API Security Handbook: How to Prevent the Top 5 Most Common Attacks

Building an API that works is easy. Building an API that is secure is a completely different challenge. With the rise of microservices, APIs have become the primary target for hackers. In this guide, we will explore the most critical security flaws and, more importantly, how to fix them before they are exploited.

1. Broken Object Level Authorization (BOLA)

BOLA is currently the #1 threat to APIs. It occurs when a user can access or modify data belonging to another user just by changing an ID in the URL.

The Attack: A user logged in as ID 123 sends a request to GET /api/orders/456 and successfully sees someone else's order.

The Fix: Never trust the ID provided in the request alone. Always validate that the resource belongs to the authenticated user in your database query.

// BAD: Trusting the URL ID
var order = db.Orders.Find(id);

// GOOD: Checking ownership
var userEmail = User.Identity.Name;
var order = db.Orders.FirstOrDefault(o => o.Id == id && o.UserEmail == userEmail);

2. Rate Limiting and Throttling

Without rate limiting, your API is vulnerable to Denial of Service (DoS) attacks or brute-force attempts. A script could hit your /login endpoint 10,000 times a minute until it finds a password.

The Solution: Implement a "Bucket" algorithm or use middleware like Express-Rate-Limit (Node.js) or AspNetCoreRateLimit (.NET) to restrict the number of requests per IP address.

3. Mass Assignment Vulnerability

This happens when an API takes a JSON object from the user and maps it directly to a database model without filtering the fields. A hacker could send {"is_admin": true} in a registration form, and if your code isn't careful, it will update that field.

The Fix: Use DTOs (Data Transfer Objects) or ViewModels. Only map the fields that the user is actually allowed to change.

// Example in C# using DTOs
public async Task<IActionResult> UpdateUser(UserUpdateDto dto) {
    var user = await _db.Users.FindAsync(CurrentUserId);
    user.DisplayName = dto.DisplayName; // is_admin is never touched!
    await _db.SaveChangesAsync();
}

4. Exposure of Sensitive Data

Does your API return the user's password hash or home address even if the frontend only displays their name? Hackers can intercept the full JSON response and find sensitive data that was never meant to be public.

The Fix: Always use a "Allow-list" approach. Explicitly define which fields the API should return instead of returning the entire database object.

The Security Checklist

Action	Benefit
Use HTTPS Only	Encrypts data in transit.
Implement OAuth2/OIDC	Industry standard for secure Auth.
Input Validation	Prevents SQL Injection and XSS.
Logging & Monitoring	Detects attacks in real-time.

Conclusion

Security is not a one-time setup; it’s a continuous process. By following these patterns, you eliminate 80% of the common vulnerabilities found in modern APIs. Keep your libraries updated and always assume that the client-side data is malicious.

Are you securing your APIs correctly? Leave a comment with the security tools you use in your daily workflow!