Ultimate Member <= 2.8.6 - Authenticated (Contributor+) Stored Cross-Site Scripting

The Ultimate Member – User Profile, Registration, Login, Member Directory, Content Restriction & Membership Plugin plugin for WordPress is vulnerable to Stored Cross-Site Scripting via the plugin's 'um_loggedin' shortcode in all versions up to, and including, 2.8.6 due to insufficient input sanitization and output escaping on user supplied attributes. This makes it possible for authenticated attackers, with contributor-level access and above, to inject arbitrary web scripts in pages that will execute whenever a user accesses an injected page.