A server at Minecraft.net handling username verifications was hacked as of December 1st.
It was unknown if any account details had been leaked, but two lists of usernames and passwords were later found being shared on 4chan. Each list contains over 20,000 usernames and many thousand accounts have been stolen already.
If you have an account on minecraft.net, I strongly advise you to see for yourself if your username and password is on the list. If you discover it on the list, change your password ASAP![link]
Please share this post with every Minecraft player you know. This must reach out before more accounts are stolen.