"We have no evidence that they were obtained directly from Yahoo’s systems. Our ongoing investigation shows that malicious computer software used the list of usernames and passwords to access Yahoo Mail accounts. The information sought in the attack seems to be names and email addresses from the affected accounts’ most recent sent emails."Yahoo is taking a proactive action and resetting the password of all the impacted accounts. Company is also implementing the Second Sign-in verification to its service. Yahoo is also working with the Federal authorities to find who is responsible for this.
This is not the first time that yahoo system has been compromised. Earlier in July 2012, also it was hacked and login credential of 450,000 users have been stolen.
Yahoo mention that, if your accounts have been affected then you will get the mail and also a SMS text to your mobile if it is linked to your account.