Building APIs is one thing, but building secure APIs is another. I've seen too many applications get compromised because basic security measures were overlooked. It's not that developers don't care about security—it's that they might not know what to look for or where to start.

When I first started building APIs, I focused on getting them to work. Security was something I'd add later, or so I thought. But security isn't something you can bolt on at the end. It needs to be part of how you build from the start.

Authentication and authorization

The first line of defense for any API is proper authentication. You need to make sure that only authorized users can access your endpoints. There are several ways to handle this, and the right choice depends on your use case.

For most modern applications, JWT tokens are a solid choice. They're stateless, which means your server doesn't need to store session information. When a user logs in, you generate a token that contains information about who they are and what they can do. The client sends this token with every request, and your server validates it.

But JWTs aren't perfect. If someone steals a token, they can use it until it expires. That's why you should keep token expiration times reasonable—maybe an hour for access tokens, and use refresh tokens for longer sessions. Also, always use HTTPS. Sending tokens over unencrypted connections is asking for trouble.

Input validation is critical

One of the most common ways APIs get compromised is through malicious input. An attacker might try to send SQL injection code, or they might send data that's way too large to crash your server. You need to validate everything that comes in.

I always validate on both the client and server side. Client-side validation gives users immediate feedback, but server-side validation is what actually protects you. Never trust data from the client. Even if your frontend validates everything perfectly, an attacker can bypass it and send requests directly to your API.

Check the types of data, the lengths, the formats. If you're expecting an email, make sure it looks like an email. If you're expecting a number, make sure it's actually a number and within reasonable bounds. This might seem tedious, but it prevents a lot of problems.

Rate limiting protects against abuse

Rate limiting is one of those things that's easy to forget but really important. Without it, someone could hammer your API with thousands of requests and either crash your server or rack up huge bills if you're using cloud services.

I usually set different rate limits for different endpoints. Public endpoints might have stricter limits, while authenticated endpoints for logged-in users can be more generous. The key is to find a balance that prevents abuse without frustrating legitimate users.

Most API frameworks have middleware for rate limiting. You can limit by IP address, by user, or by API key. I've found that combining IP-based and user-based limiting works well. If someone is making too many requests from a single IP, slow them down. If a specific user account is making too many requests, that's a red flag too.

CORS configuration matters

CORS, or Cross-Origin Resource Sharing, is one of those things that can be confusing at first. It's a browser security feature that controls which websites can make requests to your API. If you misconfigure it, you either block legitimate requests or open yourself up to attacks.

The key is to be specific about which origins you allow. Don't just allow everything with a wildcard unless you really need to. If your API is only meant to be used by your own frontend, only allow your domain. If you're building a public API, you might need to be more permissive, but still be careful.

I've seen APIs that allow all origins in development and forget to change it for production. That's a security risk. Make sure your production configuration is locked down properly.

Error messages shouldn't leak information

When something goes wrong, it's tempting to return detailed error messages. They help with debugging, and they're useful during development. But in production, detailed errors can give attackers information about your system.

Instead of returning "SQL syntax error at line 42," return something generic like "An error occurred processing your request." Log the detailed error on the server where you can see it, but don't send it to the client.

This applies to authentication errors too. Don't say "Invalid password" or "User not found." Say "Invalid credentials" for both cases. This prevents attackers from figuring out which email addresses are registered in your system.

Keep dependencies updated

Most APIs rely on third-party libraries and frameworks. These dependencies can have vulnerabilities, and when patches are released, you need to update. I've seen applications running libraries with known security flaws because no one bothered to update.

Set up automated dependency scanning if you can. Many tools can check your dependencies against databases of known vulnerabilities and alert you when updates are available. It's worth the time to set this up and keep things current.

Use HTTPS everywhere

This should go without saying, but I'll say it anyway: always use HTTPS. Never send sensitive data over unencrypted connections. Modern browsers are pretty good about warning users when they're on an insecure connection, but you shouldn't rely on that.

If you're using a service like AWS or Heroku, they usually handle SSL certificates for you. If you're managing your own server, tools like Let's Encrypt make it easy to get free SSL certificates. There's really no excuse for not using HTTPS in 2024.

Logging and monitoring

Good logging helps you catch security issues before they become major problems. Log authentication attempts, especially failed ones. If you see a pattern of failed login attempts from the same IP, that might be someone trying to brute force their way in.

Monitor your API for unusual patterns. A sudden spike in requests, especially to specific endpoints, could indicate an attack. Set up alerts so you know when something unusual is happening.

The bottom line

API security isn't something you do once and forget about. It's an ongoing process. You need to stay informed about new vulnerabilities, keep your dependencies updated, and regularly review your security practices.

The good news is that most of these practices aren't that hard to implement. Many frameworks have built-in support for authentication, rate limiting, and input validation. You just need to use them correctly.

Start with the basics: proper authentication, input validation, and HTTPS. Then add rate limiting and monitoring. As your API grows, you can add more sophisticated security measures. But don't wait until you have a security incident to start thinking about these things.