Both the United Airlines and American Airlines confirmed that some of their customers' login credentials were used to book flights and upgrade travels without their authorization. The said incidents occurred in late December.

United Airlines spokesman Luke Punzenberger told the Associated Press that about three dozen loyalty accounts were compromised, while American Airlines spokeswoman Martha Thomas said that 10,000 accounts were affected on their end. Customers were notified through email on Monday and were reminded to not use the same login credentials on more than one site.

The American Airlines have suspended most of the affected accounts and advised customers to create a new one. The United Airlines, on the other hand, said that the company would shoulder the credit watch service of the affected customers for a year.

Both airlines clarified that no credit card information was compromised during the hack.

Hackers have started targeting the travel industry and its mileage program since some airlines allow their customers to use the miles to pay for the flight, car rent, dining and shopping, according to the ComputerWorld.

The airlines affected added that the hackers were able to get into their systems by acquiring credentials from a third-party and not directly from their server. They also presume that hackers tried to use the credentials in the site hoping that they would match and inadvertently discovered that they did.