Assessing Custom Claims Security in Firebase Authentication JWT Tokens

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Determine whether custom claims in JSON Web Tokens (JWT) from Firebase Authentication can be modified (falsified) during the execution of onCall functions in Firebase.

**Conditions:**
1. Firebase Authentication is used for user authentication.
2. Firebase JWT tokens include additional custom data (claims) that define user permissions or roles.
3. It is necessary to understand whether there is a risk of falsification or modification of these custom claims by external users when invoking onCall functions.

**Criteria for Execution:**
1. Establish how Firebase protects custom claims in JWT tokens.
2. Evaluate the security of using custom claims in onCall functions.
3. Describe any potential vulnerabilities or confirm the security of the implementation.

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To solve this task, we should consider the principles of Firebase Authentication working with JWT tokens and the protection mechanisms that ensure the integrity of custom claims. Here’s a complete breakdown:

### 1. How JWT Tokens Work in Firebase Authentication
Firebase Authentication uses JSON Web Tokens (JWT) to verify user authenticity in applications. These tokens consist of three main parts:
- **Header**: Contains information about the token type and the signing algorithm.
- **Payload**: Includes standard claims (user data) and custom claims.
- **Signature**: Ensures the integrity of the data in the token and confirms that it was indeed issued by Firebase.

Each token is signed by Firebase using a secret key known only to Firebase, which protects the token from forgery.

### 2. How Custom Claims Work in Firebase
Custom claims are additional data (e.g., user roles) added to the token and used for access control. These claims are added by an administrator or server logic (e.g., Firebase Cloud Functions using the Firebase Admin SDK).

### 3. Security of Custom Claims in onCall Functions
In Firebase onCall functions, an authentication verification mechanism operates:
- When a client calls an onCall function, Firebase automatically attaches the user's authentication token to the request.
- On the server side, Firebase verifies the signature of this token to ensure that it was issued by Firebase and that its claims (including custom claims) have not been altered.
- **Thus, the client cannot change custom claims in the token**: any attempt to modify the token will result in its rejection due to an invalid signature.

### 4. How to Use Custom Claims in onCall Functions
To ensure that you have the most current claims, you should use `auth.token` after verifying the token in onCall functions. For example:

```javascript

Code: Select all

const functions = require('firebase-functions');
const admin = require('firebase-admin');
admin.initializeApp();

exports.secureFunction = functions.https.onCall(async (data, context) => {
  // Check if the user is authenticated
  if (!context.auth) {
    throw new functions.https.HttpsError(
      'unauthenticated',
      'User must be authenticated'
    );
  }

  // Retrieve custom claims
  const claims = context.auth.token;
  if (claims.role !== 'admin') {
    throw new functions.https.HttpsError(
      'permission-denied',
      'Access denied'
    );
  }

  // Allow function execution for admin
  return { message: 'Access granted' };
});

```

### 5. Conclusions
- **Falsification of custom claims in Firebase is impossible**, as they are signed by Firebase, and any modification will lead to token rejection.
- Use claims in onCall functions to manage access based on roles or privileges.
- Keep in mind that changes to claims take effect only after the token is refreshed, so after updating custom claims, the user will need to log in again to receive a new token.