CVE-2025-48953
📋 TL;DR
This vulnerability allows attackers to upload files with disallowed extensions in Umbraco CMS by manipulating API requests. It affects Umbraco installations from version 14.0.0 up to (but not including) versions 15.4.2 and 16.0.0. Attackers can bypass file extension restrictions to upload potentially malicious files.
💻 Affected Systems
- Umbraco CMS
📦 What is this software?
Umbraco Cms by Umbraco
⚠️ Risk & Real-World Impact
Worst Case
An attacker uploads malicious executable files (like .aspx, .php) that could lead to remote code execution, complete system compromise, or website defacement.
Likely Case
Attackers upload malicious scripts or web shells to gain unauthorized access, modify content, or steal data from the CMS.
If Mitigated
With proper file validation and server-side restrictions, unauthorized file uploads are blocked, limiting impact to failed upload attempts.
🎯 Exploit Status
Exploitation requires API access but the vulnerability is in the core file upload mechanism, making it relatively easy to exploit once access is obtained.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 15.4.2 or 16.0.0
Vendor Advisory: https://github.com/umbraco/Umbraco-CMS/security/advisories/GHSA-fr6r-p8hv-x3c4
Restart Required: Yes
Instructions:
1. Backup your Umbraco installation and database. 2. Update to Umbraco version 15.4.2 if using Umbraco 15.x. 3. Update to Umbraco version 16.0.0 if using Umbraco 16.x. 4. Restart the application pool/IIS. 5. Test file upload functionality.
🧯 If You Can't Patch
- Implement strict file upload validation at the web server level (IIS/nginx/Apache) to block unauthorized file extensions.
- Monitor and audit all file upload activities through logging and alert on suspicious upload attempts.
🔍 How to Verify
Check if Vulnerable:
Check your Umbraco version in the admin dashboard or web.config file. If version is between 14.0.0 and 15.4.1, or any 16.x version before 16.0.0, you are vulnerable.Check Version:
Check Umbraco version in Admin → Settings → General or examine the web.config file for version information.Verify Fix Applied:
After updating, verify the version shows 15.4.2 or higher (for 15.x) or 16.0.0 or higher (for 16.x). Test file upload functionality with disallowed extensions to confirm they are rejected.📡 Detection & Monitoring
Log Indicators:
- Failed file upload attempts with unusual extensions
- Successful uploads of files with typically blocked extensions
- API requests to file upload endpoints with manipulated parameters
Network Indicators:
- Unusual POST requests to Umbraco file upload API endpoints
- Traffic patterns showing file uploads with unexpected file types
SIEM Query:
source="umbraco.logs" AND (event="file_upload" OR uri_path="/umbraco/api/*upload*") AND (file_extension NOT IN ("jpg","png","pdf","doc","txt"))