OWASP Top 10 risks for payment systems and fintech: what they look like in real products

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

Every security vendor in Nigeria talks about the OWASP Top 10. They run a vulnerability scanner against your website. The scanner exports a PDF. The PDF lists "Cross-Site Scripting" and "Security Misconfiguration". The vendor charges you thousands of dollars for this PDF.

This approach is completely broken for fintech companies. The standard OWASP Top 10 was built for generic web applications. It was built for blogs, forums, and corporate brochures. It was not built for transaction engines that process billions of Naira daily.

When you apply the generic OWASP Top 10 to a payment system, you completely miss the fatal flaws. You miss the race conditions that allow users to double-spend. You miss the business logic bypasses that allow users to fund their wallets for free.

At Simpa Labs, we specialize in manual adversarial engineering for financial products. We map the OWASP Top 10 directly to the realities of modern payment architecture. We show you exactly how these generic risks translate into catastrophic financial loss.

The OWASP risks that hit hardest in fintech

In a fintech or payment context, the blast radius of standard web vulnerabilities is significantly larger. A broken access control finding on a corporate blog has minimal impact. An attacker reads an unpublished draft.

The exact same finding on a payment API destroys the company. It exposes every transaction in your system. It allows a user to transfer money from another user's account. It leaks the Bank Verification Numbers of your entire customer base. We focus our testing entirely on the high-impact translation of these risks.

A1: Broken Access Control

In fintech, broken access control is not just about seeing data you should not see. It is about moving money you do not own. It is the number one cause of total system compromise in our assessments. This manifests in four critical ways:

• Broken Object Level Authorization (BOLA): A customer accesses another customer's wallet or transaction history simply by changing an ID in the URL. Your server checks if they are logged in, but fails to check if they actually own the requested resource.
• KYC Tier Bypass: A Tier 1 KYC user accesses Tier 3 transaction limits by modifying request parameters in the API payload. The server blindly trusts the client-side state.
• Merchant Isolation Failure: A merchant reads another merchant's settlement data by iterating predictable, sequential merchant IDs.
• Role Boundary Enforcement: A customer support agent modifies a customer account without proper managerial approval workflows.

// VULNERABLE: no ownership check on transaction endpoint
router.get('/transactions/:txnId', authenticate, async (req, res) => {
  const txn = await Transaction.findById(req.params.txnId);
  res.json(txn); // Returns anyone's transaction to any authenticated user
});

// CORRECT: verify ownership before returning
router.get('/transactions/:txnId', authenticate, async (req, res) => {
  const txn = await Transaction.findOne({
    _id: req.params.txnId,
    userId: req.user.id // Ownership check
  });
  if (!txn) return res.status(403).json({ error: 'Access denied' });
  res.json(txn);
});

A2: Cryptographic Failures

Cryptographic failures in blogs mean leaked passwords. Cryptographic failures in payment systems mean direct financial theft. This means transmitting card data or account numbers over unencrypted HTTP channels. This means storing PINs or CVVs in plain text in the database.

It also means using weak or reversible encryption for stored payment tokens. It means using outdated hashing algorithms like MD5 or SHA-1 for data integrity checks on webhooks.

PCI DSS requirements largely codify the correct cryptographic standards for card data. But we regularly find Nigerian fintech products storing BVNs, NINs, and virtual account numbers in completely unencrypted database columns because they fall outside of strict PCI scope. We audit your entire cryptographic architecture to stop these leaks.

A3: Injection

Classic SQL injection in payment APIs is rare in modern ORMs like Django or Sequelize. But it still appears in raw query paths where developers bypass the ORM for performance reasons.

More relevant in modern fintech is NoSQL injection. Many Nigerian startups use MongoDB-backed APIs. An attacker can manipulate filter operators in the JSON payload. They bypass authentication entirely by injecting logical operators instead of strings. We test every parameterized and raw query path in your codebase.

// VULNERABLE: NoSQL injection in MongoDB
// Attacker sends: {"username": {"$ne": ""}, "password": {"$ne": ""}}
app.post('/login', async (req, res) => {
  const user = await User.findOne({
    username: req.body.username,  // Not sanitized
    password: req.body.password   // Accepts operator objects
  });
  if (user) return res.json({ token: generateToken(user) });
});

A4: Insecure Design (the OWASP risk most relevant to payment products)

This is the OWASP category that covers what we call "business logic vulnerabilities." In payment systems, insecure design is the most fatal category. Scanners cannot find these flaws. They require human intuition. Insecure design includes:

• Race Conditions: Balance checks that are not atomic. We send ten simultaneous withdrawal requests. The database checks the balance ten times simultaneously, sees enough funds, and approves all ten requests, heavily overdrafting the account.
• Client-Side Trust: Payment flows where the transaction amount is set in the frontend JavaScript and not strictly recalculated and validated server-side. We change the price of a premium feature from NGN 50,000 to NGN 1 before checkout.
• Decoupled Transactions: Loan disbursement APIs where the eligibility check and the actual disbursement are in separate, non-atomic database transactions.
• Premature Rewards: Referral bonus systems where the bonus is credited before the referral's initial payment actually settles on the blockchain or gateway.

These are not implementation bugs. They are deep architectural design flaws. The only way to find them is manual, aggressive testing by engineers who understand payment product design.

A5: Security Misconfiguration

The fintech-specific misconfiguration findings we see most often involve cloud infrastructure. We see DEBUG mode left enabled in production Django applications, exposing full stack traces with active database credentials. We see Cross-Origin Resource Sharing (CORS) misconfigurations allowing any external origin to make credentialed requests to your API.

We frequently find AWS S3 buckets containing highly sensitive KYC documents (national ID cards, utility bills, passports) with public read access. We find Swagger and OpenAPI documentation exposed publicly on production APIs without any authentication, handing attackers a complete map of your attack surface.

A7: Identification and Authentication Failures

In fintech, authentication failure is immediate account takeover. The specific patterns we test for payment products are highly targeted.

We test OTP brute force capabilities. A 6-digit OTP with no rate limiting gives an attacker exactly 1 million attempts. Our bots can guess 1 million combinations in under ten minutes. We test for SIM swap resilience. We verify that session tokens properly expire immediately after logout or password change. We test device binding logic to ensure sessions cannot be trivially transferred to attacker-controlled devices.

What OWASP does not cover for fintech

The original OWASP Top 10 was built around web applications in general. It does not specifically address the deep, complex attacks that cause massive financial loss in modern payment products. If you only look at the OWASP Top 10, you are completely blind to these attack vectors:

• Webhook replay attacks: Capturing a valid payment success webhook and replaying it to credit an account multiple times from a single actual payment event.
• Payment reference reuse: Using a valid, paid transaction reference from a NGN 100 purchase to validate a NGN 50,000 purchase.
• Synthetic Identity Fraud: Bypassing automated KYC document checks using heavily modified digital templates and deepfaked liveness checks.
• Authorised Push Payment (APP) fraud: Exploiting weak beneficiary validation flows to trick users into sending money to attacker-controlled accounts.

The OWASP API Security Top 10 and PCI DSS together cover more of the payment-specific surface. However, none of these generic lists replace a rigorous manual assessment by an offensive security team that specializes in breaking financial technology. Do not rely on generic scanners. Hire specialists.

Related reading

Guides: OWASP for fintech · Fintech security checklist

Blog: Business logic flaws in Nigerian payment platforms · BOLA in payment APIs · Webhook race conditions

Services: Penetration testing · API security testing