The WordPress Webmention plugin versions up to 5.6.2 contains a Server-Side Request Forgery vulnerability in the MF2::parse_authorpage function via the Receiver::post function. This vulnerability allows unauthenticated attackers to make arbitrary web requests from the application server, potentially enabling them to query and modify internal services. The flaw affects all versions up to and including 5.6.2 and can be exploited without authentication, making it a significant security risk for WordPress sites using this plugin.