Advertising networks have gotten better at spotting malicious downloads embedded into advertisements, so criminals began using redirects, even chains of a dozen redirects or more, to keep their malicious ads from being detected.
Advertising networks wised up to that, as well, so the latest technique is SSL redirects which has proven effective on an AOL-owned network that serves ads in many non-English-speaking countries.
According to security firm Cyphort, these malicious ads have been spotted in the wild in Vietnam, Turkey, Japan, Saudi Arabia and Germany.
Nick Bilogorskiy, Cyphort's director of security research, speculated that the attackers may be deliberately avoiding the U.S. because the top sites are being monitored pretty well.
Sites affected include readms.com, a Japanese manga comics site visited by 280,000 people a month. Another target is bisnis.com, a daily business newspaper in Jakarta visited by 4.7 million people a month. In Vietnam, the attackers have hit phununet.com, the country's 36th most popular site and Vietnam's first social network for women.
The specific ad network affected is AOL-owned adtech.de, which serves clients in 74 countries.
"We reached out to AOL but we see that the malware is still continuing," said Bilogorskiy.
He could not comment on whether the criminals purchase the ad space or hacked into the network.
"All we're seeing is that the ad networks are serving malware, but we have no idea how they're getting in there," he said.
That download typically takes advantage of a zero-day exploit and installs ransomware, Bilogorskiy said.
"There's a flood of these zero days out there right now," he said.
That doesn't mean that every user will be downloading the malware, however -- some might not be running the software or the browser that the exploit targets, he said.
"This is the first time we've seen three SSL redirectors being used," he said. "The goal is to make the chain appear legitimate, and harder to analyze."
He added that the number of victimized websites continues to grow.
"The attack is ongoing as we speak," he said.
It will end either when the attackers end their campaign, or when AOL is able to identify the malicious ads and put a stop to them.
"They serve billions of ads," he said. "It's really hard to check them all."