Protect custom backend resources with App Check on Android

You can use App Check to protect non-Google custom backend resources for your app, like your own self-hosted backend. To do so, you'll 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 this page.
  • Modify your backend to require a valid App Check token with every request, as described in Verify App Check tokens from a custom backend.

Before you begin

Add App Check to your app, using either the default Play Integrity provider, or a custom provider.

Send App Check tokens with backend requests

To ensure your backend requests include a valid, unexpired, App Check token, wrap each request in a call to getAppCheckToken(). The App Check library will refresh the token if necessary, and you can access the token in the method's success listener.

Once you have a valid token, send it along with the request to your backend. The specifics of how you accomplish this are up to you, but don't send App Check tokens as part of URLs, including in query parameters, as this makes them vulnerable to accidental leakage and interception. The recommended approach is to send the token in a custom HTTP header.

For example, if you use Retrofit:

Kotlin 

class ApiWithAppCheckExample {
    interface YourExampleBackendService {
        @GET("yourExampleEndpoint")
        fun exampleData(
            @Header("X-Firebase-AppCheck") appCheckToken: String,
        ): Call<List<String>>
    }

    var yourExampleBackendService: YourExampleBackendService = Retrofit.Builder()
        .baseUrl("https://yourbackend.example.com/")
        .build()
        .create(YourExampleBackendService::class.java)

    fun callApiExample() {
        Firebase.appCheck.getAppCheckToken(false).addOnSuccessListener { appCheckToken ->
            val token = appCheckToken.token
            val apiCall = yourExampleBackendService.exampleData(token)
            // ...
        }
    }
}
ApiWithAppCheckExample.kt

Java 

public class ApiWithAppCheckExample {
    private interface YourExampleBackendService {
        @GET("yourExampleEndpoint")
        Call<List<String>> exampleData(
                @Header("X-Firebase-AppCheck") String appCheckToken);
    }

    YourExampleBackendService yourExampleBackendService = new Retrofit.Builder()
            .baseUrl("https://yourbackend.example.com/")
            .build()
            .create(YourExampleBackendService.class);

    public void callApiExample() {
        FirebaseAppCheck.getInstance()
                .getAppCheckToken(false)
                .addOnSuccessListener(new OnSuccessListener<AppCheckToken>() {
                    @Override
                    public void onSuccess(@NonNull AppCheckToken appCheckToken) {
                        String token = appCheckToken.getToken();
                        Call<List<String>> apiCall =
                                yourExampleBackendService.exampleData(token);
                        // ...
                    }
                });
    }
}
ApiWithAppCheckExample.java

Replay protection (beta)

When making a request to an endpoint for which you've enabled replay protection, wrap the request in a call to getLimitedUseAppCheckToken() instead of getAppCheckToken():

Kotlin 

Firebase.appCheck.limitedUseAppCheckToken.addOnSuccessListener {
    // ...
}
ApiWithAppCheckExample.kt

Java 

FirebaseAppCheck.getInstance()
        .getLimitedUseAppCheckToken().addOnSuccessListener(
                new OnSuccessListener<AppCheckToken>() {
                    @Override
                    public void onSuccess(AppCheckToken appCheckToken) {
                        String token = appCheckToken.getToken();
                        // ...
                    }
                }
        );
ApiWithAppCheckExample.java