Attack Chain

1. The attacker crafts a polyglot file that is both a valid image (e.g., JPEG) and a valid PHP archive (phar).
2. The malicious phar archive contains serialized PHP objects designed to execute arbitrary code when deserialized.
3. The attacker uploads the polyglot file to the OpenMage LTS server through a vulnerable endpoint, such as product images, CMS media, or file import functionality.
4. The application stores the uploaded file in a publicly accessible directory.
5. The attacker triggers the vulnerable application logic in app/code/core/Mage/Core/Model/File/Validator/Image.php (line 72), app/code/core/Mage/Cms/Model/Wysiwyg/Images/Storage.php (line 137) or lib/Varien/Image.php (line 71), causing the application to use getimagesize() or similar functions on the uploaded file with the phar:// stream wrapper.
6. PHP attempts to read the file using the phar:// wrapper, which triggers the deserialization of the malicious metadata contained within the phar archive.
7. The deserialization process instantiates the malicious PHP objects, executing the attacker’s code.
8. The attacker achieves remote code execution on the server, allowing them to compromise the system, install malware, or exfiltrate data.

Impact

Successful exploitation allows an attacker to execute arbitrary code on the OpenMage LTS server. This can lead to complete system compromise, data theft, defacement of the website, or the installation of malware. Given the potential for unauthenticated file uploads, the impact is significant, with potential widespread compromise affecting all versions of OpenMage LTS prior to 20.16.1. The vulnerability exists in core Magento 1.x code, so all derived products are affected.

Recommendation

• Upgrade OpenMage LTS to version 20.16.1 or later to patch the vulnerability.
• Implement the recommended code fix by blocking phar:// paths before passing to vulnerable functions like getimagesize() in the affected files: app/code/core/Mage/Core/Model/File/Validator/Image.php, app/code/core/Mage/Cms/Model/Wysiwyg/Images/Storage.php, and lib/Varien/Image.php.
• Deploy the Sigma rule to detect attempts to access files using the phar:// stream wrapper (see rule “Detect Phar Stream Wrapper Access”).
• If upgrading is not immediately possible, disable the phar:// stream wrapper in the php.ini file.
• Implement strict upload validation beyond file extension checks to prevent the upload of polyglot files.