Verify App Check tokens from a custom backend

You can protect your app's non-Firebase resources, such as self-hosted backends, with App Check. To do so, you will need to do both of the following:

  • Modify your app client to send an App Check token along with each request to your backend, as described on the pages for iOS+, Android, and web.
  • Modify your backend to require a valid App Check token with every request, as described on this page.

Token verification

To verify App Check tokens on your backend, add logic to your API endpoints that does the following:

  • Check that each request include an App Check token.

  • Verify the App Check token using the Admin SDK.

    If verification succeeds, the Admin SDK returns the decoded App Check token. Successful verification indicates the token originated from an app belonging to your Firebase project.

Reject any request that fails either check. For example:

Node.js

If you haven't already installed the Node.js Admin SDK, do so.

Then, using Express.js middleware as an example:

import express from "express";
import { initializeApp } from "firebase-admin/app";
import { getAppCheck } from "firebase-admin/app-check";

const expressApp = express();
const firebaseApp = initializeApp();

const appCheckVerification = async (req, res, next) => {
    const appCheckToken = req.header("X-Firebase-AppCheck");

    if (!appCheckToken) {
        res.status(401);
        return next("Unauthorized");
    }

    try {
        const appCheckClaims = await getAppCheck().verifyToken(appCheckToken);

        // If verifyToken() succeeds, continue with the next middleware
        // function in the stack.
        return next();
    } catch (err) {
        res.status(401);
        return next("Unauthorized");
    }
}

expressApp.get("/yourApiEndpoint", [appCheckVerification], (req, res) => {
    // Handle request.
});

Python

If you haven't already installed the Python Admin SDK, do so.

Then, in your API endpoint handlers, call app_check.verify_token() and reject the request if it fails. In the following example, a function decorated with @before_request performs this task for all requests:

import firebase_admin
from firebase_admin import app_check
import flask
import jwt

firebase_app = firebase_admin.initialize_app()
flask_app = flask.Flask(__name__)

@flask_app.before_request
def verify_app_check() -> None:
    app_check_token = flask.request.headers.get("X-Firebase-AppCheck", default="")
    try:
        app_check_claims = app_check.verify_token(app_check_token)
        # If verify_token() succeeds, okay to continue to route handler.
    except (ValueError, jwt.exceptions.DecodeError):
        flask.abort(401)

@flask_app.route("/yourApiEndpoint")
def your_api_endpoint(request: flask.Request):
    # Handle request.
    ...

Go

If you haven't already installed the Admin SDK for Go, do so.

Then, in your API endpoint handlers, call appcheck.Client.VerifyToken() and reject the request if it fails. In the following example, a wrapper function adds this logic to the endpoint handlers:

package main

import (
    "context"
    "log"
    "net/http"

    firebaseAdmin "firebase.google.com/go/v4"
    "firebase.google.com/go/v4/appcheck"
)

var (
    appCheck *appcheck.Client
)

func main() {
    app, err := firebaseAdmin.NewApp(context.Background(), nil)
    if err != nil {
        log.Fatalf("error initializing app: %v\n", err)
    }

    appCheck, err = app.AppCheck(context.Background())
    if err != nil {
        log.Fatalf("error initializing app: %v\n", err)
    }

    http.HandleFunc("/yourApiEndpoint", requireAppCheck(yourApiEndpointHandler))
    log.Fatal(http.ListenAndServe(":8080", nil))
}

func requireAppCheck(handler func(http.ResponseWriter, *http.Request)) func(http.ResponseWriter, *http.Request) {
    wrappedHandler := func(w http.ResponseWriter, r *http.Request) {
        appCheckToken, ok := r.Header[http.CanonicalHeaderKey("X-Firebase-AppCheck")]
        if !ok {
            w.WriteHeader(http.StatusUnauthorized)
            w.Write([]byte("Unauthorized."))
            return
        }

        _, err := appCheck.VerifyToken(appCheckToken[0])
        if err != nil {
            w.WriteHeader(http.StatusUnauthorized)
            w.Write([]byte("Unauthorized."))
            return
        }

        // If VerifyToken() succeeds, continue with the provided handler.
        handler(w, r)
    }
    return wrappedHandler
}

func yourApiEndpointHandler(w http.ResponseWriter, r *http.Request) {
    // Handle request.
}

Other

If your backend is written in another language, you can use a general-purpose JWT library, such as one found at jwt.io, to verify App Check tokens.

