(CNN) -- A malware attack hit Yahoo's advertising server over the last few days, affecting thousands of users in various countries, an Internet security firm said.
In a blog post, Fox-IT said Yahoo's servers were releasing an "exploit kit" that exploited vulnerabilities in Java and installed malware.
"Clients visiting yahoo.com received advertisements served by ads.yahoo.com," the internet security firm said. "Some of the advertisements are malicious."
Fox-IT, which is based in the Netherlands, focuses on cyber defense.
It estimates tens of thousands of users were affected per hour.
"Given a typical infection rate of 9%, this would result in around 27,000 infections every hour," the company said. "Based on the same sample, the countries most affected by the exploit kit are Romania, Great Britain and France. At this time it's unclear why those countries are most affected, it is likely due to the configuration of the malicious advertisements on Yahoo."
If a computer infected with malware is connected to a network, attackers can often access other connected systems and servers.
Yahoo said it is aware of the security issues.
"At Yahoo, we take the safety and privacy of our users seriously," it said in a statement Saturday night. "We recently identified an ad designed to spread malware to some of our users. We immediately removed it and will continue to monitor and block any ads being used for this activity."
The security firm said it is unclear who's behind the attack, but it appears to be "financially motivated." It did not provide details.
Its investigation revealed that the earliest sign of infection was on December 30.