CVE-2025-67288: Umbraco CMS RCE Vulnerability

CVE-2025-67288 Overview

An arbitrary file upload vulnerability exists in Umbraco CMS v16.3.3 that allows attackers to execute arbitrary code by uploading a crafted PDF file. This vulnerability falls under CWE-434 (Unrestricted Upload of File with Dangerous Type), which represents a significant security risk for web applications that handle file uploads without proper validation.

It is important to note that this vulnerability is disputed by Umbraco. According to the vendor, the responsibility for file validation belongs to the system administrator implementing Umbraco CMS in their environment, not to Umbraco CMS itself. This dispute is related to CVE-2023-49279, indicating a recurring discussion around the shared responsibility model for file upload security in the platform.

Critical Impact

Successful exploitation allows remote attackers to achieve arbitrary code execution on the server by uploading malicious files disguised as PDFs, potentially leading to complete system compromise with no authentication required.

Affected Products

• Umbraco CMS version 16.3.3
• Systems implementing Umbraco CMS without custom file validation controls
• Environments allowing PDF file uploads through the CMS interface

Discovery Timeline

• 2025-12-22 - CVE-2025-67288 published to NVD
• 2026-01-08 - Last updated in NVD database

Technical Details for CVE-2025-67288

Vulnerability Analysis

This arbitrary file upload vulnerability allows attackers to bypass file type restrictions by crafting malicious files that appear to be legitimate PDFs. The core issue lies in insufficient validation of uploaded file content, enabling attackers to upload executable code that the server may process or execute.

The vulnerability is network-accessible, requires no privileges or user interaction to exploit, and can impact resources beyond the vulnerable component's security scope. This makes it particularly dangerous in internet-facing Umbraco CMS deployments where the file upload functionality is exposed.

The dispute status from Umbraco highlights an important architectural consideration: Umbraco CMS provides documentation on implementing file validation, placing the onus on system administrators to configure appropriate upload restrictions. However, the lack of secure-by-default validation means that default or minimally configured installations may be vulnerable.

Root Cause

The root cause is classified as CWE-434: Unrestricted Upload of File with Dangerous Type. This vulnerability class occurs when an application allows users to upload files without adequately validating the file type, content, or extension. In this case, attackers can craft PDF files containing malicious payloads that, when processed by the server, result in code execution.

The architectural decision to delegate file validation entirely to implementing administrators creates a security gap where default installations lack protective measures against malicious uploads.

Attack Vector

The attack is conducted over the network, targeting the file upload functionality within Umbraco CMS. An attacker crafts a malicious PDF file embedded with executable code or leverages polyglot file techniques where a file appears to be a valid PDF but contains executable payloads.

The attack flow typically involves:

1. Identifying an Umbraco CMS installation with exposed file upload functionality
2. Crafting a malicious PDF file containing embedded executable code or scripts
3. Uploading the crafted file through the CMS interface
4. Triggering execution of the malicious payload through direct access or server-side processing

Since no authentication is required and the attack complexity is low, exploitation can be automated and conducted at scale against vulnerable installations.

Detection Methods for CVE-2025-67288

Indicators of Compromise

• Unusual PDF files uploaded to the media library containing embedded scripts or executable content
• Web server logs showing access to uploaded PDF files followed by unexpected process execution
• New or modified files in web directories that were created after PDF uploads
• Unexpected outbound network connections originating from the web server process

Detection Strategies

• Implement file upload monitoring to detect files with mismatched extensions and MIME types
• Deploy web application firewalls (WAF) with rules to inspect uploaded file content for malicious patterns
• Use endpoint detection solutions like SentinelOne to monitor for suspicious process spawning from web server processes
• Analyze uploaded files with sandbox environments before allowing storage

Monitoring Recommendations

• Enable verbose logging for all file upload operations within Umbraco CMS
• Monitor the media upload directory for newly created files with executable content or suspicious characteristics
• Set up alerts for any server-side execution events triggered by files in upload directories
• Implement real-time file integrity monitoring on web server directories

How to Mitigate CVE-2025-67288

Immediate Actions Required

• Review and implement strict file validation controls as outlined in Umbraco's documentation
• Restrict file upload functionality to authenticated and authorized users only
• Configure server-side validation to check file magic bytes, not just extensions
• Consider disabling PDF uploads temporarily until proper validation is implemented
• Isolate uploaded files in a non-executable directory with restricted permissions

Patch Information

This vulnerability is currently disputed by the vendor. Umbraco maintains that file validation is the responsibility of system administrators implementing the CMS. Administrators should consult the Umbraco Official Site for documentation on implementing proper file validation controls. Additional technical details may be available in the GitHub CVE-2025-67288 Repository.

Workarounds

• Implement a whitelist-based file validation approach that checks both file extensions and magic bytes
• Use server-side content inspection to scan uploaded PDFs for embedded scripts or malicious content
• Deploy uploaded files to a separate, isolated storage location with no execute permissions
• Configure web server rules to prevent direct execution of files in upload directories
• Consider implementing antivirus scanning for all uploaded files before storage

The following example demonstrates configuring IIS to prevent execution in upload directories:

# IIS web.config example for upload directory
# Place this in the media/uploads directory

# <configuration>
#   <system.webServer>
#     <handlers accessPolicy="Read" />
#     <staticContent>
#       <remove fileExtension=".pdf" />
#       <mimeMap fileExtension=".pdf" mimeType="application/pdf" />
#     </staticContent>
#   </system.webServer>
# </configuration>