The WP Events Manager WordPress plugin before 2.2.5 does not verify that an incoming payment notification originates from the site's configured merchant account, nor that the paid amount matches the booking total, allowing unauthenticated users to mark any booking as paid without a legitimate payment reaching the merchant, including other users' bookings.
https://wpscan.com/vulnerability/08d4761f-ddc6-48f4-909e-db38438e385e/