The Add User Meta plugin for WordPress is vulnerable to Cross-Site Request Forgery in all versions up to, and including, 1.0.1. This is due to missing or incorrect nonce validation on the 'add-user-meta' page. This makes it possible for unauthenticated attackers to update settings and inject malicious web scripts 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.