The hacker discovered that if you place several withdrawals all in practically the same instant, they will get processed at more or less the same time. This will result in a negative balance, but valid insertions into the database, which then get picked up by the withdrawal daemon.
Are you kidding me? Did you do
any research on past Bitcoin exchanges hacks before auditing your code?
That exact same "hack" has been done on multiple exchanges in the past.
Another guy who's created an exchange but yet somehow doesn't know what a database transaction is... unreal.
Indeed, so let's tell him and help him out. He does have the bit most of the other exchange owners were missing, a bit of decency and moral fibre..
Put another way, most people capable of making an exceptional exchange that's technically just right, have neither the will or inclination to run one long term, and the profits involved are too low to warrant doing it.
Probably why you and I both don't have our own exchanges, eh.