Yahoo's advertising servers have been distributing malware to hundreds of thousands of users over the last few days.
The attack appears to be the work of malicious parties who have hacked Yahoo's advertising network.
Fox IT, a security firm based in the Netherlands, wrote a blog post on Friday describing the problem.
According to the Washington Post, the firm said that clients visiting yahoo.com received advertisements served by ads.yahoo.com, adding that some of the advertisements are malicious.
The firm said that instead of serving ordinary ads, Yahoo's servers reportedly sends users an 'exploit kit' that 'exploits vulnerabilities in Java and installs a host of different malware,' the report added.
Fox IT said Yahoo users have been getting infected since at least December 30.