Critical Unpatched SharePoint Zero-Day Actively Exploited: How to Protect Your Systems

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Introduction

A critical zero-day vulnerability (CVE-2025-53770) in Microsoft SharePoint Server is being actively exploited, compromising over 75 company servers. This flaw, a variant of CVE-2025-49706, allows attackers to execute arbitrary code on unpatched on-premises SharePoint deployments. Here’s what you need to know to defend your infrastructure.

Learning Objectives

  • Understand the exploit mechanics of CVE-2025-53770
  • Apply immediate mitigations if a patch isn’t available
  • Harden SharePoint Server against future attacks

You Should Know

1. Verify If Your SharePoint Server Is Vulnerable

Command (PowerShell):

Get-SPFarm | Select BuildVersion

Step-by-Step Guide:

1. Open PowerShell as Administrator.

  1. Run the command to check your SharePoint build version.
  2. Compare it with Microsoft’s advisory (linked below). Versions 16.0.10396.0 and below are vulnerable.

Mitigation: Disable custom script execution via:

Set-SPSite -Identity "<SiteURL>" -DenyAddAndCustomizePages 1

2. Block Exploit Traffic with IIS URL Rewrite

Rule (web.config):

<rule name="Block SharePoint Exploit" stopProcessing="true"> 
<match url="._layouts/15/.\.aspx.cmd=" /> 
<action type="AbortRequest" /> 
</rule>

Step-by-Step Guide:

1. Open IIS Manager.

2. Navigate to your SharePoint site.

  1. Add this rule to block malicious payloads targeting the vulnerability.

3. Enable Enhanced Logging for Attack Detection

Command (PowerShell):

Set-SPLogLevel -TraceSeverity VerboseEx

Step-by-Step Guide:

1. Enable verbose logging to capture exploit attempts.

2. Monitor logs for unusual `_layouts/15/` requests.

3. Use `Get-SPLogEvent` to filter suspicious activity.

4. Restrict SharePoint Service Accounts

Command (Windows):

net localgroup "SharePoint_Admins" /add "DOMAIN\svc_sharepoint"

Step-by-Step Guide:

1. Create a dedicated service account.

2. Assign least-privilege permissions.

3. Isolate the account from domain admin groups.

5. Deploy Emergency Network Controls

Command (Firewall):

iptables -A INPUT -p tcp --dport 80 -m string --string "_layouts/15" --algo bm -j DROP

Step-by-Step Guide:

1. Block HTTP requests containing exploit patterns.

2. Test rules before applying to production.

What Undercode Say

  • Key Takeaway 1: Patchless exploits demand proactive mitigations—assume breach and monitor aggressively.
  • Key Takeaway 2: SharePoint’s complexity makes it a prime target; segment networks and restrict administrative access.

Analysis:

The exploit’s rapid weaponization underscores gaps in legacy enterprise systems. While Microsoft works on a patch, attackers are leveraging the delay. Organizations must balance operational continuity with security, prioritizing web application firewalls (WAFs) and endpoint detection.

Prediction

This flaw will fuel ransomware campaigns targeting mid-market enterprises. Expect:
1. Escalation: More APT groups adopting the exploit within 30 days.
2. Patching Delays: Enterprises with change-freeze policies will remain vulnerable for months.
3. Cloud Shift: Accelerated migration to SharePoint Online as a stopgap.

Actionable Intel: Bookmark Microsoft’s advisory for updates:

Stay vigilant—assume your SharePoint servers are already in crosshairs.

IT/Security Reporter URL:

Reported By: Moritzsamrock Critical – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅

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