Arbitrary File Upload in Monstra CMS Files Manager Enables RCE

Executive Summary

CVE-2025-69906 is an arbitrary file upload vulnerability in Monstra CMS version 3.0.4, specifically in the Files Manager plugin.

The vulnerability exists because the application uses a blacklist-based file extension validation that can be bypassed by attackers using techniques such as case variations, double extensions, null byte injection, and non-standard PHP extensions.

Uploaded files are stored directly in a web-accessible directory without proper sanitization or renaming, allowing an attacker with authenticated access to upload malicious files that can be executed as code on the server.

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Detection Guidance

[{'type': 'paragraph', 'content': 'This vulnerability can be detected by checking for the presence of suspicious files uploaded to the web-accessible /public/files/ directory, especially files with extensions that bypass the blacklist (e.g., .PHP, .php7, .phtml, or files with double extensions like .php.jpg).'}, {'type': 'paragraph', 'content': 'You can also look for signs of remote code execution attempts or webshell activity by monitoring web server logs for requests to such uploaded files.'}, {'type': 'paragraph', 'content': 'Suggested commands to detect suspicious files include:'}, {'type': 'list_item', 'content': 'Find files with suspicious extensions or double extensions in the upload directory: find /path/to/monstra/public/files/ -type f \\( -iname "*.php*" -o -iname "*.phtml" \\)'}, {'type': 'list_item', 'content': "Search for files with null byte injection attempts in filenames (if filesystem supports it): ls -l | grep $'\\x00'"}, {'type': 'list_item', 'content': 'Check web server access logs for requests to suspicious files: grep -iE "\\.php|\\.phtml" /var/log/apache2/access.log'}, {'type': 'list_item', 'content': 'Look for recently modified or created files in the upload directory: find /path/to/monstra/public/files/ -type f -mtime -7'}] [2]

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Impact Analysis

This vulnerability can lead to remote code execution (RCE) on the affected server.

• An attacker with authenticated access to the Files Manager can upload a malicious PHP webshell.
• The webshell can be executed remotely, allowing the attacker to run arbitrary PHP code.
• Potential impacts include full server compromise, privilege escalation, and information disclosure such as reading sensitive configuration files.

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Compliance Impact

I don't know

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Mitigation Strategies

Immediate mitigation steps include:

• Disable the Files Manager plugin entirely to prevent any file uploads through the vulnerable interface.
• Implement a whitelist of allowed file extensions for uploads, such as only permitting image files like PNG or JPEG.
• Rename uploaded files using unique identifiers (e.g., using uniqid()) and enforce safe extensions like .jpg to prevent execution.
• Store uploaded files outside the web root directory to prevent direct web access and execution.
• Consider migrating to a maintained CMS platform since Monstra CMS has been unmaintained since 2018.

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