Booking.com confirmed on Monday that hackers have accessed customers’ personal data, including names, email addresses, phone numbers, and booking details. The global travel and hotel reservation giant notified customers this past week of the breach, according to several online posts.

There are reports of people receiving phishing messages via WhatsApp in the last two weeks that included “booking details and personal information.” That suggests hackers are leveraging the stolen information to target Booking.com customers.

Experts warn that this kind of data will be extremely valuable to fraudsters who are now racing to trick unwitting customers.

Cybersecurity firm Norton has dubbed the scams “reservation hijacks” because criminals have contacted Booking.com customers pretending to be hotels in order to trick victims into sending them money based on bogus reservation problems.

The advice is to ignore any calls or messages purportedly from Booking.com, hotels, or other accommodations, especially if they request money, and to contact Booking.com directly.