Swiss Toolkit For WP <= 1.4.6 - Authenticated (Author+) Arbitrary File Upload via upload_extension_files()

The Swiss Toolkit For WP plugin for WordPress is vulnerable to arbitrary file upload due to a flawed file type validation bypass in the upload_extension_files() function in all versions up to, and including, 1.4.6. The upload_extension_files() function hooks into WordPress's wp_check_filetype_and_ext filter and uses strpos() to check if a filename contains a configured extension string, rather than verifying the actual file extension. This makes it possible for authenticated attackers, with Author-level access and above, to upload arbitrary files (including PHP) on the affected site's server which may make remote code execution possible, granted the "Enhanced Multi-Format Image Support" feature is enabled with at least one extension (e.g., avif) in the allowed formats.