(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.”

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