The SlimStat Analytics plugin for WordPress is vulnerable to Stored Cross-Site Scripting via the 'User-Agent' header in all versions up to, and including, 5.4.11 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 show_complete_user_agent_tooltip setting must be explicitly enabled by an administrator (disabled by default) for the stored payload to be rendered and executed.