Yahoo quickly took action on all known security breaches from 3rd party sites, by resetting passwords.  When any security breach takes place it is beneficial for all users to reset passwords

QUOTE: Users of Yahoo mail should be aware of a security issue with the email service. Yahoo acknowledged that it has identified a “coordinated effort” by some person or group to gain “unauthorized access” to accounts. The company didn’t state how many accounts were affected, but it turns out that those impacted have been prompted to reset their passwords.

Yahoo was clear to state that it has no evidence that the attempted hacking attack came as a result of its systems being compromised. It turns out that malicious computer software was the culprit that helped someone obtain names and email addresses from affected accounts’ most recent sent emails. The company said that it was likely from a third-party database that featured the list of usernames and passwords.  So what’s being done? Yahoo is resetting passwords and implementing second sign-in verification. The company is also working with federal law enforcement agencies to find out who is responsible. Additional measures are also being implemented to help secure Yahoo’s systems.