Hey devs 👋
I run penetration tests on production APIs for a living, and I keep seeing the same seven security mistakes — even from teams that pride themselves on "shifting left." These aren't exotic flaws. They're patterns I find in 8 out of 10 engagements.
Here are the seven, in rough order of how often I find them. If your API has even three of these, you've got real exposure.

1. Trusting Client-Side Authorization This is the #1 vulnerability I find. The frontend hides the 'Admin Panel' button if the user isn't an admin — but the backend doesn't verify the role on the corresponding endpoint. If your endpoint is /api/admin/users/delete, and you're checking 'is this user an admin' in your React code but not in your Express middleware, any user can call that endpoint with curl. I've broken into more than one B2B SaaS this way during pentests. Fix: every endpoint enforces authorization at the API layer. Every. Single. One. Even if the UI 'never lets users reach it.'
2. IDOR via Sequential IDs /api/users/12345/profile is a beautiful invitation to try /api/users/12346/profile. If you don't verify that the requesting user has permission to access user 12346's data, congratulations — you have an Insecure Direct Object Reference (IDOR). This is OWASP API #1 (Broken Object Level Authorization) and it's the most common API vulnerability we see across 2026 engagements. Sequential numeric IDs make it trivial to enumerate. Fix: use UUIDs for resource IDs OR enforce strict ownership checks on every authenticated endpoint. Ideally both.
3. Verbose Error Messages Your error response says: "Database connection failed: PostgreSQL 14.2 — host db-prod-01.internal:5432 — role app_user has insufficient privileges." You just told an attacker your database type, version, internal hostname, port, and a username. They didn't even have to ask. Fix: in production, return generic error messages to the client ("Internal server error"). Log the full details server-side for debugging. Never let stack traces escape.
4. Missing Rate Limits on Authentication Endpoints Your /api/login endpoint accepts unlimited requests per IP per minute. An attacker brute-forces 10,000 password attempts in five minutes. They eventually find the user with password "Welcome@2024" — and they will. Fix: rate-limit aggressively on auth endpoints. Use exponential backoff. Lock accounts after N failed attempts (with a careful UX so you don't enable account-lockout DOS attacks). Consider CAPTCHA after 3 failures.
5. JWT Without Expiration (or with 30-Day Expiration) If your JWT lifetime is "forever" or 30 days, and a token gets compromised through XSS, a logged keystroke, or a leaked Slack screenshot — the attacker has 30 days of access. With no way to revoke. Fix: short-lived access tokens (15 minutes) + refresh tokens. Implement a token blacklist or use opaque tokens you can revoke server-side. JWTs aren't "set and forget."
6. CORS Wildcards Access-Control-Allow-Origin: * combined with Access-Control-Allow-Credentials: true is a vulnerability that browsers actually try to prevent (they'll throw an error in modern Chrome) — but I still find it in older codebases. Even more common: Access-Control-Allow-Origin: $REQUEST_ORIGIN — i.e., echoing back whatever Origin the request claims. This is functionally a wildcard and lets any malicious site make authenticated requests on behalf of your user. Fix: explicit allowlist of trusted origins. Hardcode them. Don't echo the request header.
7. No API Authentication for Server-to-Server Endpoints Internal microservice endpoints often start as "only accessible from inside the VPC" and that becomes the entire security model. Then someone misconfigures a load balancer, or you move to a service mesh, or a developer adds an /admin/health endpoint — and suddenly your unauthenticated internal API is on the public internet. Fix: every API endpoint authenticates, even internal ones. mTLS for service-to-service. Defense in depth means assuming the network IS the attacker. Quick Self-Audit Checklist Run through this checklist on your most-used API: Every endpoint enforces both authentication AND authorization at the API layer Resource IDs use UUIDs OR strict ownership checks are present Production error responses are generic (no stack traces, no internal details) Authentication endpoints have rate limits and account lockout Access tokens expire in <30 minutes; refresh tokens are revocable CORS uses an explicit allowlist (no wildcards, no Origin echoing) Internal/server-to-server APIs require authentication If you can't tick all seven boxes, you've got at least one open vulnerability waiting to be exploited. Final Thoughts Most API security failures aren't sophisticated zero-day attacks. They're operational gaps that accumulate over time as teams move fast. The fix isn't more tools — it's better defaults, security training for engineers, and a regular external pentest to catch what you've started ignoring. If you want a fresh set of eyes on your APIs, the team I work with at SecureRoot's API security insights runs structured API VAPT engagements covering all OWASP API Top 10 categories. They've worked with clients including the Ministry of Justice (Kuwait), OmanTel, and several Indian fintech and SaaS firms. The reports are JSON-structured and integrate directly into Jira — none of that 200-page-PDF nonsense. What API security mistakes have I missed? Drop them in the comments — I'd love to add to my list of "most common findings." — Vara, on behalf of the SecureRoot security team