The Pixabay Images plugin for WordPress is vulnerable to arbitrary file uploads due to missing file type validation in the pixabay_upload function in all versions up to, and including, 3.4. This makes it possible for authenticated attackers, with Author-level access and above, to upload arbitrary files on the affected site's server which may make remote code execution possible.
https://plugins.trac.wordpress.org/browser/pixabay-images/trunk/pixabay-images.php#L177