I don't know what kind of damage you can cause with this information, but I do think that posting this publicly was pretty unethical. You should have contacted Yahoo and then disclosed the information after.
EDIT: And here comes the downvotes. If you disagree with what I said then you really should post a response because you're preventing me from understanding why I might be wrong. If I knew your house door was unlocked, would it be okay for me to tell everyone in the world before I let you know?
EDIT: And here comes the downvotes. If you disagree with what I said then you really should post a response because you're preventing me from understanding why I might be wrong. If I knew your house door was unlocked, would it be okay for me to tell everyone in the world before I let you know?