Updated at 5:50 p.m. PDT September 14 with explanation from The New York Times.
The New York Times' Web site is grappling with problems created by an "unauthorized advertisement," but it is unknown how the ads managed to appear on the site and whether the site had been compromised.
The rogue ad warns readers that their computer may be infected with a virus and redirects them to a site that purports to offer antivirus software, according to a note posted to the newspaper's Media & Advertising section:
Some NYTimes.com readers have seen a pop-up box warning them about a virus and directing them to a site that claims to offer antivirus software. We believe this was generated by an unauthorized advertisement and are working to prevent the problem from recurring. If you see such a warning, we suggest that you not click on it. Instead, quit and restart your Web browser.
The site, best-antivirus03.com, is a so-called hijacker that uses fraudulent strategies to promote fake security software, according to security site GeekPolice.net.
One CNET reader described how the pop-up ad essentially hijacked his browser, preventing him from navigating away from the site.
"They took me to an 'antivirus site,' which kept attempting to scan my computer and install software. Using the back button kept reloading the virus page," the reader said. "It was not possible to close the page, necessitating a force quit."
Update with explanation from The New York Times:
The New York Times said the offending ad was provided by someone posing as a national advertiser with a legitimate-looking advertising product. Over the weekend that ad being served up was swapped out so that the offending ad would appear, the Times said.
"As soon as we were made aware of the situation, we took aggressive steps, suspending all third-party advertisements on the site," Diane McNulty, executive director of Community Affairs and Media Relations, said in a statement. "We now know how it occurred and have taken steps to prevent a similar situation from happening."