Matt Krantz and Beth Belton

USA TODAY

Yahoo ads infected with malware

Users who click on ads are redirected to infected site

Yahoo working to fix problem

Visitors to Yahoo's Web pages who viewed ads the past few days are potentially at risk of having their computers infected by malware, according to published reports Sunday.

Fox IT -- an Internet security firm that discovered the alleged malware infection -- says 300,000 users were visiting the infected ads every hour. That means roughly 27,000 computers and devices were being infected every hour since typically 9% of computers are actually infected after visiting the site. Most computer users either use software that combats such infections or may have configured their computers to be resistant to the attacks.

Malware, short for malicious software, is software used to disrupt computer operation, gather sensitive information, or gain access to private computer systems. Computers connected to a network can spread the malware onto many more computers. The malware may have started spreading on Dec. 30.

This is just the latest technical problem to hit the struggling Yahoo as it attempts to become more relevant as online services proliferate. The company's e-mail service experienced widespread outages and problems in late December.

Consumers should know that this Yahoo malware attack works by redirecting visitors on Yahoo's pages to an infected site, which then uses security holes in Oracle's Java to install malware. Java is a commonly used "plug in" designed to add additional computational capability to Internet browsers.

The infected site proceeds to install a variety of malware to the user's device including those called ZeuS, Andromeda, Dorkbot, Tinba or Necurs, Fox IT says. Most of the users affected have been in Great Britain, France and Romania.

In a statement, Yahoo said it is aware of the security issues. "On Friday, January 3 on our European sites, we served some advertisements that did not meet our editorial guidelines, specifically they spread malware. We promptly removed these advertisements. Users in North America, Asia Pacific and Latin America were not served these advertisements and were not affected," according to an updated Yahoo statement released Sunday evening. The infection rate has declined significantly, indicating that Yahoo is making adjustments to fix the problem, Fox IT says.

Computer users can protect themselves from this and other similar attacks. Perhaps the easiest form of defense is turning off the Java plug in, which is commonly installed in most browsers including Internet Explorer, Chrome, Firefox and Safari. Internet Explorer users can easily turn off Java by clicking on the icon that looks like a gear in the upper right-hand corner of the screen and selecting "Manage add-ons." Under the "Add-on Types," look for a section titled Oracle America, Inc. Right-click on any entry that starts with the word Java, and choose Disable. Oracle took ownership of Java following its acquisition of Sun Microsystems closed in 2010.

Any computer users concerned they might be infected should run a scan of their computer. Microsoft provides a free scanning tool called the Microsoft Safety Scanner that is able to detect and remove most malware.