Yahoo Mail was recently the target of a cyber-attack, the company revealed in a blog post Thursday.
Yahoo said it recently identified a coordinated effort by hackers who tried to log into many email accounts with stolen usernames and passwords. The note by Yahoo products executive Jay Rossiter did not immediately say how many accounts were affected.
The company declined to comment further but said it has teamed up with federal law enforcement to investigate the attack.
The credentials were likely taken from a third-party database, Yahoo said.
Yahoo said it reset passwords for users who were impacted. The company sent them text messages Wednesday night warning of "unusual activity on the network."