Ultimate Member <= 2.13.1 - Unauthenticated Stored Cross-Site Scripting via 'form_id' Parameter

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 'form_id' parameter in all versions up to, and including, 2.13.1 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. The injected payload is stored in the registering user's 'submitted' usermeta via update_user_meta() and is only triggered when an administrator opens the affected user record in the wp-admin Users modal, which inserts the unescaped output via jQuery .html().