Yahoo had some fun poking fun at Gmail when it went down last week, but it looks like karma gives quite a punch herself because the just got attacked. They took to Tumblr to acknowledge a recent mass of security attacks with the vaguest details possible.

Attacks on Yahoo mail accounts are becoming a "regular occurrence".
Recently, we identified a coordinated effort to gain unauthorized access to Yahoo Mail accounts. Upon discovery, we took immediate action to protect our users, prompting them to reset passwords on impacted accounts.
Yahoo claims that usernames and passwords that may have been compromised came from some "third-party database" and it "has no evidence that they were obtained directly from Yahoo's systems". The attackers, it seems, were looking to acquire the names and email addresses of the affected accounts' most recent sent messages.

If your account was affected, Yahoo should be resetting your password, and you'll be prompted to change it the next time you log in. [Yahoo]