Api Security

API Security

Web-app and API security overlap but are not the same. APIs ship with different defaults (CORS-permissive, no CSRF tokens, often no rate-limiting), different consumers (mobile apps, integrations, scripts — not just browsers), and a different attack surface (object IDs in URLs, JSON bodies, scoped tokens). The OWASP API Security Top 10 (2023 edition) is the canonical reference; this skill walks each item with concrete detection and fix patterns.

When to invoke

• Designing a new REST or GraphQL API
• Reviewing an existing API before scaling user count
• After abuse — scraping, account takeover, suspicious 4xx/5xx patterns
• Adding a public-facing endpoint to a previously internal service
• An API is feeding a mobile or single-page app where the client cannot be trusted
• Periodic API audit (quarterly is reasonable)

API01:2023 — Broken Object Level Authorization (BOLA)

The #1 API vulnerability and not even close. Every endpoint that takes an ID and returns the corresponding resource must check the caller is allowed to see that specific object.

// Bad — any authenticated user reads any invoice
app.get('/api/invoices/:id', requireAuth, async (req, res) => {
  const invoice = await db.invoices.findUnique({ where: { id: req.params.id }});
  res.json(invoice);
});

// Good — scoped to the requester's ownership
app.get('/api/invoices/:id', requireAuth, async (req, res) => {
  const invoice = await db.invoices.findFirst({
    where: { id: req.params.id, userId: req.user.id }
  });
  if (!invoice) return res.status(404).json({ error: 'not found' });
  res.json(invoice);
});

Detection in a codebase:

# Every parameterized route is a BOLA candidate. Walk them.
grep -rEn ":id|:slug|:uuid" src/routes src/api app 2>/dev/null

# Routes that fetch by primary key alone — high-suspicion pattern
grep -rEn 'findUnique\\(\\{\\s*where:\\s*\\{\\s*id:' --include='*.{ts,js}' . | head

Patterns that make BOLA harder to introduce:

• Always include userId or tenantId in the where clause, not just the resource ID
• Use database row-level security (RLS) as a backstop — Postgres RLS plus per-request SET LOCAL app.current_user. See [postgres-hardening](../postgres-hardening/SKILL.md).
• Use opaque IDs (UUIDs or hashids), not sequential integers. Doesn't fix BOLA but slows enumeration if it slips through.

API02:2023 — Broken Authentication

Auth on APIs has its own failure modes beyond web-app auth.

• Token validation skipped on health/info/debug endpoints that grew to leak data
• JWTs validated for signature but not for iss, aud, exp — pin all three
• No revocation path for issued tokens — long-lived JWTs are session cookies you cannot recall
• API keys with no expiry, no rotation, no scope — issued once, lives forever, total power
• Bearer tokens in URLs (showing up in access logs, referer headers) — always in Authorization header

For password-based API auth specifically:

• Same patterns as web auth — see [auth-hardening](../auth-hardening/SKILL.md)
• Per-IP and per-account rate-limiting on /api/auth/login
• MFA flow that works over API (TOTP code as a field; WebAuthn via session)
• Generic error on auth failure — never "user not found"

API03:2023 — Broken Object Property Authorization

Two flavors:

Mass assignment

// Bad — user can set their own role to admin
app.patch('/api/users/me', requireAuth, async (req, res) => {
  const updated = await db.users.update({
    where: { id: req.user.id },
    data: req.body,                    // ☠️ accepts arbitrary keys including 'role'
  });
  res.json(updated);
});

// Good — explicit allowlist of editable fields
const PROFILE_FIELDS = ['displayName', 'avatarUrl', 'timezone'];

app.patch('/api/users/me', requireAuth, async (req, res) => {
  const data = Object.fromEntries(
    Object.entries(req.body).filter(([k]) => PROFILE_FIELDS.includes(k))
  );
  const updated = await db.users.update({ where: { id: req.user.id }, data });
  res.json(updated);
});

Better: validate with a schema library (Zod, Valibot, class-validator) and let it reject unknown keys.

Excessive data exposure

API returns the whole DB row when the consumer needs three fields. Browser-facing this leaks PII; mobile-app-facing the same leak is in production traffic forever.

// Bad
const user = await db.users.findUnique({ where: { id }});
return user;   // includes password_hash, email_verified_at, internal_notes, ...

