Attackers who have slipped malicious advertisements onto major websites over the last month have potentially compromised large numbers of computers.
Several security vendors have documented attacks involving malicious advertisements, which automatically redirect victims to other websites or pages that silently attack their computer and install malware.
"We certainly see malvertising on the rise," said Nick Bilogorskiy, head of security research at Cyphort, a security vendor in Santa Clara, California. "We see it is going to be a major channel of delivering malware this year."
For the second time in about a month, Cyphort found malicious advertisements popping up on major websites including the Huffington Post and the LA Weekly between Thursday and Monday. The attack is likely a continuation of the first one, Bilogorskiy said.
The malvertisements were distributed by Adtech.de, an AOL-owned online advertising company, and two other companies, adxpansion.com and Ad.directrev.com. The bad ads appear to have been removed from Adtech, Bilogorskiy said, who has been in touch with its security team. He couldn't reach the other two companies.
The malicious advertisements redirected users through several domains before finally dumping them on pages hosting an exploit kit, an attack tool that scans for software vulnerabilities. It appears this campaign uses the Sweet Orange exploit kit, Bilogorskiy said.
If a vulnerability is found, malware is automatically delivered, a dangerous type of attack known as a drive-by download. "It's the worst case," Bilogorskiy said.
The malware installed is called Kovter, which is used to fraudulently generate ad impressions.
It can be difficult for online advertising companies to keep bad ads out of their systems. The companies "are getting millions of ads submitted to them, and any one of them could be malware," Bilogorskiy said.
"They try their best to detect and filter, but it is challenging," he said.
Attackers, for example, may enable malicious payloads after their ads have been approved. Other times, they may only attack every 10th user. The ads, Bilogorskiy said, have to be repeatedly checked to ensure they're not malicious.
On Tuesday, Cisco's Talos security research group wrote it had analyzed another large malvertising campaign that uses the Angler exploit kit, a potent one known for its quick employment of the latest Flash vulnerabilities.
More than 1,800 legitimate domains were being used as part of that campaign, wrote Nick Biasini, a Cisco threat researcher. It appeared the attackers had gained control of the domains' accounts, many of which were registered through GoDaddy, he wrote.
The attackers created subdomains on those accounts. People who viewed a malicious ad were redirected to a newly-created subdomain, which then redirected to another subdomain that served up the exploit kit.
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