Cache Poisoning in Fiber with Api Keys

Cache poisoning occurs when an attacker manipulates cached responses so that subsequent users receive malicious or incorrect data. In Fiber, this risk can become more tangible when API keys are handled in ways that bypass cache normalization or leak into cache keys inadvertently.

When API keys are passed in HTTP headers and those headers are not excluded from cache normalization, a caching layer (or reverse proxy) may treat each unique key as a distinct cache entry. This can lead to a cache that contains user-specific or role-specific responses stored under keys that an attacker can probe or guess. For example, if a response for an administrative endpoint is cached together with the requester’s API key in the cache key, a lower-privilege user might be able to reuse or infer cached entries that should have been isolated per principal.

Additionally, if an API key is reflected in URLs or cache-related headers (e.g., Vary) without strict normalization, an attacker can intentionally vary the key to probe whether different keys produce cache hits for sensitive data. This can expose data that should remain segregated, effectively turning the cache into a cross-user data leakage channel. The unauthenticated attack surface that middleBrick scans can surface such normalization issues by correlating OpenAPI spec definitions with runtime behavior, highlighting endpoints where authentication parameters influence cacheability.

Another scenario involves caching of error or redirect responses that include API key material in query strings. If those responses are cached and later served to other clients, the key may be exposed in logs or to unauthorized users. Proper Vary semantics and strict header-based cache rules are essential to ensure that sensitive routing material does not inadvertently populate shared cache entries.

To mitigate cache poisoning risks related to API keys in Fiber, ensure that cache keys exclude sensitive headers and that responses with key-dependent data are not cached broadly. Below are concrete examples showing how to configure routes and middleware in Fiber to avoid these pitfalls.

Example 1: Skipping caching for requests with an API key header

Use middleware to bypass caching when an API key header is present:

const { app } = require('express-rate-limit'); // illustrative; use fiber-native patterns
const fiber = require('fiber');
const app = fiber();

// Middleware to skip cache when API key is present
app.use((req, res, next) => {
  if (req.headers['x-api-key']) {
    res.set('Cache-Control', 'no-store');
  } else {
    res.set('Cache-Control', 'public, max-age=3600');
  }
  next();
});

app.get('/data', (req, res) => {
  res.json({ message: 'public data' });
});

app.listen(3000);

Example 2: Explicitly controlling Vary to avoid key-based fragmentation

Ensure responses are not unnecessarily fragmented by API key by setting Vary only on non-sensitive headers:

const fiber = require('fiber');
const app = fiber();

app.get('/items', (req, res) => {
  // Do NOT vary on X-API-Key to prevent cache fragmentation by key
  res.set('Vary', 'Accept-Encoding');
  res.json({ items: ['item1', 'item2'] });
});

app.listen(3000);

Example 3: Using a reverse proxy or gateway to strip API keys from cache lookups

While Fiber handles application logic, offload cache normalization to a gateway that strips or hashes API keys before caching. In Fiber you can enforce no-cache for authenticated routes:

const fiber = require('fiber');
const app = fiber();

// Enforce no-store for authenticated endpoints
const ensureAuth = (req, res, next) => {
  const apiKey = req.headers['x-api-key'];
  if (apiKey) {
    // Mark as private/no-cache to prevent shared caching
    res.set('Cache-Control', 'no-cache, no-store, must-revalidate');
    res.set('Pragma', 'no-cache');
    res.set('Expires', '0');
  }
  next();
};

app.get('/secure', ensureAuth, (req, res) => {
  res.json({ secret: 'value' });
});

app.listen(3000);

General recommendations

• Do not include API keys in URLs or in headers that influence cache key generation.
• Set explicit Cache-Control and Vary headers to avoid caching sensitive or user-specific responses in shared caches.
• Validate and normalize incoming headers to prevent cache key injection via malformed or duplicate header names.
• Use middleware to enforce no-store for responses that contain or depend on API key context.

These practices reduce the likelihood that cached responses are incorrectly shared across users or used in cache poisoning attempts that exploit API key handling.