Cache Poisoning in Nestjs with Api Keys

Cache poisoning occurs when an attacker causes a cache to store malicious content, leading other users to receive that content. In NestJS applications that use API keys for authentication, this risk arises when responses are cached without considering the authorization context provided by the API key. If a cache key is derived only from the request path and query parameters, but not the validated identity or scopes encoded in the API key, responses intended for one set of permissions may be served to another user or application. This can expose sensitive data or enable privilege abuse when cached responses are later retrieved by different clients.

For example, consider a GET endpoint that returns user-specific data, such as profile information. If this endpoint is cached based solely on the URL and query parameters, and the API key is passed only in headers, the caching layer may not differentiate between users. An attacker who can observe or guess another user’s cached response might infer information about that user’s data, or if the response includes sensitive headers or cookies, that content could be replayed. In a microservice architecture where caching is performed at an edge or gateway layer, this misalignment between API key validation and cache key composition can lead to cross-user data exposure, a pattern that aligns with BOLA/IDOR findings that middleBrick reports when runtime behavior mismatches authorization expectations.

Another scenario involves query parameters that influence data selection but are not included in the cache key, while the API key merely gates access at the perimeter. Suppose an endpoint supports filtering by tenant ID via a query parameter, and the API key encodes tenant claims. If the cache key excludes the tenant filter, a request for one tenant could be served to another, violating tenant isolation. This is particularly relevant when APIs use API keys tied to roles or scopes, and caching ignores those attributes. The scanner checks such authorization mismatches across 12 parallel security checks, including BOLA/IDOR and Property Authorization, to highlight where runtime responses differ from expected authorization boundaries defined by the API specification.

With unauthenticated scanning, middleBrick tests the surface without credentials and can identify whether caching behavior appears to ignore authorization headers by analyzing responses and spec definitions. When OpenAPI specifications include security schemes based on API keys, but runtime responses show data or behaviors not aligned with those schemes, findings are surfaced with severity and remediation guidance. This helps teams recognize that cache-related logic must incorporate the full context of authenticated requests, including API key attributes, to avoid unintentionally sharing or altering cached content across distinct clients.

To remediate cache poisoning risks when using API keys in NestJS, ensure cache keys incorporate the identity and scope expressed by the API key. This prevents cached responses from being shared across users or tenants with different authorization contexts. Below are concrete code examples showing how to integrate API key validation and caching logic safely.

import { Injectable, NestMiddleware } from '@nestjs/common';
import { Request, Response } from 'express';

@Injectable()
export class ApiKeyMiddleware implements NestMiddleware {
  private readonly apiKeys = new Set(['trusted-key-abc123', 'trusted-key-xyz789']);

  use(req: Request, res: Response, next: () => void) {
    const key = req.headers['x-api-key'] as string | undefined;
    if (!key || !this.apiKeys.has(key)) {
      res.status(401).send('Forbidden');
      return;
    }
    // Attach normalized identity to request for downstream cache and handler use
    req.apiKey = {
      key,
      tenant: this.extractTenant(key),
      scopes: this.scopesFor(key),
    };
    next();
  }

  private extractTenant(key: string): string {
    // Map key to tenant, e.g., via lookup or prefix
    if (key.startsWith('tenantA-')) return 'tenantA';
    if (key.startsWith('tenantB-')) return 'tenantB';
    return 'unknown';
  }

  private scopesFor(key: string): string[] {
    if (key === 'trusted-key-abc123') return ['read:profile'];
    if (key === 'trusted-key-xyz789') return ['read:profile', 'read:admin'];
    return [];
  }
}

In your caching layer, include tenant and scopes in the cache key. For example, if using a simple in-memory or external cache, structure the key to reflect the validated API key context:

import { Injectable } from '@nestjs/common';

@Injectable()
export class ProfileService {
  private cache = new Map();

  getProfile(userId: string, apiKeyMeta: { tenant: string; scopes: string[] }) {
    const cacheKey = `profile:${apiKeyMeta.tenant}:${apiKeyMeta.scopes.join(',')}:${userId}`;
    const cached = this.cache.get(cacheKey);
    if (cached !== undefined) {
      return cached;
    }
    // Simulate fetching data
    const data = { userId, tenant: apiKeyMeta.tenant, role: apiKeyMeta.scopes };
    this.cache.set(cacheKey, data);
    return data;
  }
}

Additionally, configure your caching strategy to vary by relevant request dimensions that the API key implies. If your API specification defines security requirements via API keys, ensure runtime behavior honors those requirements in cache lookups. The middleBrick CLI can be used to scan endpoints and verify whether caching behavior aligns with declared security schemes. By running middlebrick scan <url>, you can detect potential mismatches where cached responses may leak data across tenants or privilege levels, and the report will include findings mapped to frameworks such as OWASP API Top 10 to guide remediation.

For teams using the Pro plan, continuous monitoring and CI/CD integration can help detect regressions where caching logic inadvertently ignores authorization context. The dashboard allows tracking of scan results over time, and the GitHub Action can fail builds if risk scores exceed configured thresholds, ensuring that future changes do not reintroduce cache poisoning risks related to API key handling.