The OPEN-BRAIN plugin for WordPress is vulnerable to Stored Cross-Site Scripting via the 'API Key' settings field in all versions up to, and including, 0.5.0. This is due to insufficient input sanitization and output escaping. The plugin uses sanitize_text_field() which strips HTML tags but does not encode double quotes or other HTML-special characters needed for safe attribute context output. The API key value is saved via update_option() and later output into an HTML input element's value attribute without esc_attr() escaping. This makes it possible for authenticated attackers, with Administrator-level access, to inject arbitrary web scripts via attribute breakout payloads (e.g., double quotes followed by event handlers) that execute whenever a user accesses the plugin settings page.