EmailKit < 1.6.4 - Authenticated (Administrator+) Path Traversal via 'emailkit-editor-template' REST API Parameter

Description

The EmailKit – Email Customizer for WooCommerce & WP plugin for WordPress is vulnerable to arbitrary file read via path traversal in all versions up to, and including, 1.6.3. This is due to the action() function in the TemplateData class passing user-supplied input from the 'emailkit-editor-template' REST API parameter directly to file_get_contents() without any path validation, sanitization, or restriction to an allowed directory. This makes it possible for authenticated attackers, with Administrator-level access, to read arbitrary files on the server (such as /etc/passwd or wp-config.php) by supplying a traversal path. The file contents are stored as post meta and can subsequently be retrieved via the fetch-data REST API endpoint. Notably, the CheckForm class in the same plugin implements proper path validation using realpath() and directory restriction, demonstrating that the developer was aware of the risk but failed to apply the same protections to the TemplateData endpoint.