UnderConstructionPage PRO <= 5.76 - Authenticated (Subscriber+) Arbitrary File Read via template_thumbnail Parameter

The UnderConstructionPage PRO plugin for WordPress is vulnerable to Arbitrary File Read in all versions up to, and including, 5.76. This is due to the plugin accepting arbitrary local file paths in the template_thumbnail parameter and copying their contents into a publicly accessible uploads file. This makes it possible for authenticated attackers, with Subscriber-level access and above, to read arbitrary files on the server, which can contain sensitive information.