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The vulnerability in Umbraco CMS v16.3.3 arises from inadequate validation mechanisms during file upload processes, specifically targeting PDF files. Attackers craft a malicious PDF file that embeds server-side code or scripts, exploiting weaknesses in the upload handler. When a user uploads a file, the CMS fails to properly verify the file’s content, MIME type, or extension, allowing the crafted PDF to bypass security filters. This is often due to insufficient sanitization of file names, such as allowing double extensions (e.g., .pdf.aspx) or null bytes, which can trick the system into saving the file with an executable extension. The uploaded file is stored in web-accessible directories, like media or temp folders, where it can be directly requested via HTTP. If the server interprets the file as executable—due to misconfigurations or vulnerable libraries—the embedded code runs, leading to remote code execution (RCE). This typically involves leveraging server-side scripting engines like ASP.NET, as Umbraco is built on .NET, enabling attackers to execute arbitrary commands, access databases, or install backdoors. The vulnerability may stem from flaws in the PDF parsing library or the upload module’s failure to restrict file types based on actual content rather than extensions. Attackers with minimal permissions, such as content editors, can exploit this, escalating privileges to compromise the entire CMS instance. The crafted PDF might contain hidden scripts that activate when processed by server components, exploiting trust in uploaded media files. This allows unauthorized access, data theft, or system takeover, highlighting critical lapses in input validation and secure file handling practices.
Platform: Umbraco CMS
Version: v16.3.3
Vulnerability: Arbitrary file upload
Severity: Moderate
Date: Dec 22, 2025
Prediction: Patch by January 2026
What Undercode Say:
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!/bin/bash
Check Umbraco version
curl -s http://target/umbraco/version | grep “16.3.3”
Upload crafted PDF
curl -X POST -F “[email protected]” http://target/umbraco/api/upload
Verify exploit
curl http://target/uploads/malicious.pdf.aspx
Python code for PDF crafting:
import PyPDF2
with open(‘exploit.pdf’, ‘wb’) as f:
f.write(b’%PDF-1.4\n‘)
how Exploit:
Attacker uploads a PDF with embedded ASPX code via the Umbraco media interface, bypassing validation by using a crafted MIME type or filename. The file is saved as .pdf.aspx in a web-accessible directory. When accessed, the server executes the ASPX code, allowing remote command execution via HTTP requests.
Protection from this CVE
Update to patched version, restrict upload permissions, implement file type verification using content detection, sanitize filenames, disable execution in upload directories, and use web application firewalls.
Impact:
Remote code execution, data breach, system compromise, unauthorized access.
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Sources:
Reported By: github.com
Extra Source Hub:
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