WP Live Chat Support Pro <= 8.0.26 - Arbitrary File Upload

The WP Live Chat Support Pro plugin through 8.0.26 for WordPress contains an arbitrary file upload vulnerability. This results from an incomplete patch for CVE-2018-12426. Arbitrary file upload is achieved by using a non-blacklisted executable file extension in conjunction with a whitelisted file extension, and prepending "magic bytes" to the payload to pass MIME checks. Specifically, an unauthenticated remote user submits a crafted file upload POST request to the REST api remote_upload endpoint. The file contains data that will fool the plugin's MIME check into classifying it as an image (which is a whitelisted file extension) and finally a trailing .phtml file extension.