Most fintech founders think about security the way most people think about insurance: important, later. The problem is that in payments, 'later' usually means 'after the incident'. By then you're not buying security, you're buying crisis management.
This is the checklist we run against our own fintech builds before they touch real money. None of it requires an enterprise budget. All of it requires deciding, before launch, that trust is part of the product.
1. Treat secrets like they're already leaking
API keys for your payment provider, database credentials, and signing secrets should never live in your codebase: not in a config file, not in a commented-out line, not in the git history. Use environment variables managed by your hosting platform, rotate anything that has ever been committed, and give production keys to as few people as possible.
The most common fintech breach we see in small teams isn't an exotic exploit. It's a live secret sitting in a public or semi-public repository.
2. Verify every webhook, make every payment idempotent
Payment providers like Paystack and Stripe sign their webhooks. Verify that signature on every event, an unverified webhook endpoint is an open invitation to mark unpaid orders as paid.
Idempotency is the other half: networks fail mid-request, users double-click, providers retry. Every money-moving operation needs an idempotency key so a retried request can never charge or credit twice. Build reconciliation in from day one. A daily job that compares your ledger against the provider's records will catch what your logic misses.
3. Collect less KYC data than you think you need
KYC data is toxic inventory: valuable to attackers, expensive to protect, and regulated under Nigeria's data protection regime and its equivalents elsewhere. Collect the minimum your compliance obligations require, encrypt it at rest, restrict who and what can read it, and set a retention policy so you're not guarding data you no longer need.
4. Rate-limit like an adversary is watching
Login endpoints, OTP requests, transfer initiations, and airtime or wallet conversions all get probed. Rate limits per account and per IP, escalating delays on failures, and alerts on anomalous patterns turn an automated attack from a breach into a log entry.
5. Log actions, not just errors
When a dispute or an incident happens, the question is always 'who did what, when?', and application error logs can't answer it. Keep an append-only audit trail for sensitive actions: logins, KYC changes, transfers, admin overrides. Store it separately from application data, and treat admin actions as the most important entries in it.
6. Decide your incident response before you need it
An incident response plan for a small fintech fits on two pages: who gets woken up, how you freeze money movement, how you rotate credentials, what you tell users, and who talks to the payment provider. Write it while you're calm. Practicing it once, even as a tabletop conversation, is worth more than any tooling you'll buy this year.
Security isn't a feature you add at the end. It's a posture you either build with or retrofit at ten times the cost. If you're building something that touches money and want a second pair of eyes on it, that's exactly the work we do.
Building something this applies to?