Enlarge (credit: Malwarebytes)

An ongoing operation that’s installing ransomware and other malware on the computers of unsuspecting website visitors is one of the most potent drive-by attack campaigns researchers have seen in recent memory.

The attacks install three pieces of malware using an exploit kit called GreenFlash Sundown, which researchers identified in 2015 and have continued to follow since. Attacks in recent weeks have spiked again as ShadowGate—one of the names given to the hacker group behind the campaign—has unleashed a highly revamped version of the exploit kit on hacked ad servers run by Web publishers. The most notable compromise is of an ad server belonging to onlinevideoconverter[.]com, a site with more than 200 million visitors per month that converts YouTube videos into video files that can be stored on a computer hard drive.

“They are ongoing and with a scale we haven’t seen in a couple of years when it comes to exploit kit-related attacks,” Jérôme Segura, a Malwarebytes researcher tracking the campaign, said of the attacks on onlinevideoconverter[.]com visitors. “We literally noticed a huge spike in our telemetry starting a few days ago, which is very unusual. Given what we see in our telemetry, this is the most successful drive-by campaign we have seen in quite a while, so we can infer many people were affected by it.”