OWASP API Security Top 10: Complete Technical Breakdown (2026)

APIs are the core foundation of modern web, mobile, and AI applications. However, over 80% of enterprise web traffic now flows through APIs, making them the #1 attack vector for malicious threat actors.

At TrustLayerLabs, our offensive security team manually audits thousands of API endpoints every year. Here is a technical breakdown of the top API vulnerabilities and how to fix them in production.

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1. API1:2023 — Broken Object Level Authorization (BOLA / IDOR)

BOLA occurs when an API endpoint exposes an object identifier (e.g., /api/v1/invoices/94821) but fails to verify if the requesting user owns that object.

The Attack Vector

An attacker changes the invoice ID parameter from 94821 to 94822 in Burp Suite to view another customer's sensitive financial data.

Remediation Code (Node.js/TypeScript)

// VULNERABLE CODE
app.get('/api/v1/invoices/:id', async (req, res) => {
  const invoice = await db.invoices.findById(req.params.id);
  return res.json(invoice); // No ownership check!
});

// PATCHED CODE
app.get('/api/v1/invoices/:id', async (req, res) => {
  const invoice = await db.invoices.findOne({
    _id: req.params.id,
    tenantId: req.user.tenantId // Enforce row-level tenant boundary
  });
  if (!invoice) return res.status(404).json({ error: "Invoice not found" });
  return res.json(invoice);
});

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2. API2:2023 — Broken Authentication & JWT Signature Exploits

API authentication flaws allow attackers to compromise auth tokens or exploit weak JWT implementations (e.g., accepting "alg": "none" or failing to check token expiration).

Remediation Best Practices

• Never use symmetric secret keys weaker than 256 bits (HS256). Prefer asymmetric RS256 / ES256 signatures.
• Enforce strict audience (aud) and issuer (iss) claims verification.
• Implement short token lifetimes (15 minutes max) paired with secure HTTP-only refresh cookies.

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3. API5:2023 — Broken Function Level Authorization (BFLA)

BFLA happens when administrative endpoints (e.g., /api/admin/delete-user) are accessible to low-privilege standard accounts simply by guessing or brute-forcing administrative URLs.

How We Test BFLA

Our pentesters capture normal user session cookies and execute requests against administrative REST and GraphQL endpoints to verify role-based access control (RBAC) boundaries.

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Conclusion & Next Steps

Preventing API breaches requires more than static vulnerability scanners. Manual penetration testing is essential to discover complex business logic flaws.

Read our full API Security Testing Service page or download our free Redacted Sample VAPT Report.