Yahoo Mail was hacked. Details are sketchy in terms of just how many Yahoo Mail accounts have been compromised. Yahoo suggests that the attackers most likely gained access to the data through a third-party database outside of Yahoo control. Regardless of how the compromise occurred, there is a lesson to be learned here…again.
Hacks happen, but If you’ve followed basic security practices and aren’t using the same login credentials for multiple sites and services, a compromised Yahoo Mail shouldn’t put anything at risk other than your Yahoo Mail account.