All though Yahoo did not confirmed The Name of the third part app which was compromised neither they said anything on the numbers of users effected by the breach.
In a statement made by the Company on their blog, They confirmed the attack Made by the Hackers on a third part app,
“Recently, we identified a coordinated effort to gain unauthorized access to Yahoo Mail accounts. Upon discovery, we took immediate action to protect our users, prompting them to reset passwords on impacted 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. We have no evidence that they were obtained directly from Yahoo’s systems.” the company said in the statement.
The company is already taking security measures, and have notified the effected users to reset their password, via mail or Mobile SMS.
Moreover Yahoo is working with federal law enforcement agencies, and is investing to find the culprits behind the breach. Additional measures to block attack over yahoo systems have already been implied.
If you are one of the effected user and is also using the same set of username and password for other accounts then in addition to resetting Your yahoo mail password you should also change passwords of other accounts too.