Cache Poisoning in Loopback

Cache poisoning in Loopback applications typically occurs through manipulation of cache keys or cache values that are derived from untrusted user input. In Loopback's data layer, this vulnerability can manifest in several ways:

// Vulnerable Loopback model method
async function getCachedUserData(req) {
  const userId = req.query.userId || 'default';
  const cacheKey = `user:${userId}`;
  
  // Attacker can control cacheKey via userId parameter
  // If userId = '../admin', cacheKey becomes 'user:../admin'
  // This could overwrite critical cache entries
  
  let cachedData = await cache.get(cacheKey);
  if (!cachedData) {
    cachedData = await User.findById(userId);
    await cache.set(cacheKey, cachedData);
  }
  return cachedData;
}

The most common Loopback-specific cache poisoning vectors include:

• Loopback Explorer/Discovery cache poisoning: Loopback's API Explorer caches OpenAPI specifications and model metadata. Attackers can manipulate cache keys through crafted requests to overwrite critical API documentation or model definitions.
• Loopback Boot script cache manipulation: Boot scripts that cache configuration data without proper validation can be poisoned when the configuration includes user-controlled parameters.
• Loopback Relations cache corruption: When caching related model data, improper handling of relation IDs can allow attackers to overwrite cache entries for other users' data.

A particularly dangerous pattern in Loopback involves caching database query results without proper key normalization:

// DANGEROUS: Cache key injection possible
const cacheKey = `query:${model}:${filter.raw}`;
// If filter.raw contains '../admin', cache poisoning occurs

Loopback's default caching mechanisms (Redis, memory, or database) don't automatically sanitize cache keys or validate cache values, making applications vulnerable to cache poisoning when user input influences cache operations.

Detecting cache poisoning in Loopback requires examining both the application code and runtime behavior. Here's how to identify this vulnerability:

Code Analysis: Look for these patterns in your Loopback application:

// Patterns to search for:
// 1. User input directly used in cache keys
const cacheKey = `user:${req.query.id}`;

// 2. Dynamic cache key construction
const cacheKey = 'data:' + someUserControlledValue;

// 3. Caching without input validation
await cache.set(userInput, sensitiveData);

Runtime Detection: Use middleBrick's API security scanner to detect cache poisoning vulnerabilities in your Loopback endpoints. middleBrick tests for cache manipulation by:

• Analyzing cache key construction patterns in your Loopback API responses
• Testing for cache key injection through parameter manipulation
• Checking for insecure cache value serialization that could allow code injection

middleBrick Scan Example:

npm install -g middlebrick
middlebrick scan https://your-loopback-app.com/api/users

The scan will identify specific endpoints vulnerable to cache poisoning, providing severity levels and remediation guidance. middleBrick's cache poisoning detection includes testing for:

• Path traversal in cache keys
• Parameter pollution affecting cache operations
• Insecure cache serialization formats

Manual Testing: Test your Loopback endpoints by attempting to manipulate cache keys through various input vectors like path parameters, query strings, and headers. Look for cache-related headers in responses and test if you can influence cache behavior through crafted requests.

Securing Loopback applications against cache poisoning requires implementing proper input validation and secure cache key construction. Here are Loopback-specific remediation strategies:

Input Validation and Sanitization:

const { sanitize } = require('loopback-context');

async function getCachedUserData(req) {
  const userId = sanitize(req.query.userId);
  
  // Validate userId format before using in cache key
  if (!/^[a-zA-Z0-9_-]+$/.test(userId)) {
    throw new Error('Invalid user ID format');
  }
  
  const cacheKey = `user:${userId}`;
  let cachedData = await cache.get(cacheKey);
  
  if (!cachedData) {
    cachedData = await User.findById(userId);
    await cache.set(cacheKey, cachedData, { ttl: 3600 });
  }
  return cachedData;
}

Loopback Boot Script Security: Secure your boot scripts that handle caching:

// server/boot/cache-security.js
module.exports = async function(app) {
  const cache = app.cache;
  
  // Create a secure cache wrapper
  const secureCache = {
    get: async (key) => {
      // Validate cache key format
      if (!/^[a-zA-Z0-9:._-]+$/.test(key)) {
        throw new Error('Invalid cache key format');
      }
      return await cache.get(key);
    },
    set: async (key, value, options) => {
      // Validate key and value
      if (!/^[a-zA-Z0-9:._-]+$/.test(key)) {
        throw new Error('Invalid cache key format');
      }
      // Optionally validate value structure
      await cache.set(key, value, options);
    }
  };
  
  // Replace default cache with secure wrapper
  app.cache = secureCache;
};

Loopback Model Method Security: Implement secure caching in model methods:

// common/models/user.js
module.exports = function(User) {
  User.cache = async function(id, options) {
    const cache = User.app.cache;
    const cacheKey = `user:${id}`;
    
    // Validate ID format
    if (!/^[a-fA-F0-9]{24}$/.test(id)) { // MongoDB ObjectId format
      throw new Error('Invalid user ID');
    }
    
    let cached = await cache.get(cacheKey);
    if (cached) return cached;
    
    const user = await User.findById(id);
    await cache.set(cacheKey, user, { ttl: 600 });
    return user;
  };
  
  User.remoteMethod('cache', {
    accepts: { arg: 'id', type: 'string' },
    returns: { arg: 'user', type: 'object' },
    http: { path: '/:id/cache', verb: 'get' }
  });
};

Loopback Explorer Security: Secure the API Explorer cache:

// server/middleware.json
{
  "explorer": {
    "params": {
      "proxy": false,
      "cache": {
        "maxAge": 300000, // 5 minutes
        "validateCacheKeys": true
      }
    }
  }
}

Rate Limiting Integration: Combine cache security with rate limiting to prevent abuse:

const rateLimit = require('express-rate-limit');

const cacheLimiter = rateLimit({
  windowMs: 15 * 60 * 1000, // 15 minutes
  max: 100, // limit each IP to 100 requests
  keyGenerator: (req) => {
    // Use sanitized cache key for rate limiting
    const userId = sanitize(req.query.userId);
    return `cache-limit:${userId}`;
  },
  message: 'Too many cache requests from this ID'
});

By implementing these Loopback-specific security measures, you can effectively prevent cache poisoning attacks while maintaining the performance benefits of caching in your Loopback applications.