Almost 6,000 online shops have been compromised by hackers who added specially crafted code that intercepts and steals payment card details.
By March the number of infected shops grew by almost 30 percent to 4,476, and by September, it reached 5,925. More than 750 online stores that were unwillingly skimming payment card details for attackers in 2015 are still doing so today, showing that this type of activity can go undetected for months, the researcher said in a blog post.
De Groot's data suggests there are multiple groups engaged in online skimming. While in 2015, there were variants of the same malware code, today there are three distinct malware families with a total of nine variants.
"The first malware just intercepted pages that had checkout in the URL," the researcher said. "Newer versions also check for popular payment plugins such as Firecheckout, Onestepcheckout, and Paypal."
The malicious code is obfuscated and is deployed using known vulnerabilities in content management solutions or e-commerce software that website owners have failed to patch.
What's worse is that some shop owners don't seem to grasp the seriousness of these issues or understand their impact. De Groot gives some examples of the worst answers he has received from companies when he attempted to inform them about the compromises.
"We don’t care, our payments are handled by a 3rd party payment provider," one unnamed shop owner said.
"Our shop is safe because we use HTTPS," said another.
HTTPS protects against man-in-the-middle attacks, where the attacker is in a position on the network to intercept traffic between a user and a server. However, in this case, the malicious code runs on the server itself and is served over HTTPS, so it can see whatever information users enter into websites.
The good news is that some shop owners are taking action, with 334 stores fixed in a 48-hour period. On the other hand, during the same time period, 170 new stores were hacked.
De Groot has published the list of compromised websites on Github.