We really should have learned by now. Using web-based email is convenient, easy and free but it is also risky. Yahoo! Mail got hacked a few days ago and the accounts of over 450,000 accounts were compromised. These accounts (and their log in credentials, i.e. passwords, were also published online.
For people who rely on their Yahoo! mail this is terrible news. To check if your account has been compromised, check out Sucuri Labs's checker or even better just go and change your password already, this time make it secure. These recent webmail breaches are serious and should make people consider getting their personal or business email handled by more secured services. Below is Yahoo's official statement on the matter.
At Yahoo we take security very seriously and invest heavily in protective measures to ensure the security of our users and their data across all our products. We confirm that an older file from Yahoo Contributor Network (previously Associated Content) containing approximately 450,000 Yahoo and other company users names and passwords was compromised yesterday, July 11. Of these, less than 5 percent of the Yahoo accounts had valid passwords. We are taking immediate action by fixing the vulnerability that led to the disclosure of this data, changing the passwords of the affected Yahoo users and notifying the companies whose users' accounts may have been compromised. We apologize to all affected users. We encourage users to change their passwords on a regular basis and also familiarize themselves with our online safety tips at security.yahoo.com.