Rate Limiting Bypass in Fiber with Api Keys

Rate limiting in Fiber can be bypassed when API keys are handled inconsistently between the rate limiter and authentication layers. This typically occurs when keys are validated after rate limit checks, or when multiple key formats (e.g., query, header, cookie) are not enforced uniformly. An attacker can exploit this by cycling through valid keys before the limiter refreshes, or by omitting the key from the rate-limited request path to avoid triggering the limit.

Consider a Fiber route where an API key is read from a header but the rate limiter is configured only on IP. If the key is validated later in middleware, an unauthenticated attacker can send many requests from the same IP without a key, consuming the rate limit budget intended for authenticated clients. Even when keys are used, if the rate limiter does not scope limits per key, a single compromised key can exhaust the quota and impact all users sharing that key. Compounded by non‑uniform enforcement—such as accepting keys in query parameters but only checking headers in the limiter—an attacker can switch injection points to stay under the threshold.

In practice, this maps to common weaknesses such as CWE‑770 (Insufficient Resource Management) and aligns with OWASP API Top 10:2023 Broken Object Level Authorization and Excessive Resource Consumption. A scanner testing unauthenticated endpoints can detect missing per‑key rate limiting and inconsistent key usage by observing whether responses differ when keys are included or omitted, and whether limits reset on key rotation.

Leveraging OpenAPI/Swagger spec analysis (2.0, 3.0, 3.1) with full $ref resolution, middleBrick cross‑references spec definitions with runtime findings to highlight mismatches between documented authentication and actual rate limiting behavior. This helps identify whether security checks are applied at the correct layers and whether the spec accurately reflects key usage in path, query, and header parameters.

Remediation focuses on consistent key handling and scoping limits to the key itself. In Fiber, ensure the rate limiter inspects the same key source used for authentication, and apply limits per key rather than globally or per IP. Avoid deferring key validation; validate before entering any rate‑limited route group. Use a deterministic key extraction function so all middleware see the same identifier.

Example: define a middleware that reads the API key from the Authorization header and attaches it to the context, then configure the rate limiter to use that context value.

import { Router, Request, Response, NextFunction } from 'express';
import { rateLimit } from 'express-rate-limit';

const app = express();

// Key extraction middleware
app.use((req: Request, res: Response, next: NextFunction) => {
  const key = req.get('x-api-key') || req.query.api_key as string | undefined;
  (req as any).apiKey = key || 'unauthenticated';
  next();
});

// Per‑key rate limiter
const keyLimiter = rateLimit({
  windowMs: 60_000,
  delayMs: 0,
  keyGenerator: (req: Request) => (req as any).apiKey,
  handler: (_req, res) => {
    res.status(429).json({ error: 'Too many requests' });
  },
});

app.use('/api', keyLimiter);

app.get('/api/endpoint', (req: Request, res: Response) => {
  res.json({ ok: true });
});

app.listen(3000);

If your API keys are stored server‑side, validate them within the same middleware chain before the limiter runs, and ensure the rate limiter uses a stable key identifier (e.g., the key ID or a normalized token) rather than the raw secret. For multi‑tenant scenarios, scope limits to tenant or key IDs to prevent cross‑tenant exhaustion. Rotate keys promptly on detection of abuse and monitor per‑key metrics to detect anomalies.

middleBrick can support this remediation by scanning your endpoints to verify that authentication and rate limiting are consistently applied per key. With the CLI (middlebrick scan <url>) or via the GitHub Action to fail builds on risky configurations, you can detect inconsistencies before deployment. The dashboard helps track scores over time, and the Pro plan’s continuous monitoring can alert you when per‑key rate limiting is missing or uneven across endpoints.