In a blog post, fraud researcher Joseph Chan said in May the company had discovered a case where an ecommerce provider used by American and Canadian universities was hit by a group it had named Mirrorthief.

The hotel-booking sites were found this month, and had been injected with JavaScript that loaded a remote script on their payment pages.

"When we first checked the script’s link, it downloaded a normal JavaScript code. However, we found that the same link could also download a different script when we requested it from mobile devices like Android or iOS phones," Chen wrote.

Both sites had been developed by Spanish firm Roomleader and the malicious code was injected via a module called viewHotels which had been provided by the development firm.

"Despite the seemingly small number of affected sites, we still consider the attack significant given that one of the brands has 107 hotels in 14 countries while the other has 73 hotels in 14 countries," Chen said.

Commenting on the incident, Ron Burley, the head of Global Security at cyber security service provider Instart, said: "Web skimming attacks that are being engineered by various Magecart groups, like Mirrorthief are our new reality, and this latest attack on hotel booking sites is a sign that these threats are continuing to grow.

"The biggest problem with WAFs (Web application firewalls) is that they’re ineffective when it comes to today’s Web-skimming attacks because third-party JavaScript runs in the customer’s browser - out of a WAF’s reach.

"The only defence from this type of attack is to prevent all JavaScript from unauthorised access of sensitive data by adopting a zero-trust approach.

"In this environment, the stakes are incredibly high and consumer loyalty is everything - especially when it means protecting sensitive credit card and personal information because they are actively being exploited by vulnerabilities in third-party JavaScript."