Authy founder here (we do Two-Factor Auth for Coinbase).

Looks like you didn't have Two-Factor enabled https://news.ycombinator.com/item?id=6947037). Enable it now. We've stopped lots of Coinbase account password compromises. Most of the time we see that the e-mail was hacked.

Do the following:

1. Enable Two-Factor Authentication on your e-mail.

2. If you use GMail, go to Settings -> Forwarding POP/Imap. Check that no "weird" addresses are added to your account.

3. Change your E-mail password.

4. Change your Coinbase password.

If you have Two-Factor enabled we can also temporarily block your account if you suspect a hacker is trying to get into it. Contact us at support@authy.com and we'll block it.