The Booking Calendar plugin for WordPress is vulnerable to Sensitive Information Exposure in all versions up to, and including, 11.4.3 via the wpbc_is_show_popover_in_flex_timeline() function. This makes it possible for unauthenticated attackers to extract sensitive data including names, email addresses, and phone numbers of customers who have made bookings.
https://plugins.trac.wordpress.org/changeset/3633943/booking