The WP Fastest Cache WordPress plugin before 1.5.1 does not include a set of tracking-related query parameters in its page-cache key while still caching pages requested with them, allowing unauthenticated attackers to have a page rendered under their own request context stored under, and served from, the clean URL's cache entry to every subsequent visitor.
https://wpscan.com/vulnerability/b6f8be96-13d9-497a-8e57-5a65b519958b/