The New York Times has warned readers to be wary of ads for antivirus software on its website.

The so-called rogue antivirus products that have been spotted on the website, often promoted by Eastern European criminal organisations, are either ineffective or actually end up infecting the computers of people who purchase them.

"Some readers have seen a pop-up box warning them about a virus and directing them to a site that claims to offer antivirus software," the Times said in a 'Note to Readers' on its website.

"We believe this was generated by an unauthorised advertisement and are working to prevent the problem from recurring." The newspaper did not respond to a request for more information on the issue.

Because online advertisements are usually sold through networks, sites such as often have to rely on other companies to make sure that the ads they carry are appropriate.

Blogger Troy Davis was hit with the ad on Saturday night. After taking a closer look, he discovered that JavaScript code in a New York Times ad redirected him to a website that popped up a browser Window designed to look like it is conducting a scan of the system. The window warns, ‘Your computer is infected'.

"It's a fake page for a non-existent antivirus app, which is actually malware," Davis wrote in his analysis of the issue.

In the past three months, rogue antivirus software has emerged as a major online problem, according to Paul Ferguson, a researcher with antivirus vendor Trend Micro. "It's pervasive," he said in an instant message interview. "Right now, they are going full-tilt."

Criminals use a variety of tricks to get people to shell out for the bogus products: They use search engine optimisation techniques to get search engines like Google to list websites that display the pop-up ads, or they'll flog them through social media sites like Twitter or Facebook . They even use malicious Trojan horse programs to pop up error messages in hopes that people will buy.

"It's a multimillion dollar business," Ferguson said.

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See also: Is your PC botnet infested?