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updated 2/4/2013 5:17:25 PM ET 2013-02-04T22:17:25

This story was updated at 5 p.m. ET with additional information.

Nope, that wasn't a massive malware attack that Google Chrome and Mozilla Firefox users encountered earlier today (Feb. 4).

"Danger: Malware Ahead!" read a big red warning page that Chrome users saw when trying to access individual pages on many news sites, including this one.

"Google Chrome has blocked access to this page on [site URL]. Content from cmi.netseer.com, a known malware distributor, has been inserted into this Web page. Visiting this page now is very likely to infect your computer with malware."

Mistaken identity

What happened was that the public website of Netseer, an online ad-placement service rather than a "known malware distributor," was itself corrupted with malware, perhaps in an attempt to instigate drive-by downloads.

"This morning at about 5:30am PT, our 3rd party-hosted corporate website (netseer.com) was infected with a malware [sic]," said a Netseer email sent to websites that used the company's ad service. "Consequently, Google added our domain to the list of malware-affected websites.

"Our ad-serving infrastructure is completely different from the corporate website, but shares the same domain (netseer.com). So although the malware never impacted the ad serving, all our ad-serving partners saw Chrome and other browsers flagging malware warnings to users."

In other words, because the Netseer public page was infected, Google blocked all content coming from that domain name, including ads. No actual malware was transmitted across the ad network.

Sites affected by Google's block included those of the New York Times, Washington Post, Los Angeles Times, Huffington Post, Guardian, TheStreet.com, Hollywood Reporter and IMDb.com, among many others.

Friendly fire

Ironically, many of the ads that caused Google Chrome to block the pages were themselves served up by Google ad networks.

Since ads are served up differently every time a single page is loaded, Google didn't block all pages on affected sites, and refreshing pages that were blocked often took care of the problem.

Internet Explorer handles possibly infected pages differently and did not block any Netseer-served pages.

Users of all browsers should nevertheless protect themselves from browser-based malware by having constantly updated anti-virus software, by not clicking on links in unsolicited emails, by disabling Java browser plug-ins and by updating Adobe Flash Player and Adobe Reader only from the Adobe website.

UPDATE: Netseer sent an email to its ad-service users just before 5 p.m. ET Monday (Feb. 4) noting t the page blocks had begun again.

"The malware issue has started occurring again on Chrome as of the last few minutes. We are actively working to resolve it," said the email. "The malware issue seems much larger and is apparently impacting [a] lot of networks today."

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