I was in a similar situation a few years ago. It involved write access to other people's brokerage accounts.
FINRA, SEC, and FBI are all good points of contact and they have a straightforward complaint/action process. Assuming that you mailed a letter to the CEO first. Otherwise, I just now post live exploits to my blog at http://privacylog.blogspot.com... and usually give the vendor a heads up.
You will not get credit for the find. The TLAs will not invite you to give a speech. You will not get a career out of this, or even consulting money. Your end game is getting the thing fixed and moving on. Do this by posting your story which proves how innocent you are and giving the people an honest chance to fix it. Imagine you are in front of a jury of idiots. If you are saying "I wrote down this URL, then I typed it back in and some else's bank records came up... then I found out I made a typo". This is a perfectly reasonable story, there is nothing to be afraid of.