The Meow Gallery plugin for WordPress is vulnerable to unauthorized modification of data due to a missing capability check on the REST API endpoint /wp-json/meow-gallery/v1/save_shortcode in all versions up to, and including, 5.4.4 This makes it possible for authenticated attackers, with Author-level access and above, to arbitrarily create or overwrite existing gallery shortcode records by supplying a user-controlled id value. The endpoint performs database update operations without verifying that the requesting user is authorized to modify the referenced gallery record or create their own.
