WP Statistics <= 14.16.8 - Unauthenticated Stored Cross-Site Scripting via 'utm_campaign' Parameter

The WP Statistics – Simple, privacy-friendly Google Analytics alternative plugin for WordPress is vulnerable to Stored Cross-Site Scripting via the 'utm_campaign' parameter in all versions up to, and including, 14.16.8 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 payload can be planted without authentication via the public /wp-statistics/v2/hit REST endpoint, because the required signature is exposed on the public homepage and a base64-encoded page_uri POST parameter overrides the previously sanitized REQUEST_URI, allowing the malicious utm_campaign value to bypass sanitization and be stored in the database.