The Tutor LMS – eLearning and online course solution plugin for WordPress is vulnerable to Reflected Cross-Site Scripting via the 'tab' parameter in all versions up to, and including, 4.1.0 due to insufficient input sanitization and output escaping. This makes it possible for unauthenticated attackers to inject arbitrary web scripts in pages that execute if they can successfully trick a user into performing an action such as clicking on a link. Exploitation requires the targeted lesson page to have an empty nav-items array — a condition that occurs naturally when a lesson has no post content, no attachments, and comments are disabled or the viewer is not logged in — which bypasses the in_array() tab validation and allows the raw attacker-controlled value to reach the sink.