Cache Poisoning in Restify with Api Keys

Cache poisoning occurs when an attacker manipulates cached responses so that malicious content is served to other users. In Restify, this risk can emerge when API keys are handled in ways that cause cache keys to vary by sensitive values without appropriate normalization, or when keys are inadvertently reflected in response bodies or headers. If a Restify service uses the API key as part of the cache key but does not strip or isolate it before storage, responses containing user-specific data may be cached under a key that other users can trigger, leading to information disclosure across users.

Consider a scenario where a Restify endpoint appends the API key to a downstream HTTP request and caches the result based on the full request URL including the key. Two different clients with different API keys would produce different cache entries, but if the service also caches error or metadata responses by a non-keyed path, an attacker who can observe or influence the key (for example through logs or error messages) may learn whether a given key exists or infer usage patterns. More critically, if the API key is used as a cache identifier without hashing or normalization, cached entries may not be shared appropriately, causing excessive cache misses and increasing exposure to cache-based side channels.

In practice, this combination violates the principle of separating authentication context from cache identity. Restify middleware that adds the API key to downstream headers or query parameters may inadvertently cause cached responses to be keyed on values that should remain private. An attacker who can cause the server to request the same logical resource with two different keys may observe timing differences or error messages that reveal whether the cache was populated, leading to inference or cache poisoning attacks. These issues align with the broader OWASP API Top 10 category of Broken Object Level Authorization (BOLA) and can be surfaced by middleBrick as a BOLA/IDOR finding when the scanner detects inconsistent authorization across cached resources.

Additional risk arises when responses include sensitive headers or tokens that should not be cached. If Restify caches responses that contain the API key in a header or body, and later serves that cached response to another caller, the key may be unintentionally exposed. For example, a caching layer that does not strip authorization-related headers may return a response that includes a key-derived token to an unauthorized user. middleBrick’s Data Exposure and Property Authorization checks are designed to detect such inconsistencies by correlating runtime behavior with OpenAPI specifications, highlighting endpoints where sensitive data appears in cacheable contexts.

To summarize, the vulnerability is not caused by Restify or API keys alone, but by the interaction between caching logic and how keys are used in request processing and cache key construction. When keys are treated as part of the cacheable request identity without proper isolation or normalization, the attack surface expands to include cross-user data exposure and potential cache poisoning. Security scanning that maps findings to compliance frameworks such as OWASP API Top 10 helps prioritize these risks.

Remediation focuses on ensuring that API keys never directly influence cache keys or cached content, and that responses containing keys are never stored or shared. Below are concrete Restify patterns that address these concerns.

1. Normalize cache keys by stripping sensitive headers

Before caching a response, remove or hash headers that contain API keys. This ensures that different keys for the same logical request map to the same cache entry only when appropriate, and prevents key leakage via cache storage.

const restify = require('restify');
const crypto = require('crypto');

function normalizeCacheKey(req) {
  // Copy incoming headers to avoid mutating the original request
  const headers = { ...req.headers };
  // Remove or replace sensitive authorization headers
  delete headers['x-api-key'];
  delete headers.authorization;
  // Create a stable string for caching
  const base = req.method + ':' + req.url + ':' + JSON.stringify(headers);
  return crypto.createHash('sha256').update(base).digest('hex');
}

const server = restify.createServer();
server.use((req, res, next) => {
  const key = normalizeCacheKey(req);
  // Use key with your caching layer, e.g., redis.get(key)...
  req.cacheKey = key;
  return next();
});

2. Do not propagate API keys to downstream services when caching

If your service forwards requests to another API, avoid including the client’s API key in forwarded queries or headers when the response may be cached. Instead, use a server-side credential or remove the key for cacheable requests.

const restify = require('restify');
const axios = require('axios');

server.get('/data', async (req, res, next) => {
  const clientKey = req.headers['x-api-key'];
  // For cacheable endpoints, do not forward the client key
  const upstreamUrl = 'https://internal-service.example.com/data';
  const response = await axios.get(upstreamUrl, {
    headers: {
      // Use a server-side identifier instead
      'X-Server-Key': process.env.SERVER_KEY
    },
    // Ensure query does not include client key
    params: req.query
  });
  res.send(response.data);
  return next();
});

3. Strip sensitive headers from cached responses

Ensure that responses stored in cache do not contain API keys or tokens. Use a middleware layer to remove sensitive headers before caching.

function stripSensitiveHeaders(obj) {
  const headers = obj.headers || {};
  delete headers['x-api-key'];
  delete headers.authorization;
  delete headers['set-cookie'];
  return { ...obj, headers };
}

// Example integration point depends on your caching layer
server.on('after', (req, res, route, err) => {
  const safeRes = stripSensitiveHeaders(res);
  // Store safeRes in cache using req.cacheKey
});

4. Use short TTLs for sensitive endpoints

For endpoints that must include keys for routing or auditing, reduce cache lifetime or disable caching entirely to limit exposure windows.

server.get('/account/:id', (req, res, next) => {
  // Explicitly disable caching for sensitive routes
  res.setHeader('Cache-Control', 'no-store');
  // Proceed with request handling
  return next();
}, handler);

These patterns align with remediation guidance that can be part of a Pro plan’s continuous monitoring and GitHub Action integration, where PR gates can enforce that cache-related code does not reintroduce key exposure. The Dashboard can track changes over time to confirm that sensitive headers are consistently omitted from cached flows.