The attack surface nobody's watching
Every modern application is built on APIs — connecting frontends to backends, services to each other, and businesses to their partners. That connective tissue is also one of the most consistently exploited attack surfaces, precisely because it's often built fast and reviewed less carefully than user-facing code.
The mistakes that lead to real breaches are rarely exotic. They're the same handful of issues, repeated across countless codebases.
The recurring mistakes
Broken object-level authorization
The single most common API vulnerability: an endpoint checks that a user is authenticated, but not that they're authorized to access the specific resource they're requesting. Change an ID in the URL from /orders/1001 to /orders/1002, and if the server doesn't verify ownership, you're looking at someone else's data.
Excessive data exposure
An endpoint returns an entire database object — including internal fields, password hashes, or other users' data — because it was faster to build than filtering the response to only what the frontend actually needs. The frontend hiding a field in the UI does nothing if the API response still contains it.
No rate limiting
Without limits on how many requests an endpoint accepts, it becomes trivial to brute-force passwords, scrape entire datasets, or overwhelm the server — all through a completely "legitimate" authenticated or public endpoint.
Trusting client-side validation
Validation logic that only exists in the frontend JavaScript is not security — it's a UX nicety. Anyone can bypass the frontend entirely and send requests directly to the API. Every validation rule that matters for security has to be enforced server-side.
Verbose error messages
Detailed stack traces or database error messages returned to the client are a gift to an attacker — they reveal your technology stack, table structure, and exact failure points, turning a blind probe into a guided one.
Unversioned, undocumented internal APIs treated as "not public"
An API that's technically reachable but "not linked from anywhere" isn't hidden — it's just undiscovered yet. Security through obscurity fails the moment someone runs a basic endpoint scanner.
The baseline every API should have
- Authorization checks on every single endpoint, verifying not just "is this user logged in" but "does this user own or have rights to this specific resource."
- Explicit response shaping — return only the fields the client actually needs, never a raw database object.
- Rate limiting on every public and authenticated endpoint, not just login forms.
- Server-side validation on everything, treating all client input as untrusted regardless of what the frontend already checked.
- Generic error messages to the client, with the real detail logged server-side for debugging, not exposed in the response.
The takeaway
API security failures are rarely sophisticated attacks — they're overlooked basics, exploited by anyone who thinks to check. The good news is that the fixes are well-understood and not expensive to implement; they just require treating API design as a security surface from the start, not an afterthought bolted on before launch.
Building or auditing an API and want a second set of eyes on the authorization logic specifically? That's usually where the real risk hides.