Visiting family for the holidays? So is everyone else. Last year, a record-shattering 107 million people traveled between Thanksgiving and New Year. In December and early January, travel spiked by 20%. That’s millions of people booking flights, grabbing hotels, and buying train tickets.

A good chunk of these transactions happen on “Travel Tuesday,” the day after Cyber Monday. That’s when many airlines and hotels have sales to compensate for the post-Thanksgiving travel slump. It’s also when airlines tend to experience price drops before the holiday rush. Still, Travel Tuesday doesn’t hold a monopoly on holiday ticket sales; millions of transactions pepper the whole holiday season.

But travelers aren’t the only ones who are active during the holidays: fraudsters are too. Travel bookings are uniquely ripe for fraud. For one, they’re intangible, so they don’t have to be picked up or physically resold. They can be purchased instantaneously. Travel bookings are also perishable. As soon as the date of the booking passes, they lose their value. That means once a fraudster cashes out the transaction or uses the fraudulent ticket or room, they’re in the clear.

How does travel fraud work? Many fraudsters book flights or hotels using stolen credit cards or bank account info. In fact, same-day bookings are four times more likely to be fraudulent because the merchant has so little time to screen the purchase for fraud. Other scammers buy expensive travel bookings with stolen account credentials and then try to return them for a cash refund. And some fraudsters pose as travel agencies to scam unsuspecting consumers.

Given the number of people who travel over the holidays, you might be surprised to learn that the fraudiest day of the year for travel isn’t between Thanksgiving and New Year. It’s June 5! But holiday travel fraud is a persistent, far-reaching problem.

Fraudsters who prey on holiday travelers have an advantage. To understand why, it helps to take a look at how fraud analysts operate over the holidays. From Thanksgiving to Christmas, e-commerce sites generally experience three major changes:

• Overall order volume goes up. People are scrambling to buy gifts for their friends and loved ones, so most businesses see an increasingly sharp increase in orders as the holidays draw nearer.
• Customers make more expensive purchases. Americans have been spending more on holiday gifts every year since 2008. This year, consumers are expected to spend a total of $720.89 billion.
• People who usually ship items to their own homes start shipping gifts to friends and family across the country or internationally.

Fraud prevention typically relies on anomaly detection. To find fraudsters, analysts seek and investigate unusual user behavior. The problem, of course, is that over the holidays, the line between good user behavior and suspicious user behavior starts to grow indistinct. For instance, fraudsters often use stolen credentials to spend more than the average customer, often buying multiple tickets to far-off places. But during the holidays, buying expensive tickets, making multiple purchases within a small window of time, and booking vacations on the other side of the world are par for the course.

Fraud analysts are in a double bind. If they adjust their rules or machine learning systems to block more “risky” travel bookings over the holidays, they’ll wind up with a massive false positive rate. The last thing they want to do is alienate stressed-out Christmas shoppers. And many airlines have a high false positive rate as it is: they reject as many as 8-25% of legitimate orders. So, analysts instead adjust their systems to let in more “risky”-looking orders, since many of those orders aren’t actually risky at all. But because overall order volume is so high — and that includes a spike in fraud — they do end up letting in more actual fraud, as well.

It’s an expensive problem for travelers, hotels, and businesses alike. On average, fraudsters steal between $283 and $588 per fraudulent transaction. Airlines alone stand to lose up to $4.8 billion to credit card fraud every year.

As travel continues to boom over the holidays, businesses are making it easier for consumers to book their trips. But a frictionless experience for customers can often mean a frictionless experience for fraudsters. Ultimately, it falls on businesses to secure their users’ trust by ensuring a quick, pleasant, and uneventful shopping experience — especially over the holidays.