You'll never think of everything a hacker might try, or rather it is safer to assume so. My advice is to make sure the project has a properly-planned alpha phase where in-house testers follow a test plan and perhaps even deliberately hack the service before it goes on general release. This is not just to tackle security but other aspects of a business critical release. And after that an additional beta phase can check the satisfaction of a small group of selected customers whose feedback would be even more valuable.

Such lifecycle decisions are not normally taken by programmers, but it covers your back if you at least recommended it.