The SMTP by BestWebSoft plugin for WordPress is vulnerable to arbitrary file uploads due to missing file type validation in the 'save_options' function in all versions up to, and including, 1.1.9. This makes it possible for authenticated attackers, with Administrator-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/changeset/3250935/
https://plugins.trac.wordpress.org/browser/bws-smtp/tags/1.1.8/includes/class-bwssmtp-settings.php