The Webmention plugin for WordPress is vulnerable to Server-Side Request Forgery in all versions up to, and including, 5.6.2 via the 'Tools::read' function. This makes it possible for authenticated attackers, with Subscriber-level access and above, to make web requests to arbitrary locations originating from the web application and can be used to query and modify information from internal services.
https://plugins.trac.wordpress.org/changeset/3494831/webmention
https://plugins.trac.wordpress.org/browser/webmention/tags/5.6.2/includes/class-tools.php#L81