The WP-UserOnline plugin for WordPress is vulnerable to Stored Cross-Site Scripting via the ‘templates[browsingpage][text]' parameter in versions up to, and including, 2.87.6 due to insufficient input sanitization and output escaping. This makes it possible for authenticated attackers with administrative capabilities and above to inject arbitrary web scripts in pages that will execute whenever a user accesses an injected page. The only affects multi-site installations and installations where unfiltered_html is disabled.