Newegg is clearing up its website after a month-long data breach.

Hackers injected 15 lines of card skimming code on the online retailer’s payments page which remained for more than a month between August 14 and September 18, Yonathan Klijnsma, a threat researcher at RiskIQ, told TechCrunch. The code siphoned off credit card data from unsuspecting customers to a server controlled by the hackers with a similar domain name — likely to avoid detection. The server even used an HTTPS certificate to blend in.

The code also worked for both desktop and mobile customers — though it’s unclear if mobile customers are affected.

The online electronics retailer removed the code on Tuesday after it was contacted by incident response firm Volexity, which first discovered the card skimming malware and reported its findings.

Newegg is one of the largest retailers in the US, making $2.65 billion in revenue in 2016. The company touts more than 45 million monthly unique visitors, but it’s not known precisely how many customers completed transactions during the period.

In an email to customers, Newegg chief executive Danny Lee said the company has “not yet determined which customer accounts may have been affected.” When reached, a Newegg spokesperson did not immediately comment.

Klijnsma called the incident “another well-disguised attack” that looked near-identical to the recent British Airways credit card breach, and earlier, the Ticketmaster breach. Like that breach, RiskIQ attributed the Newegg credit card theft to the Magecart group, a collective of hackers that carry out targeted attacks against vulnerable websites.

The code used in both skimming attacks was near identical, according to the research.

“The breach of Newegg shows the true extent of Magecart operators’ reach,” said Klijnsma. “These attacks are not confined to certain geolocations or specific industries—any organization that processes payments online is a target.”

Like previous card skimming campaigns, he said that the hackers “integrated with the victim’s payment system and blended with the infrastructure and stayed there as long as possible.”

Anyone who entered their credit card data during the period should immediately contact their banks.