Summary
APIs are how modern applications talk to each other, which makes them a prime target for attacks. Most developers know they need authentication on their endpoints, but authentication alone isn't enough. Between rate limiting, input validation, proper authorization, and protecting against injection attacks, there's a lot more to API security than just checking if someone has a valid token.
Authentication Is Just the First Step
Yes, you need to verify who's making requests to your API. OAuth 2.0, JWT tokens, API keys – whatever authentication method you choose, it's essential. But here's what catches people: authentication just tells you who someone is. It doesn't tell you what they're allowed to do.
That's where authorization comes in. Just because a user is authenticated doesn't mean they should access every endpoint or every piece of data. A user might be logged in legitimately, but they shouldn't be able to access another user's data, modify resources they don't own, or call admin-only endpoints.
The fix is implementing proper authorization checks at every endpoint. Don't just check if a token is valid – check if that specific user is allowed to perform that specific action on that specific resource. This needs to happen on the server side, not just in your frontend. Never trust the client to enforce authorization rules.
Rate Limiting Isn't Optional
Without rate limiting, your API is vulnerable to abuse. An attacker can flood your endpoints with requests, either trying to brute force credentials, scrape all your data, or just bring your service down through resource exhaustion. Even legitimate users with buggy code can accidentally create problems if they're making thousands of requests per second.
Implement rate limiting at multiple levels. You might have a global rate limit for all unauthenticated requests, a higher limit for authenticated users, and even higher limits for premium customers. Different endpoints might need different limits too – maybe you allow more read requests than write requests.
When someone hits the rate limit, return a proper HTTP 429 status code and tell them when they can try again using the Retry-After header. Good rate limiting makes your API more reliable for everyone by preventing a few users from monopolizing resources.
Input Validation and Sanitization
Never trust data coming into your API. Users will send you all kinds of unexpected input, whether by mistake or maliciously. If you're not validating and sanitizing that input, you're opening yourself up to injection attacks, data corruption, and system crashes.
Validate everything: data types, formats, lengths, ranges. If an endpoint expects an integer between 1 and 100, enforce that. Don't just hope users will send valid data. Use schema validation libraries that let you define exactly what valid requests look like, and reject anything that doesn't match.
Be especially careful with data that gets used in database queries, system commands, or gets embedded in other places. SQL injection is still a thing today because people are still building queries from unvalidated user input. Use parameterized queries or ORMs that handle escaping for you. Same goes for any other kind of injection – command injection, LDAP injection, XML injection. If user input is going somewhere, sanitize it first.
Protecting Against Common API Attacks
APIs face some specific attack patterns you need to defend against. Mass assignment is one that catches people constantly. This is where an attacker includes extra fields in their request that modify things they shouldn't be able to change. Maybe your user update endpoint only shows fields for name and email, but behind the scenes your code just updates whatever fields are in the request. An attacker adds an "isAdmin" field set to true, and suddenly they're an administrator.
The solution is to explicitly whitelist which fields can be modified through each endpoint. Only update the fields you specifically allow, and ignore everything else. Many frameworks have built-in features for this, but you need to actually use them.
Another common issue is exposing too much data in your responses. Just because you have all of a user's information doesn't mean you should return it all in every API call. Only send what the client actually needs. This reduces the impact if your API gets compromised and prevents information leakage.
Logging and Monitoring for Security
You need visibility into what's happening with your API. Log all authentication attempts, especially failures. Log authorization failures. Log unusual patterns like someone accessing endpoints in a weird order or making requests that barely avoid your rate limits.
But be careful what you log. Don't log passwords, tokens, or other sensitive data. Make sure your logs don't become a security vulnerability themselves. Store them securely and limit who can access them.
Set up alerts for suspicious patterns. Multiple failed authentication attempts from the same IP might be a brute force attack. Sudden spikes in traffic could be a DDoS attempt or data scraping. Someone accessing hundreds of different user records in rapid succession is probably not legitimate activity. The sooner you detect these patterns, the sooner you can respond.
API Versioning and Deprecation
Security improvements often require breaking changes to your API. You fix a vulnerability, but the fix changes how an endpoint works. If you don't have a versioning strategy, you're stuck choosing between security and not breaking existing clients.
Version your API from day one. This lets you make security improvements in new versions while giving clients time to migrate from old versions. When you do deprecate insecure endpoints, give plenty of warning and set a hard deadline. Then actually shut down the old version. Keeping insecure legacy endpoints running indefinitely defeats the purpose of fixing the security issues.
Concluding Remarks
API security requires thinking about threats at every layer. Authentication, authorization, input validation, rate limiting, proper error handling, logging – each of these protects against different attack vectors. Skip one, and you've left a door open for attackers.
The good news is that most API security issues are well understood, and there are established patterns for addressing them. You don't need to invent new security measures; you just need to implement the existing ones properly. Use security-focused frameworks and libraries where possible. They've already solved a lot of these problems.
Test your API security regularly. Try to break your own endpoints. See if you can access data you shouldn't. Check if you can bypass rate limits. Better that you find these issues during testing than having attackers find them in production. API security is not a one-time thing you do at launch; it's an ongoing process of testing, monitoring, and improving.