// Good — explicit projection
const user = await db.users.findUnique({
  where: { id },
  select: { id: true, displayName: true, avatarUrl: true },
});

For GraphQL: the schema is the contract. Treat every field on every type as "is this safe to expose to this kind of caller?"

API04:2023 — Unrestricted Resource Consumption

DoS via expensive operations. APIs are easier to DoS than web apps because each request can deliberately ask for more work.

Defenses, all of which you need:

• Rate limiting — per IP, per user, per endpoint. Cheap to add, eliminates the easy attacks.

``ts // Express + express-rate-limit import rateLimit from 'express-rate-limit'; app.use('/api/', rateLimit({ windowMs: 60_000, max: 100, standardHeaders: 'draft-7', legacyHeaders: false, })); app.use('/api/auth/login', rateLimit({ windowMs: 60_000, max: 5, skipSuccessfulRequests: true })); ``

• Body size limits — app.use(express.json({ limit: '100kb' })). The default of 1MB is too generous for most APIs.
• Request timeout — 10–30s max for any endpoint. Long-running work goes to a queue.
• Pagination caps — limit=10000 is not pagination, it is "give me everything." Cap server-side at 100 or so.
• Query complexity for GraphQL — graphql-depth-limit + graphql-query-complexity. Without these, { user { friends { friends { friends { ... } } } } } brings the DB down.
• Cost-aware operations — anything that touches an external paid API (email send, SMS, AI) needs its own budget cap per user per time window.

API05:2023 — Broken Function Level Authorization

Admin / privileged endpoints reachable by non-admins. Common when:

• Admin routes are nested under /api/admin/ but the auth check is at middleware level only — and a route was added later that bypasses middleware
• Mobile-app endpoints hide admin features client-side; the API still serves them to anyone with a token

Detection:

grep -rE '/api/admin' src 2>/dev/null
grep -rE 'role.*admin|isAdmin|requireAdmin' src 2>/dev/null
# Every admin route should have the role check in the handler, not just middleware

Same rule as elsewhere in this repo: never rely on middleware alone. Each privileged handler does its own check.

API06:2023 — Unrestricted Access to Sensitive Business Flows

A specific subclass of resource consumption: even with rate limits, an attacker can scrape your "show me product X price" endpoint at 1 RPS for weeks. The data eventually leaves your system.

Examples in the wild:

• Booking sites scraped for hotel inventory
• E-commerce APIs scraped for product catalogs
• Review/ratings APIs scraped for user data
• "Refer a friend" flows automated for promo abuse

Mitigations require business-context decisions:

• CAPTCHAs / proof of work for high-value flows after first hit
• Behavioral signals (user-agent diversity, mouse movement patterns, ASN reputation) — Cloudflare Bot Fight / Turnstile is a free starting point
• Account-level rate limiting beyond IP — one user creating 1000 referrals per hour is suspicious even if from 1000 IPs
• Pricing / catalog data served via signed, time-limited URLs after auth, not as free GET endpoints

API07:2023 — Server-Side Request Forgery (SSRF)

// Bad — fetches whatever the user says, including internal services
app.get('/api/preview', async (req, res) => {
  const url = req.query.url as string;
  const r = await fetch(url);
  res.send(await r.text());
});

Attacker submits http://169.254.169.254/latest/meta-data/ (AWS metadata) or http://localhost:6379/ (your Redis) and your server happily proxies.

Fix:

• Allowlist destination hostnames — only https://known-customer-domains.example.com/*
• Resolve and validate the IP before fetching — reject private ranges (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16, 127.0.0.0/8, 169.254.0.0/16, ::1, fc00::/7)
• Disable redirects or revalidate at each hop — an allowed domain can 302 to a bad one
• Run the fetcher in a network-isolated process without access to internal services

For URL-fetching features, consider routing through a dedicated proxy service (e.g. Cloudflare Workers) that enforces the rules outside the app's trust boundary.

