CVE-2021-38651
📋 TL;DR
CVE-2021-38651 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. Attackers can exploit this to conduct phishing attacks or redirect users to malicious sites.
💻 Affected Systems
- Microsoft SharePoint Server
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Successful exploitation could lead to credential theft, malware installation, or data exfiltration through convincing phishing campaigns that appear to originate from legitimate SharePoint sites.
Likely Case
Attackers create convincing phishing emails with SharePoint links that redirect to malicious sites, potentially compromising user credentials or delivering malware.
If Mitigated
With proper URL filtering, user awareness training, and network segmentation, the impact is limited to potential user confusion or minor inconvenience.
🎯 Exploit Status
Exploitation requires user interaction (clicking a link). The vulnerability is in how SharePoint handles URL validation.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Security updates released in September 2021 patch cycle
Vendor Advisory: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-38651
Restart Required: Yes
Instructions:
1. Download the September 2021 security update for SharePoint Server from Microsoft Update Catalog. 2. Apply the update to all SharePoint servers. 3. Restart SharePoint services. 4. Test functionality after patching.
🔧 Temporary Workarounds
URL Filtering and Monitoring
allImplement web filtering to block suspicious SharePoint URLs and monitor for unusual redirect patterns.
User Awareness Training
allTrain users to verify URLs before clicking, especially for SharePoint links in emails.
🧯 If You Can't Patch
- Implement strict URL filtering at network perimeter to detect and block spoofed SharePoint URLs
- Enable enhanced logging on SharePoint servers and monitor for suspicious URL patterns or redirects
🔍 How to Verify
Check if Vulnerable:
Check SharePoint Server version against affected versions (2019 and Subscription Edition pre-September 2021 patches).Check Version:
Get-SPFarm | Select BuildVersion (PowerShell) or check Central Administration > Upgrade and Migration > Check product and patch installation statusVerify Fix Applied:
Verify that September 2021 security updates are installed via Windows Update history or SharePoint Central Administration.📡 Detection & Monitoring
Log Indicators:
- Unusual URL patterns in SharePoint logs
- Multiple redirects from SharePoint URLs to external domains
- Increased failed authentication attempts following SharePoint link clicks
Network Indicators:
- HTTP 302 redirects from SharePoint to unexpected domains
- Unusual outbound traffic patterns following SharePoint access
SIEM Query:
source="sharepoint_logs" AND (url="*redirect*" OR url="*spoof*" OR status=302)