I know that double-spending, ddos what else ?
do you plan to make one ?
Hire an professional whitehat hacker if u want to keep it secure.
some Possible attacks and vulnerabilities;
XSS, CSRF, SQL-Injection, no ssl, broken Authentication, misconfiguration, unvalidated redirects and forwards and brutforce attacks.
if you are not into it, you probably produce bad and unsecure code.