Here's the thing about APIs that makes them uniquely dangerous: they're built to be talked to. A database can hide behind a private network. A server can sit in a locked-down subnet. But an API's entire job is to be reachable and to respond to requests, which means it's the one part of your system you deliberately expose, and therefore the one an attacker gets to poke at all day. So the security of your API isn't a nice-to-have layer on top. It's the door, and the door is standing open by design.
Let me walk through where API security actually goes wrong, because it's rarely the exotic stuff.
Authentication and authorization are different, and the second one is where you'll get breached
People conflate these constantly. Authentication is "who are you." Authorization is "are you allowed to do this specific thing." Most teams get authentication roughly right, there's a login, there's a token. Then they under-do authorization, and that's where the actual breaches live.
The classic failure: an API checks that you're a valid, logged-in user, and then trusts you to only ask for your own data. So the request comes in for /users/123/orders, the API confirms you're logged in, and hands over user 123's orders, without checking whether you are user 123. Change the number to 124 and you get someone else's data. This is called broken object level authorization, it's one of the most common API vulnerabilities in existence, and it comes down to authenticating the user but never authorizing them against the specific resource they asked for. Every endpoint that returns a specific object needs to check not just "are you logged in" but "are you allowed to see this exact thing."
