[Bug] iOS Phone Auth fails with about:blank on Unity 6 (6000.0.68+) due to UnityScene not forwarding URLs to FIRAuth

[TTfujita]

Description

Summary

iOS Phone Authentication fails on Unity 6 (6000.0.68f1 and later). After the user completes the reCAPTCHA image challenge, the SafariViewController gets stuck at about:blank and the SMS code is never sent. No errors are logged.

Root Cause

Starting from Unity 6000.0.68f1, Unity automatically inserts UIApplicationSceneManifest into Info.plist to support Apple's UIScene-based lifecycle (in preparation for the future iOS UIScene requirement). With this manifest, iOS routes incoming URLs to UnityScene.scene(_:openURLContexts:) instead of the legacy AppDelegate.application(_:open:options:).

However, UnityScene does not forward received URLs to the AppDelegate chain. Since Firebase Auth's swizzling mechanism hooks into the AppDelegate chain (and cannot intercept UISceneDelegate methods due to Apple's architecture), the reCAPTCHA callback URL never reaches FIRAuth, leaving the SafariViewController stuck at about:blank.

This is documented for native iOS developers at https://firebase.google.com/docs/auth/ios/phone-auth (see the SceneDelegate snippet calling [FIRAuth.auth canHandleURL:url]), but the equivalent handling is not provided in the Firebase Unity SDK, and the Unity Phone Auth documentation (https://firebase.google.com/docs/auth/unity/phone-auth) does not mention this requirement.

Workaround

Removing UIApplicationSceneManifest from Info.plist via a PostProcessBuild script restores the legacy AppDelegate routing and fixes the issue:

[PostProcessBuild(999)]
public static void OnPostProcess(BuildTarget target, string path)
{
    if (target != BuildTarget.iOS) return;
    var plistPath = path + "/Info.plist";
    var plist = new PlistDocument();
    plist.ReadFromFile(plistPath);
    if (plist.root.values.ContainsKey("UIApplicationSceneManifest"))
        plist.root.values.Remove("UIApplicationSceneManifest");
    plist.WriteToFile(plistPath);
}

This is a temporary workaround that will not survive Apple's UIScene mandate (expected in the iOS release following iOS 26).

Related Issues

• Unity Issue Tracker UUM-135497 (partially fixed in 6000.0.71f1, but URL forwarding to AppDelegate chain still missing): https://issuetracker.unity3d.com/issues/ios-application-dot-deeplinkactivated-does-not-get-invoked-while-app-is-running-when-uiapplicationscenemanifest-is-added-in-info-dot-plist
• Firebase iOS SDK Issue #4415 (AppDelegate proxy swizzling does not work with SceneDelegate): Twitter/OAuth Provider successful login callback not being not received firebase-ios-sdk#4415
• Firebase iOS SDK Release 11.2.0 (Phone Auth + SceneDelegate fixes for native iOS): https://github.com/firebase/firebase-ios-sdk/releases/tag/11.2.0

Request

Please consider adding UIScene support to the Firebase Unity SDK so that URL handling required by Firebase Auth works out of the box on Unity 6 projects. This will also become mandatory once Apple enforces the UIScene lifecycle in a future iOS release.

Other SDKs (e.g., AppLovin MAX's ATT prompt flow) appear to be affected by the same UIScene transition, suggesting this is a broader concern for Unity iOS apps.

Reproducing the issue

Environment

• Unity 6000.0.72f1 (also reproducible on 6000.0.68f1 through latest; NOT reproducible on 6000.0.67f1 or earlier)
• Xcode 16.1
• iOS device (real device required — Phone Auth cannot be tested on simulator)

Steps

1. Create a Unity 6 project (6000.0.68f1 or later) with Firebase Auth installed.
2. Configure Firebase Phone Authentication in the Firebase Console (enable Phone provider, set SMS region policy, upload APNs key).
3. Implement Phone Auth via PhoneAuthProvider.VerifyPhoneNumber in Unity.
4. Build for iOS with Xcode 16.x and deploy to a real device.
5. Run the app and trigger the phone authentication flow with a real phone number.
6. The reCAPTCHA WebView opens. Complete the image challenge (e.g., "Select all images with cars").
7. Observe: After completing the challenge, the WebView navigates to a firebaseapp.com URL and then becomes stuck at about:blank. The SafariViewController is never dismissed, and no SMS is sent.

Note: Reproducible with both real phone numbers and test phone numbers configured in Firebase Console — test numbers still go through the reCAPTCHA flow and hit the same wall, indicating the issue is not in the SMS sending logic but in the URL callback handling.

Expected behavior: SafariViewController should dismiss automatically after reCAPTCHA succeeds, and the SMS verification code should be sent to the provided phone number.

Actual behavior: SafariViewController remains stuck at about:blank. No SMS sent. No errors logged.

Firebase Unity SDK Version

12.10.0, 13.6.0

Unity editor version

6000.0.72f1

Installation Method

Unity Package Manager

Problematic Firebase Component(s)

Analytics, App Check, Authentication, Crashlytics

Other Firebase Component(s) in use

No response

Additional SDKs you are using

No response

Targeted Platform(s)

Apple Platforms

Unity editor platform

Mac

Scripting Runtime

IL2CPP

Release Distribution Type

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

Relevant Log Output

If using CocoaPods for Apple platforms, the project's Podfile.lock

Expand Podfile.lock snippet

👀 Replace this line with the contents of your Podfile.lock!

[TTfujita]

This is a summary compiled by Claude based on our investigation. Thank you in advance for looking into this.

[niccolo-hare]

Hi @TTfujita. Thanks for bringing this to our attention, and I apologize for not responding sooner. I tried this out with a sample project (with the latest SDK version) and was able to reproduce the same behavior. As an action, I've reached out to our engineers to confirm if this is a bug, and to possibly help expedite improvements to the SDK.

Let me know if you have questions.