Anti-Spam by CleanTalk <= 6.89.1 - Unauthenticated Stored Cross-Site Scripting via 'user_email' Registration Parameter

The Anti-Spam by CleanTalk – Spam Protection Without CAPTCHA plugin for WordPress is vulnerable to Stored Cross-Site Scripting via the 'user_email' parameter in all versions up to, and including, 6.89.1 due to insufficient input sanitization and output escaping. This makes it possible for unauthenticated attackers to inject arbitrary web scripts in pages that will execute whenever a user accesses an injected page. Successful exploitation requires user registration to be open on the site, and script execution is triggered when an Administrator subsequently runs the plugin's user-spam scan and views the resulting Non-checkable users or Spam users tab — the plugin's own documented workflow.