Yahoo users have been hit by one of the biggest malware attacks infecting 27,000 users per hour due to malicious advertisement on Yahoo's European websites.

After a dreadful end to 2013, Yahoo has not got off on a very good note in 2014. Yahoo was a part of last year's major hack attack that left over 2 million passwords from various social media sites exposed to the hackers. Yahoo accounted for 60,000 passwords that were stolen in the attack, earlier last month. Yahoo Mail users had an awful long week of no service last month, which was termed as "scheduled maintenance." Following these troubles, Yahoo has made the headlines for yet another disastrous malware attack.

Fox-IT, a Netherlands-based computer security firm that focuses on cyber defense, wrote in a blog that Yahoo's European websites were exposed to malicious software via its advertisement base, affecting thousands of users. The attack was limited to European countries, with maximum impact on users from Romania, Great Britain and France. Users in Asia Pacific, North America and Latin America along with Mac and mobile users were not affected by the attack, Yahoo said.

Fox-IT listed some of the malware, namely ZeuS, Andromeda, Dorkbot/Ngrbot, Advertisement clicking malware,Tinba/Zusy, Necurs that were included in the "exploit kit" released by the Yahoo servers. Malware is software that collects sensitive information stored on a computer, disrupts the computer's operations and helps gain access to home networks.

The cyber security firm estimated the attack affected 27,000 users per hour, at a typical infection rate of 9 percent.

"It is unclear which specific group is behind this attack, but the attackers are clearly financially motivated and seem to offer services to other actors," Fox-IT wrote in the January 3 blog post. "The exploit kit bears similarities to the one used in the brief infection of php.net in October 2013."

Yahoo said it is aware of the attack and is taking necessary precautions for a minimal impact.

"At Yahoo, we take the safety and privacy of our users seriously," it said in a statement Sunday night, CNN reports. "On Friday ... on our European sites, we served some advertisements that did not meet our editorial guidelines, specifically they spread malware. We promptly removed these advertisements.