SPFarm | Select BuildVersionVerify Fix Applied:
Verify that June 2021 security updates are installed via Windows Update history or SharePoint Central Administration📡 Detection & Monitoring
Log Indicators:
- Unusual URL redirect patterns in SharePoint logs
- Multiple failed authentication attempts following SharePoint link clicks
Network Indicators:
- HTTP redirects from SharePoint to external domains
- Unusual outbound traffic patterns after SharePoint access
SIEM Query:
source="sharepoint*" AND (url_redirect OR http_302) AND dest_domain NOT IN (allowed_domains)