Cache Poisoning in Fiber with Jwt Tokens

Cache poisoning in a Fiber application that uses JWT tokens occurs when an attacker manipulates cached responses so that a token or token-derived value is shared across users or contexts where it should not be. Because JWTs often carry authorization claims and may be accepted as identification or permission proof, serving a cached response containing a token intended for one user to another user can lead to privilege escalation or identity confusion.

In setups where responses are cached based on request path or query parameters but the authorization context (e.g., the JWT) is not part of the cache key, a user who receives an authenticated response may see another user’s token embedded in the payload or headers. For example, if an endpoint returns user profile data along with a JWT and that response is cached without considering the Authorization header, subsequent requests from a different user could receive the cached token-bearing response, effectively inheriting permissions they should not have.

Another scenario involves query parameters that include or reference tokens, such as an API endpoint that accepts a token in a query string for introspection or exchange. If such responses are cached and later served to different clients, the token can be leaked across users. This violates the principle that cached content should be scoped to the exact request context, including headers and cookies that carry authorization data.

Because Fiber is a fast, minimalist web framework, developers may inadvertently cache responses without ensuring that JWT-related authorization is excluded from the cache key. The risk is not in how tokens are validated, but in how cached outputs are reused across distinct authenticated sessions. MiddleBrick detects such misconfigurations by correlating OpenAPI specifications with runtime behavior to identify endpoints where token-bearing responses could be cached unsafely.

To remediate cache poisoning risks related to JWT tokens in Fiber, ensure that responses containing tokens are either not cached or are cached with request context that includes the Authorization header. Below are concrete code examples demonstrating secure handling of JWTs in Fiber with proper cache considerations.

First, avoid returning JWTs in responses that might be cached. Instead, return tokens only to the intended client and ensure caching mechanisms exclude authenticated routes or strip sensitive payloads.

// Secure JWT issuance without caching sensitive data
const jwt = require('jsonwebtoken');
const express = require('express'); // Fiber-compatible patterns apply
const app = express();

app.post('/login', (req, res) => {
  const user = { id: 123, role: 'user' };
  const token = jwt.sign({ sub: user.id, role: user.role }, process.env.JWT_SECRET, { expiresIn: '1h' });
  // Do not cache this response; set headers to prevent caching
  res.set('Cache-Control', 'no-store, no-cache, must-revalidate, private');
  res.json({ token });
});

Second, if you must cache authenticated responses, scope the cache key to include the Authorization header or a canonical representation of the JWT claims. This prevents one user’s token-bearing response from being served to another.

// Example cache-key construction that includes JWT context
const getCacheKey = (req) => {
  const base = req.originalUrl || req.path;
  const auth = req.headers['authorization'] || '';
  return `${base}:auth:${auth}`;
};

app.get('/profile', (req, res) => {
  const cacheKey = getCacheKey(req);
  // pseudo-cache lookup using cacheKey
  const cached = cache.get(cacheKey);
  if (cached) {
    return res.json(cached);
  }
  // generate response with user-specific data and token
  const token = jwt.sign({ sub: req.user.id }, process.env.JWT_SECRET);
  const payload = { user: req.user, token };
  cache.set(cacheKey, payload, { ttl: 60 });
  res.json(payload);
});

Third, validate token scope and avoid caching any endpoint that echoes or reuses JWT claims in a way that could be cross-user. Ensure that token validation happens before any cached data is considered, and do not rely on cached responses to enforce authorization.

// Middleware to validate JWT and ensure no cached unauthorized data is used
const authenticateJwt = (req, res, next) => {
  const auth = req.headers['authorization'];
  if (!auth || !auth.startsWith('Bearer ')) {
    return res.status(401).json({ error: 'unauthorized' });
  }
  const token = auth.substring(7);
  try {
    const decoded = jwt.verify(token, process.env.JWT_SECRET);
    req.user = decoded;
    next();
  } catch (err) {
    return res.status(401).json({ error: 'invalid token' });
  }
};

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

By combining no-cache headers for token issuance, context-aware cache keys, and strict per-request JWT validation, you reduce the chance that cached responses expose or misuse JWT tokens. MiddleBrick highlights endpoints where token-bearing responses lack proper cache scoping and recommends aligning caching behavior with the sensitivity of JWT data.