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.
But everybody has violated this practice it at some point. It's just so much easier to remember your login credentials if you use the same email address and password for everything. The problem is that the bad guys know you're probably doing it.
"Diversifying your passwords for each account is essential to protecting all of your online information," says W. Hord Tipton, executive director of (ISC)2. "Once a password has been stolen, hackers often attempt to access multiple accounts; compounding the potential damage."
Tipton recommends users follow established practices for choosing strong passwords--use at least eight characters with a combination of upper and lowercase letters, numbers, and special characters. Don't use words that can be found in the dictionary--not even if you replace the letter "e" with a "3," or the letter "a" with an "@" symbol. You should change your passwords every 60 to 90 days, and you shouldn't re-use previous passwords since that undermines the point of changing your passwords.
The most important element, though, when it comes to safeguarding your larger online identity is make sure you use different usernames and passwords for all of the various sites and services you log into. Using the same password everywhere is like having one key that unlocks your house, your car, your safety deposit box, your desk drawer at work, and your locker at the gym. They each have different keys for a reason.