The Bricksforge plugin for WordPress is vulnerable to unauthenticated arbitrary file upload in versions up to, and including, 3.1.8.9. This is due to insufficient validation of the attacker-controlled URL field in the 'temporaryFileUploads' parameter during form submission. An unauthenticated attacker can first obtain a valid nonce via the bricksforge_regenerate_nonce AJAX endpoint, then upload a GIF/PHP polyglot file to the temporary upload directory where MIME type validation is correctly performed. Subsequently, the attacker can submit a form with a crafted 'temporaryFileUploads' parameter where the server-side file path points to the validated GIF file, but the attacker-controlled url field ends with a .php extension. This makes it possible for unauthenticated attackers to upload and execute arbitrary PHP code on the server.
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