Internet giant Yahoo said it identified a "coordinated effort" to hack its email service, intensifying fears about cyber security.
The company's ongoing investigation into the matter showed that malicious computer software used the list of usernames and passwords to access Yahoo Mail accounts.
"Based on our current findings, the list of usernames and passwords that were used to execute the attack was likely collected from a third-party database compromise," the company said in a statement.
"We have no evidence that they were obtained directly from Yahoo's systems."
The hackers seem to access names and email addresses from the affected accounts' most recent sent emails, Yahoo added.
In remedial measures, the company prompted its users to reset passwords on impacted accounts. In addition, it is using second sign-in verification to allow users to re-secure their accounts.
Yahoo said it has implemented additional measures to block attacks against Yahoo's systems and is working with federal law enforcement to find and prosecute the hackers.
Using Same Password for Different Internet Services
People tend to use same passwords for multiple internet services, enabling hackers to successfully use credentials obtained from one hacking on various sites.
Yahoo claimed it became victim of such an attack.
"In addition to adopting better password practices by changing your password regularly and using different variations of symbols and characters, users should never use the same password on multiple sites or services," the company advised.
"Using the same password on multiple sites or services makes users particularly vulnerable to these types of attacks."
The company regretted the hacking and assured its users that it takes the security of their data very seriously.
The issue comes days after Yahoo CEO Marissa Mayer praised company divisions such as Yahoo Mail and Flickr as base for its revenue growth, despite a 6% decline in revenues compared to last year.
Earlier in December, Yahoo's e-mail service went down for several days, affecting about 1 million users of the service.
With about 273 million accounts worldwide, Yahoo is the second-largest email service after Google's Gmail, according to the research firm comScore.