Most API breaches are not clever. They are the slow result of a few small habits that never quite got picked up. Here are five that pay for themselves many times over.
Treat every input as hostile
Assume that anything arriving from outside your system is wrong until proven otherwise. Validate types, lengths, and ranges right at the edge, and reject early. The goal is simple. Bad data should never reach the part of your code that trusts it.
Authenticate first, authorize second
Knowing who someone is and knowing what they are allowed to do are two different questions. Answer the first at the door, then check the second on every single action. Skipping that second step is how one account quietly becomes every account.
The cheapest security fix is always the one you build in before the first real user ever arrives.
Rate limit everything
A login form without a limit is an open invitation. Put sensible ceilings on requests per user and per address, and make them tighter on the routes that matter most. It costs almost nothing and shuts down a whole category of abuse.
Log enough to investigate
When something goes wrong, and one day it will, your logs are the only honest account of what happened. Record who did what and when, keep it tamper resistant, and never write secrets into it. Future you will be grateful.
Read your own API like an attacker
Once a quarter, sit down and try to break in. Look at your own endpoints the way someone with bad intent would. That one habit turns security from a one time checklist into something that keeps pace with how your product grows.