[Question][Auth] Can FirebaseAuth.CurrentUser be initialized from an existing ID token + refresh token?

[qomo-wq]

What is your question?

I want to know whether the Firebase Unity SDK has any supported way to initialize FirebaseAuth.CurrentUser from an already-issued Firebase Auth session.

My intended flow is:

1. My trusted backend uses Firebase Admin SDK to create a custom token.
2. My backend calls the Identity Toolkit signInWithCustomToken endpoint.
3. The backend receives the Firebase ID token, refresh token, and expiration.
4. The backend returns that session information to the Unity client.
5. I want FirebaseAuth in Unity to adopt that session so FirebaseAuth.CurrentUser is populated and Firebase services such as Firestore continue working normally.

I specifically want to avoid calling SignInWithCustomTokenAsync() from the Unity client, because the custom-token exchange and future Secure Token refreshes would be handled by my backend.

Is there currently any supported API in the Firebase Unity SDK to initialize/hydrate FirebaseAuth from:

• ID token
• refresh token
• expiration time

If not, is there another supported way to achieve this while keeping FirebaseAuth.CurrentUser and direct Firestore SDK access working normally?

Why I am asking for this?
The reason for this architecture is that I am investigating using VPC Service Controls to restrict direct client access to identitytoolkit.googleapis.com and securetoken.googleapis.com, while allowing only my trusted backend to access those endpoints. If that architecture is viable, I would need Unity Firebase Auth to adopt the ID token + refresh token returned by my backend without calling those Auth endpoints directly.

Firebase Unity SDK Version

13.9.0

Unity editor version

6000.4.0f1

Installation Method

.unitypackage

Problematic Firebase Component(s)

No response

Other Firebase Component(s) in use

No response

Additional SDKs you are using

No response

Targeted Platform(s)

Android, Apple Platforms

Unity editor platform

Mac

Scripting Runtime

IL2CPP

Release Distribution Type

Pre-built SDK from https://firebase.google.com/download/unity

[AustinBenoit]

Hi @qomo-wq,

The Firebase Unity SDK (and underlying C++/native SDKs) currently does not support injecting or hydrating an existing ID token and refresh token into FirebaseAuth.CurrentUser.

FirebaseAuth manages the token lifecycle internally and acts as the credential provider for downstream SDKs like Firestore. There is no API to inject externally-obtained tokens into the client SDK. Additionally, token refreshes rely on hardcoded calls to securetoken.googleapis.com (and identitytoolkit.googleapis.com), with no supported way to redirect those endpoints through a custom proxy in production.

If your VPC Service Controls strictly block client access to Google Auth endpoints, you might want to consider handling database operations through your own backend service (e.g., querying Firestore via the Firebase Admin SDK inside your VPC) rather than connecting directly from the client SDK.

--Austin