API08:2023 — Security Misconfiguration

A grab bag, but the high-frequency ones:

• CORS too permissive — Access-Control-Allow-Origin: * on endpoints that return user data, or echoing back the Origin header unchanged
• Verbose errors in production — stack traces, DB errors, internal paths in 500 responses
• OPTIONS / TRACE enabled when not needed
• Missing security headers — Strict-Transport-Security, X-Content-Type-Options, etc. See [site-server-audit](../site-server-audit/SKILL.md).
• Default credentials somewhere in the stack (admin tools, DB admin UI, etc.)
• Debug endpoints accessible in production (/debug, /__profile__, /_admin/inspect)

Detection:

grep -rEn 'cors.*\\*|Access-Control-Allow-Origin.*\\*' --include='*.{js,ts,py}' .
grep -rEn 'NODE_ENV.*development|DEBUG.*True|app\\.debug' --include='*.{js,ts,py}' .
curl -sI https://api.example.com | grep -iE 'strict-transport|x-content-type|x-frame'

API09:2023 — Improper Inventory Management

The API you forgot about is the one that gets exploited:

• Old /v1/* endpoints still live after the app moved to /v2/*
• Staging environments with prod-like data accessible from the internet
• Mobile-app endpoints still serving older app versions long after they're deprecated, with weaker auth
• Internal endpoints that became externally reachable when networking changed

Mitigations:

• Inventory all hosts and endpoints quarterly. dig, subfinder, your DNS records, your reverse-proxy config
• Sunset old API versions with concrete dates communicated to consumers
• Stage-prod separation at the network layer, not just hope
• Robots/sitemap discipline — your sitemap should not list /api/*

API10:2023 — Unsafe Consumption of APIs

Your API calls third-party APIs (Stripe, SendGrid, OpenAI, weather, etc.). Treat third-party responses as untrusted input.

• Validate the response — schema-check, range-check, length-check. Do not blindly pipe into your DB.
• TLS verification on — never rejectUnauthorized: false in production
• Timeouts — third-party slowness becomes your outage if you wait forever
• Circuit breakers for repeatedly-failing third parties — fail fast, fall back gracefully
• Secret hygiene — third-party API keys are secrets, treat them as such ([secret-hygiene](../secret-hygiene/SKILL.md))

GraphQL-specific patterns

GraphQL adds attack surface even when REST patterns are addressed:

• Introspection on in production — turn off unless you need the playground for a reason
• Field-level authorization is the explicit responsibility of resolvers — @auth directives are good
• N+1 query patterns become DoS — DataLoader is essentially mandatory
• Query depth limit — most schemas should cap at 8–10 levels
• Query complexity limit — assign cost per field; total cap per request
• Aliasing for amplification — one query can request the same expensive field 100 times with aliases; limit alias count too

import depthLimit from 'graphql-depth-limit';
import { createComplexityRule } from 'graphql-query-complexity';

const server = new ApolloServer({
  schema,
  validationRules: [
    depthLimit(10),
    createComplexityRule({ maximumComplexity: 1000, /* ... */ }),
  ],
  introspection: process.env.NODE_ENV !== 'production',
});

API design patterns that prevent classes of issues

• Resource scoping in URLs — /users/me/invoices/:id instead of /invoices/:id. Forces the design to think about who owns the resource.
• Standard pagination — cursor-based, server-controlled page size, with documented limits
• Consistent error format — { error: { code, message } } with sanitized messages, codes mapped to client behavior
• Versioning explicit in the URL — /api/v1/, /api/v2/. Forces deprecation discipline.
• OpenAPI / GraphQL schema as contract — every endpoint is documented; what isn't documented isn't supposed to be reachable.
• Content-Type: application/json enforced on POST/PUT/PATCH — rejects naive form-encoded probes

Quick audit checklist

Walk every endpoint against:

• [ ] Auth required? Owner/role check in the handler (not just middleware)?
• [ ] Input validated against a schema?
• [ ] Output projection explicit — no leaking internal fields?
• [ ] Rate limit applied at per-endpoint or per-user level?
• [ ] Body size capped?
• [ ] Timeouts set?
• [ ] No SSRF: user-supplied URLs allowlisted before fetching?
• [ ] CORS scoped to known origins, not *?
• [ ] Errors are generic in production?
• [ ] If GraphQL: depth + complexity limits, introspection off in prod?
• [ ] Versioned and dated for sunset?
• [ ] Documented in your API catalog?

Anything you cannot answer → finding.

What this skill will not do

• Help test or exploit APIs you do not own
• Recommend disabling auth, CORS, or rate-limiting "for the dev environment that's also publicly reachable"
• Endorse trusting the client (browser, mobile app) for security-relevant checks

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: GoldenWing-360
• Source: GoldenWing-360/claude-security-skills
• License: MIT
• Homepage: https://goldenwing.at

Install and usage instructions live in the source repository linked above.