Google on Tuesday hauled out a tool it last used nearly a year ago to warn users infected with the "DNSChanger" malware.
Starting Tuesday, special messages will be displayed at the top of a Google search results page to people whose Windows PCs and Macs have been infected with malicious code that hijacks their clicks.
"Our goal with this notification is to raise awareness of DNSChanger among affected users," said Damian Menscher, a Google security engineer, in a post to a company blog. "We believe directly messaging affected users on a trusted site and in their preferred language will produce the best possible results."
One security expert appreciated Google's effort.
"Let's face it, Google is basically a central piece of infrastructure now, and as such they have a responsibility to do their part to keep the pipes clean," said Andrew Storms, director of security operations at nCircle Security.
DNSChanger silently redirects clicks by modifying victimized computers' domain name system (DNS) settings. The users are sent to hacker-created websites that resemble the real domains.
At its peak, DNSChanger infected more than four million Windows PCs and Macs, a situation that led to a major botnet takedown last November organized by the U.S. Department of Justice.
As part of the "Operation Ghost Click" takedown, the FBI seized more than 100 command-and-control (C&C) servers hosted at U.S. data centers. To replace those servers, a federal judge approved a plan where substitute DNS servers were deployed by the Internet Systems Consortium (ISC), the non-profit group that maintains the popular BIND DNS open-source software.
Without the server substitutions, DNS Changer-infected systems would have been immediately severed from the Internet.
Originally, the substitute servers were to be turned off March 8, but the judge extended the deadline to July 8.
Although Internet service providers (ISPs) have been notifying infected users, there are still an estimated 500,000 machines harboring the software, Google said. Security company KindSight recently estimated that 0.25% of all household PCs were infected with DNSChanger.
Google has begun putting this warning at the top of its search results when it detects a PC or Mac infected with DNSChanger. Unless the malware is eradicated, those computers will be cut off from the Web in July.
Most of those computers will lose their Internet connection in seven weeks unless DNSChanger is scrubbed from their hard drives.
Google knows which computers are infected with DNSChanger because it instructs the servers now maintained by the ISC to point affected users to a unique IP address when they do a search. Because only computers plagued with the malware are being served DNS (domain name system) requests by the ISC, only infected PCs and Macs display the message.
That message -- "Your computer appears to be infected" -- also includes a link to a page on Google's own Help site, where users will find more information about the threat and download links to several free tools that will remove it.
Google first used the messaging tactic in July 2011 when it warned customers whose systems were infected with fake antivirus software, often dubbed "scareware. In that instance, Google became suspicious when it uncovered "unusual search traffic" while doing maintenance at one of its data centers.
At the time, some experts questioned the tactic, pointing out that security warnings have been a hacker ploy for ages.
Storms sympathized with Google's spot.
"It's like they're damned if they do, damned if they don't," he said. "Sure a message like this could be falsified, but frankly, that could happen just about anywhere."
Gregg Keizer covers Microsoft, security issues, Apple, Web browsers and general technology breaking news for Computerworld. Follow Gregg on Twitter at @gkeizer, on Google+ or subscribe to Gregg's RSS feed. His email address is [email protected].
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