Yahoo was one of more than 30 companies hit by a sophisticated online attack from China, designed to steal intellectual property and collect information on Chinese dissidents, several news sources have reported.
The news follows revelations Tuesday that Silicon Valley giants Google and Adobe had also been targeted by the hackers, who are thought to be from China. According to sources familiar with the matter, 34 companies in total have been affected - and more names are expected to come to light in the next few days.
Google has threatened to effectively pull its business out of China, in part because of the incident.
Bloomberg and other news outlets named Yahoo as a victim Thursday, citing unnamed sources familiar with the situation.
Yahoo sold off its China business in 2005, but is a major shareholder in Alibaba.com, the company that now runs Yahoo China. The company did not respond to requests for comment Thursday, but had previously declined to comment on whether it had been hacked.
On Tuesday, the company called the Google hack "deeply disturbing".
"Yahoo is committed to protecting human rights and takes our users' privacy and security very seriously," the company said in a statement. "We condemn any attempts to infiltrate company networks to obtain user information."
Because Google reported that the hackers had sought information on Gmail users, security experts had said that companies such as Yahoo and Microsoft, which operate web-based e-mail services, were also likely targets.
The attackers, who also tried to steal valuable intellectual property from their victims, were extremely sophisticated, according to sources familiar with the situation. On Thursday, antivirus vendor McAfee disclosed that the hackers used a previously unknown 'zero-day' flaw in Internet Explorer to break into some companies.
"Our investigation has shown that Internet Explorer is vulnerable on all of Microsoft's most recent operating system releases, including Windows 7," McAfee said in a note on its Web site. "Microsoft has been working with us on this matter and we thank them for their collaboration."