The WP Post Author – Author Box, Multiple Authors, Guest Authors & Custom Avatars plugin for WordPress is vulnerable to generic SQL Injection via the 'wpma_metabox_authors_list' parameter in all versions up to, and including, 3.9.1 due to insufficient escaping on the user supplied parameter and lack of sufficient preparation on the existing SQL query. This makes it possible for authenticated attackers, with author-level access and above, to append additional SQL queries into already existing queries that can be used to extract sensitive information from the database. Exploitation is a two-step chain: an attacker first saves a crafted guest-author token via the wpma_metabox_authors_list parameter during post creation or editing, then triggers the injection when any admin user loads the post list screen at /wp-admin/edit.php, causing the injected SQL result to be rendered in the Authors column.