The WP Fastest Cache plugin for WordPress is vulnerable to Sensitive Information Exposure in all versions up to, and including, 1.5.3. The plugin caches the response of Core’s wp-activate.php without honoring its no-store/no-cache headers, the wp-activate-* cookie, or the WP_INSTALLING state, writing the credential-bearing activation response to a publicly accessible cache file. This makes it possible for unauthenticated attackers to retrieve the plaintext username and twelve-character password of the most recently activated Multisite registrant from that cached wp-activate.php response. Exploitation requires a Multisite subdirectory installation with public user and site registration enabled, and the credentials are only exposed after a genuine registrant follows their activation link.
https://plugins.trac.wordpress.org/changeset/3749897/wp-fastest-cache
https://plugins.trac.wordpress.org/browser/wp-fastest-cache/tags/1.5.2/inc/cache.php#L83
https://plugins.trac.wordpress.org/browser/wp-fastest-cache/tags/1.5.2/inc/cache.php#L567
https://plugins.trac.wordpress.org/browser/wp-fastest-cache/tags/1.5.2/inc/cache.php#L452
https://plugins.trac.wordpress.org/browser/wp-fastest-cache/tags/1.5.2/inc/cache.php#L1400
https://plugins.trac.wordpress.org/browser/wp-fastest-cache/tags/1.5.2/inc/cache.php#L1027