Yahoo said it has been the target of a security attack aimed at Yahoo Mail users. Yahoo identified a coordinated effort to gain control of email accounts. Yahoo did not specify how many users or accounts were affected.

The Yahoo Mail usernames and passwords used in the hack were likely collected from a compromised third-party database, the company said in a blog post. Yahoo is resetting passwords of affected accounts and using two step authentication, including SMS notifications to to further secure accounts. 

The company said it is working with federal law enforcement to determine who was responsible for the hack.