Your token verification logic must complete the following steps:

  1. Obtain the Firebase App Check public JSON Web Key (JWK) Set from the App Check JWKS endpoint: https://firebaseappcheck.googleapis.com/v1/jwks
  2. Verify the App Check token’s signature to ensure it is legitimate.
  3. Ensure that the token’s header uses the algorithm RS256.
  4. Ensure that the token’s header has type JWT.
  5. Ensure that the token is issued by Firebase App Check under your project.
  6. Ensure that the token has not expired.
  7. Ensure that the token’s audience matches your project.
  8. Optional: Check that the token’s subject matches your app’s App ID.

The capabilities of JWT libraries can differ; be sure to manually complete any steps not handled by the library you choose.

The following example performs the necessary steps in Ruby using the jwt gem as a Rack middleware layer.

require 'json'
require 'jwt'
require 'net/http'
require 'uri'

class AppCheckVerification
def initialize(app, options = {})
    @app = app
    @project_number = options[:project_number]
end

def call(env)
    app_id = verify(env['HTTP_X_FIREBASE_APPCHECK'])
    return [401, { 'Content-Type' => 'text/plain' }, ['Unauthenticated']] unless app_id
    env['firebase.app'] = app_id
    @app.call(env)
end

def verify(token)
    return unless token

    # 1. Obtain the Firebase App Check Public Keys
    # Note: It is not recommended to hard code these keys as they rotate,
    # but you should cache them for up to 6 hours.
    uri = URI('https://firebaseappcheck.googleapis.com/v1/jwks')
    jwks = JSON(Net::HTTP.get(uri))

    # 2. Verify the signature on the App Check token
    payload, header = JWT.decode(token, nil, true, jwks: jwks, algorithms: 'RS256')

    # 3. Ensure the token's header uses the algorithm RS256
    return unless header['alg'] == 'RS256'

    # 4. Ensure the token's header has type JWT
    return unless header['typ'] == 'JWT'

    # 5. Ensure the token is issued by App Check
    return unless payload['iss'] == "https://firebaseappcheck.googleapis.com/#{@project_number}"

    # 6. Ensure the token is not expired
    return unless payload['exp'] > Time.new.to_i

    # 7. Ensure the token's audience matches your project
    return unless payload['aud'].include? "projects/#{@project_number}"

    # 8. The token's subject will be the app ID, you may optionally filter against
    # an allow list
    payload['sub']
rescue
end
end

class Application
def call(env)
    [200, { 'Content-Type' => 'text/plain' }, ["Hello app #{env['firebase.app']}"]]
end
end

use AppCheckVerification, project_number: 1234567890
run Application.new

Replay protection (beta)

To protect an endpoint from replay attacks, you can consume the App Check token after verifying it so that it can be used only once.

Using replay protection adds a network round trip to the verifyToken() call, and therefore adds latency to any endpoint that uses it. For this reason, we recommend that you enable replay protection only on particularly sensitive endpoints.

To use replay protection, do the following:

  1. In the Cloud console, grant the "Firebase App Check Token Verifier" role to the service account used to verify tokens.

    • If you initialized the Admin SDK with the Admin SDK service account credentials you downloaded from the Firebase console, the required role is already granted.
    • If you're using 1st generation Cloud Functions with the default Admin SDK configuration, grant the role to the App Engine default service account. See Changing service account permissions.
    • If you're using 2nd generation Cloud Functions with the default Admin SDK configuration, grant the role to the Default compute service account.

  2. Then, to consume a token, pass { consume: true } to the verifyToken() method and examine the result object; if the alreadyConsumed property is true, reject the request or take some kind of corrective action, such as requiring the caller to pass other checks.

    For example:

    const appCheckClaims = await getAppCheck().verifyToken(appCheckToken, { consume: true });

if (appCheckClaims.alreadyConsumed) {
    res.status(401);
    return next('Unauthorized');
}

// If verifyToken() succeeds and alreadyConsumed is not set, okay to continue.

    This verifies the token and then flags it as consumed. Future invocations of verifyToken(appCheckToken, { consume: true }) on the same token will set alreadyConsumed to true. (Note that verifyToken() does not reject a consumed token or even check if it's consumed if consume is not set.)

When you enable this feature for a particular endpoint, you must also update your app client code to acquire consumable limited-use tokens for use with the endpoint. See the client-side docs for Apple platforms, Android, and web.