Application security testing for fintech: what to test, when to test it, and what a real test finds

Akande Simpa Lead Security Engineer & Cofounder, Simpa Labs Comprehensive Security Partner (CSP)

In the Nigerian fintech ecosystem, the stakes for application security are brutally high. If your mobile app has a broken authorization flow, you aren't just leaking an email address—you are allowing attackers to drain wallets, spoof KYC documents, or intercept settlement webhooks from Paystack or Flutterwave.

The market is flooded with massive consulting firms offering "security audits." What they actually deliver is an automated PDF from a generic vulnerability scanner. Scanners cannot understand the business logic of a peer-to-peer lending app. Scanners cannot detect when an API allows a user to withdraw NGN 50,000 from a wallet containing NGN 500. For fintech, automated scanning is not security. It is compliance theatre.

Real application security testing requires manual, adversarial engineering. It requires ripping apart compiled mobile binaries, intercepting API traffic, and intentionally breaking the complex business logic that governs your payment flows.

What application security testing covers for fintech

Application security testing is the umbrella that covers everything a security practitioner does to find exploitable vulnerabilities in your product. For a fintech application, that includes:

The testing priorities for different fintech stages

A pre-seed startup building an MVP does not need the same security testing as a Tier-1 microfinance bank processing billions of Naira daily. Testing must be scoped to your current risk profile.

Pre-launch

Focus on the highest-risk flows: payment processing, user authentication, and API authorisation. A focused test of your core money-moving functionality is more valuable than a broad shallow scan at this stage. Fix the issues that could cause financial loss or data exposure on day one.

At this stage, you should not be wasting budget on exhaustive infrastructure audits if your core API allows unauthenticated password resets. Your priority is to ensure the core transaction engine is bulletproof.

Pre-launch security test scope (minimum):
[ ] Payment initiation and verification
[ ] Webhook signature validation (Paystack/Flutterwave/Monnify)
[ ] User registration, OTP flows, and authentication
[ ] API authorisation (BOLA/IDOR) between user roles
[ ] Admin panel access controls and segmentations
[ ] Data encryption for stored PII (BVN, NIN, account numbers)
[ ] Environment variable and secret management in production

Post-launch, pre-scale

Full application security test covering all user roles, all API endpoints, mobile app binaries, and internal admin tools. This is the point where you establish your security baseline and fix the structural issues before they are embedded in a codebase that 20 engineers are shipping to daily.

If you are onboarding your first 10,000 users, you must test your internal dashboards. Often, a customer support agent's internal portal has an over-privileged database connection that can edit any user's KYC tier or wallet balance.

Growth stage

Regular testing cadence: quarterly deep dives or continuous testing for fast-shipping teams. Feature-specific tests when you launch major new functionality (lending, cards, marketplace, international transfers). Vendor security assessments when you add new third-party integrations.

At the growth stage, the attack surface expands massively. You are integrating with multiple banking partners (NIP, NIBSS, Open Banking), each introducing new trust boundaries. We test how your application handles malicious responses or malformed data from these external services.

Enterprise and regulated

CBN-mandated annual penetration testing and biannual vulnerability assessments. PCI DSS quarterly scans if you process card data. SOC 2 evidence if you serve enterprise clients. NDPA compliance audit support. The testing is no longer optional; it is a licensing requirement and a prerequisite for enterprise partnerships.

What a real application security test finds (that scanners miss)

To understand why large consulting firms fail fintechs, you must understand the limitations of automated scanners (like Nessus, Acunetix, or Burp Suite Enterprise). A scanner crawls an application and looks for patterns: missing security headers, outdated software versions, or simple SQL injection payloads.

A scanner does not understand that a POST /api/transfer endpoint is supposed to deduct money from Account A and add it to Account B. Therefore, a scanner cannot tell you if an attacker can manipulate the amount parameter to transfer negative money, effectively adding funds to their own account.

This is the difference between a vulnerability scan and a manual application security test. We manually test the business logic that governs your platform.

// Example: scanner finds "missing Content-Security-Policy header"
// Real tester finds: wallet balance manipulation via race condition

// Scanner output:
// [INFO] Missing Content-Security-Policy header on /api/v1/dashboard
// Impact: Low  |  CVSS: 3.1

// Manual tester finding:
// [CRITICAL] Concurrent withdrawal requests bypass balance check
// Impact: Direct financial loss
// Reproduction:
//   1. Account has NGN 10,000 balance
//   2. Fire 10 simultaneous POST /api/v1/withdraw requests for NGN 10,000
//   3. 4 of 10 succeed, withdrawing NGN 40,000 from a NGN 10,000 balance
//   4. Account balance: -NGN 30,000
// Fix: Use database-level row locking (SELECT FOR UPDATE) on balance check

Advanced finding: The Idempotency Bypass

Many Nigerian fintechs rely on webhooks from payment gateways to credit user wallets. A common flaw we find during manual testing is the lack of idempotency validation. If an attacker intercepts a legitimate successful payment webhook and replays it 50 times in rapid succession, does your application credit the user's wallet 50 times?

A manual tester will intentionally capture and replay these webhooks. We will test if your database properly flags the transaction reference as "processed" before authorizing subsequent credits. A scanner will never find this.

Advanced finding: Broken Object Level Authorization (BOLA)

BOLA is the most critical API vulnerability in modern fintech. An authenticated user intercepts the API request that fetches their bank statement: GET /api/statements?user_id=12345. The attacker changes the ID to 12346.

If the backend checks that the user has a valid JWT token, but fails to check if the user actually owns account 12346, the attacker can systematically scrape the financial data of every single user on the platform. Scanners struggle to find complex BOLA issues because they require mapping authorization hierarchies across multiple authenticated sessions. We find them constantly.

How Simpa Labs executes application security testing

We do not hand you a 200-page PDF generated by a tool. We act as an adversarial engineering team. We decompile your mobile binaries to find hardcoded AWS keys. We proxy your API traffic to manipulate JSON payloads. We brute-force your OTP fallback mechanisms. We test your application exactly how an organized financial crime syndicate would attack it.

When we deliver our findings, we provide the exact HTTP requests needed to reproduce the exploit, the exact line of code where the vulnerability exists, and the exact remediation strategy required to fix it permanently. We help you pass your CBN audits, close enterprise deals, and sleep soundly knowing your transaction logic is mathematically sound.

Related reading

Blog: Fintech pentest · Fintech API security testing · When does your startup need a security review

Guides: Fintech security checklist · Pentest cost Nigeria

Services: Penetration testing · API security testing · Mobile app testing