Cache Poisoning in Fiber with Basic Auth

Cache poisoning occurs when an attacker manipulates a cache key so that malicious content is stored and later served to other users. In Fiber, this risk can emerge when authentication information such as Basic Auth credentials is included directly in the request URL or in headers that influence cache key generation. Because many caching layers use the full request URI or specific headers as the cache key, credentials embedded in the URL can fragment the cache in unintended ways or cause the server to cache user-specific authenticated responses in shared caches.

When Basic Auth is used with Fiber, the Authorization header is typically base64-encoded but not encrypted; if the application includes the Authorization header or the user:password portion of the URL in the cache key, a poisoned cache entry may be created for one user and reused for another. This can lead to horizontal privilege exposure, where one authenticated user receives another user’s cached response. Additionally, if the server-side route logic in Fiber conditionally varies responses based on Authorization headers without excluding those headers from cache normalization, an unauthenticated attacker may be able to probe endpoints and observe whether cache behavior changes, aiding in inferring valid credentials or account existence.

For example, consider a Fiber route that caches user profile data without normalizing the Authorization header out of the cache key. A request like GET /api/profile with Authorization: Basic dXNlcjpwYXNz might be cached separately from the same request without the header, causing inconsistent cache states. If an attacker can register or guess a valid Basic Auth credential, they might coerce the cache to store sensitive data under their credential and later retrieve it through a shared cache path. MiddleBrick’s LLM/AI Security checks include detection of unauthenticated LLM endpoints and active prompt injection testing, but for API cache risks, scanning with middleBrick can surface misconfigurations where authentication headers affect cache behavior, providing prioritized findings and remediation guidance to address the root cause.

In a black-box scan, middleBrick tests the unauthenticated attack surface and can identify whether responses vary significantly based on the presence or value of authentication headers, indicating potential cache poisoning vectors. Because scanning with middleBrick requires no credentials, it can safely probe public endpoints to detect inconsistencies that might be exploited when authentication is involved. Developers should ensure that cache keys exclude sensitive headers and that authenticated responses are marked appropriately to prevent storage in shared caches.

To mitigate cache poisoning when using Basic Auth in Fiber, ensure that authentication credentials are never used to compute cache keys and that authenticated responses are prevented from being stored in shared caches. The following practices and code examples illustrate how to secure Fiber routes that rely on Basic Auth.

First, avoid embedding credentials in the URL. Instead, always send Basic Auth credentials via the Authorization header. When implementing middleware or route handlers in Fiber, explicitly strip or ignore the Authorization header before any caching logic. Below is a minimal Fiber example that shows how to parse Basic Auth without allowing it to influence caching:

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

// Middleware to strip Authorization header from cache-sensitive logic
app.use((req, res, next) => {
  const auth = req.get('Authorization');
  if (auth && auth.startsWith('Basic ')) {
    // Do not use auth for cache key; remove or ignore it
    req.headers['x-auth-ignored'] = 'true';
  }
  next();
});

app.get('/api/profile', (req, res) => {
  // Perform authentication separately, e.g., validate credentials against a store
  const auth = req.get('Authorization');
  if (!auth || !isValidBasicAuth(auth)) {
    return res.status(401).send('Unauthorized');
  }
  const user = getCurrentUser(auth);
  // Ensure response is not cached in shared caches
  res.set('Cache-Control', 'no-store');
  res.json({ user: user.id, name: user.name });
});

function isValidBasicAuth(header) {
  try {
    const base64 = header.split(' ')[1];
    const decoded = Buffer.from(base64, 'base64').toString('utf8');
    const [user, pass] = decoded.split(':');
    return user && pass && /* validate against secure store */ true;
  } catch (e) {
    return false;
  }
}

function getCurrentUser(auth) {
  const base64 = auth.split(' ')[1];
  const decoded = Buffer.from(base64, 'base64').toString('utf8');
  const [user, pass] = decoded.split(':');
  // Replace with secure lookup
  return { id: 1, name: user };
}

Second, configure your caching layer to exclude the Authorization header from cache keys. If you use a reverse proxy or a CDN, ensure that the cache normalization rules omit the Authorization header. For Fiber applications that set custom cache headers, enforce no-store or private for authenticated responses, as shown above. This prevents sensitive or user-specific data from being stored where it could be served to others, effectively mitigating cache poisoning risks.

Finally, consider migrating from Basic Auth to token-based authentication where feasible, storing tokens in secure, httpOnly cookies or Authorization headers with short lifetimes. While not a direct code fix for Fiber’s caching behavior, reducing the surface area of credentials in URLs and headers lowers the chance of cache key collisions and makes cache poisoning significantly harder to achieve. middleBrick’s dashboard and CLI can be used to scan endpoints and verify that authentication headers are not influencing cache behavior, integrating into CI/CD pipelines via the GitHub Action to fail builds if risky configurations are detected.