"This is the largest such attack known to date," said researchers from Cisco in a blog on Monday.
In the attack, victims are emailed an alert that contains a link with a fictitious social media contact request. As of Monday, these messages accounted for as much as 24 percent of all spam sent within a 15-minute interval, according to Cisco.
Clicking the link takes victims to a Web page that says, "PLEASE WAITING.... 4 SECONDS," and redirects them to Google. During those four seconds, the victim's PC is infected with the Zeus data-theft malware via a drive-by download, the Cisco researchers said. Zeus embeds itself in the victim's Web browser and captures personal information, such as online banking credentials, and is widely used by criminals to pilfer commercial bank accounts.
This is not the first time criminals have subverted brands associated with online social media, Cisco noted. The criminals controlling the Cutwail botnet routinely send email messages impersonating major social networks and governmental organizations.
"What makes this attack unique is the combination of the extremely high volume of messages transmitted, the focus on business users, and the use of the ZeuS data-theft malware," Cisco said. "This strongly suggests that the criminals behind this attack are most interested in employees with access to financial systems and online commercial bank accounts."
"The LinkedIn attacks are just a new medium for the same attackers," says Matt Jonkman, founder of Emerging Threats. "They go where the people are, whether that's Windows, Facebook, LinkedIn, or dating sites. One shift here was the size of the attack.
"Generally, a spam/phishing run will be on a smaller scale and changed often," Jonkman says. "This attack was a one-shot, large-scale run, likely because the attackers knew LinkedIn would respond quickly, so they would have just one very effective attack. The best advice to give to the end user is this: Be very skeptical of everything you get, especially if it's either unexpected or out of the ordinary for that service."
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