Thousands of Yahoo.com visitors were served with malicious software due to a security breach on the Yahoo! ad network.
Security blog Fox-IT discovered the breach on Friday, writing: "Clients visiting yahoo.com received advertisements served by ads.yahoo.com. Some of the advertisements are malicious."
Fox-IT also estimates that about 9 percent of visitors to the malicious site through a Yahoo! ad were infected by the malware, which is about 27,000 infections per hour. The first sign of infection dates back to Dec. 30.
Good news for U.S. residents, though: None of the attacks seem to have affected Yahoo! visitors in America. Fox-IT says users in Romania, Great Britain and France were most affected.
Yahoo! responded to the security concerns in a statement, writing: "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."
Yahoo! is just the latest major company to suffer a severe security breach.
In December last year, Target suffered the second largest heist of credit and debit card data in the country, affecting more than 40 million Target customers.
And Snapchat suffered a security breach where 4.6 million of its users had their usernames and phone numbers leaked online.
As for Yahoo!'s breach, The Washington Post blames Java programming.
"The fact that the malware targeted flaws in the Java programming environment is an important reminder that the software has become a security menace."
According to Fox-IT, since Yahoo! discovered the problem on Friday the infections of malware to Yahoo! users have tapered off.