Broken Authentication in Nestjs with Firestore

Broken Authentication in a NestJS application using Google Firestore typically arises from a mismatch between session management, token validation, and Firestore security rules. When authentication logic relies on opaque session identifiers or poorly scoped Firebase tokens, attackers can escalate privileges or impersonate users.

NestJS does not enforce a specific authentication style, so developers commonly integrate Firebase Admin SDK or custom JWT guards. If the guard validates a token but does not re-check Firestore-based user state (such as disabled flags or role changes), a formerly valid token can remain accepted after a user is disabled or roles are revoked. This creates a BOLA/IDOR vector where one user can act on another’s resources if ID references are exposed in URLs or API responses.

Firestore security rules must explicitly enforce ownership and role checks. Rules that allow broad read or write access (e.g., allowing read on a user’s document without verifying UID equality) amplify the impact of broken authentication. For example, a rule like allow read: if true; on user profiles lets any authenticated request enumerate data once access is obtained. Misconfigured CORS in Firestore rules can also expose endpoints to unauthorized origins, enabling token leakage via browser-side scripts.

Another vector involves token binding. If the NestJS backend issues its own JWTs without validating the Firebase ID token on each request, an attacker who obtains a token can reuse it indefinitely. Firestore does not natively revoke issued ID tokens; they expire based on Firebase settings. Without short-lived tokens and refresh strategies, stolen credentials remain valid across sessions. This is especially risky when combined with excessive agency patterns, where a token with elevated scopes is used across multiple services.

Insecure deserialization or unsafe consumption of Firestore data in NestJS controllers can also lead to authentication bypass. If a controller merges request parameters directly into Firestore queries without validation, an attacker can manipulate identifiers to access records outside their scope. For instance, using a user-supplied UID string to fetch documents without verifying it matches the authenticated UID enables BOLA. Logging or error messages that expose internal UIDs or tokens further aid enumeration.

Finally, unauthenticated LLM endpoints are not relevant here, but the broader principle of verifying every request context applies. In NestJS, always validate the Firebase ID token on each request, map it to a Firestore document, and enforce rule-level ownership checks. Combine this with rate limiting and strict CORS to reduce the attack surface of authentication flows in this stack.

Remediation centers on strict token validation, precise Firestore rules, and defensive coding in NestJS. Use the Firebase Admin SDK to verify ID tokens and map them to user state stored in Firestore. Never trust client-supplied identifiers alone; always cross-check with authenticated context.

Example: validating an ID token and retrieving user data in a NestJS guard:

import { Injectable, CanActivate, ExecutionContext } from '@nestjs/common';
import { FirebaseAuth } from 'firebase-admin/auth';
import { Firestore } from '@google-cloud/firestore';

@Injectable()
export class FirebaseAuthGuard implements CanActivate {
  constructor(private readonly auth: FirebaseAuth, private readonly firestore: Firestore) {}

  async canActivate(context: ExecutionContext): Promise {
    const request = context.switchToHttp().getRequest();
    const idToken = request.headers.authorization?.split('Bearer ')[1];
    if (!idToken) return false;

    try {
      const decoded = await this.auth.verifyIdToken(idToken);
      const userDoc = await this.firestore.collection('users').doc(decoded.uid).get();
      if (!userDoc.exists) return false;
      request.user = { uid: decoded.uid, ...userDoc.data() };
      return true;
    } catch {
      return false;
    }
  }
}

Example: Firestore security rule enforcing ownership and role checks:

rules_version = '2';
service cloud.firestore {
  match /databases/{database}/documents {
    match /users/{userId} {
      allow read, update: if request.auth != null && request.auth.uid == userId && request.auth.token.role == 'user';
      allow delete: if request.auth != null && request.auth.uid == userId && request.auth.token.role == 'admin';
    }
    match /posts/{postId} {
      allow read: if request.auth != null && resource.data.visibility == 'public';
      allow write: if request.auth != null && request.auth.uid == request.resource.data.userId;
    }
  }
}

In NestJS controllers, avoid using raw request parameters for Firestore queries. Instead, derive document IDs from the authenticated context:

import { Controller, Get, Param, UseGuards } from '@nestjs/common';
import { FirebaseAuthGuard } from './auth.guard';

@Controller('users')
@UseGuards(FirebaseAuthGuard)
export class UsersController {
  constructor(private readonly firestore: Firestore) {} 

  @Get(':id')
  async getUser(@Param('id') id: string, @Req() req: any) {
    if (req.user.uid !== id) {
      throw new ForbiddenException('Access denied');
    }
    const doc = await this.firestore.collection('users').doc(id).get();
    return doc.exists ? doc.data() : null;
  }
}

Enable short-lived ID tokens in Firebase console (e.g., 1 hour) and implement token refresh in your frontend. Use custom claims for roles and keep them updated via admin SDK. Add per-request rate limiting in NestJS to mitigate brute-force attempts against authentication endpoints. Combine these practices with regular scans using tools that check API security posture to detect drift in rules or configuration.