The Autoptimize plugin for WordPress is vulnerable to Stored Cross-Site Scripting via REQUEST_URI Path in all versions up to, and including, 3.1.15.1 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. Exploitation requires the Critical CSS feature to be active with a valid API key configured, as this is the precondition for unauthenticated frontend requests to trigger queue entries via ao_ccss_enqueue().