The Responsive Lightbox & Gallery plugin for WordPress is vulnerable to Server-Side Request Forgery in all versions up to, and including, 2.5.3 via the get_image_size_by_url function. This is due to insufficient validation of user-supplied URLs when determining image dimensions for gallery items. This makes it possible for authenticated attackers, with Author-level access and above, to make web requests to arbitrary locations originating from the web application which can be used to query and modify information from internal services.
The web server receives a URL or similar request from an upstream component and retrieves the contents of this URL, but it does not sufficiently ensure that the request is being sent to the expected destination.