Symantec, a security company, found flaws on hundreds of hotel websites, which were leaking sensitive information like names, phone numbers, passport numbers and addresses in confirmation emails.
Candid Wueest, a threat researcher at Symantec, said he looked at more than 1,500 hotel websites in 54 countries and found the issues among two-thirds of them.
Hotels are a primary target for cyberattacks, as they hold treasure troves of data on guests during vacation season.
They are frequently hacked, as cyberattacks on Sheraton, Westin, Starwood, Marriott and Wyndham hotels over the last few years show.
Last November, Marriott disclosed that hackers had stolen records from up to 383 million guests in one of the largest personal data breaches in history.
One major issue stems from the URL that they send to guests in emails.