Security researchers have discovered hundreds of vulnerabilities across major hotel and airline and travel booking websites, some of which have already suffered major breaches.

UK-based consumer rights group Which? and tech consultancy 6point6 studied 98 travel sector companies, probing websites, subdomains, employee portals and other web properties with lawful online tools.

They found Marriott-owned websites were riddled with 497 bugs including over 100 assessed to be “high” (96) or “critical” (18). Some of these could have allowed an attacker to target users and their data, Which? said.

“We reported our findings directly to Marriott (as we did with all the five providers in our snapshot test) and it said that it had ‘no reason to believe’ that its customer systems or data had been compromised,” Which? explained.

“It also claimed that some findings were ‘not attributable to Marriott,’ while others ‘could not be validated.’ It didn’t supply any specific examples of mitigations, but said that it would be ‘taking a closer look at and addressing Which?’s findings’.”

Marriott is facing a large fine from regulator the Information Commissioner’s Office (ICO) after last year revealing a historic breach of 339 million customers’ data.

Airline easyJet, which this year revealed a breach affecting nine million customers, was found to have 222 vulnerabilities across nine web domains, including one critical bug that could allow an attacker to hijack users’ browsing sessions.

The firm apparently took three domains offline and remediated the disclosed vulnerabilities on the other six sites.

British Airways was found to have 115 vulnerabilities on its websites including 12 judged to be critical. Although most of the issues identified were thought to be related to running old versions of software, the carrier gave no indication in its response to Which? that they would be updated.

BA famously exposed the details of around 500,000 customers to Magecart attackers last year, in an incident which could also land it a major fine from the ICO.

Elsewhere there were 291 potential vulnerabilities found at American Airlines, and a critical vulnerability at which could allow attackers to create fake log-in accounts.

“Our research suggests that Marriott, British Airways and easyJet have failed to learn lessons from previous data breaches and are leaving their customers exposed to opportunistic cyber-criminals,” argued Which? Travel editor, Rory Boland.

“Travel companies must up their game and better protect their customers from cyber-threats, otherwise the ICO must be prepared to step in with punitive action, including heavy fines that are actually enforced.”