The Peter’s Custom Anti-Spam plugin for WordPress is vulnerable to Cross-Site Request Forgery in all versions up to, and including, 3.2.3. This is due to missing nonce validation on the cas_register_post() function. This makes it possible for unauthenticated attackers to blacklist emails via a forged request granted they can trick a site administrator into performing an action such as clicking on a link.
The web application does not, or cannot, sufficiently verify whether a request was intentionally provided by the user who sent the request, which could have originated from an unauthorized actor.