Crash in FUIAuth when custom FUIAuthProvider uses signInWithCustomToken (Resulting Credential is nil)

[fangyuxi]

Step 1: Describe the issue

I am implementing a custom FUIAuthProvider to support a third-party login (e.g., Line Login) that integrates via a custom backend flow. The authentication process involves:

1. Getting an access token from the third-party SDK.
2. Exchanging it for a Firebase Custom Token via my backend.
3. Signing in using [[FIRAuth auth] signInWithCustomToken:...].

The Issue:
According to the Firebase SDK documentation, signInWithCustomToken successfully signs the user in but returns a FIRAuthDataResult where the credential property is nil. This is expected behavior for custom tokens as they don't generate an OAuth credential.

However, when I pass this nil credential to the FUIAuthProviderSignInCompletionBlock (to signal a successful login), FirebaseUI crashes internally.

It appears that FUIAuth or VerifyAssertionRequest assumes that a successful login (non-nil error) must be accompanied by a non-nil FIRAuthCredential object. It attempts to access properties on this nil object (likely provider), resulting in a Bad Access crash.

Step 2: Steps to reproduce

1. Create a class that conforms to FUIAuthProvider.
2. Implement signInWithDefaultValue:presentingViewController:completion:.
3. Inside the implementation, perform a sign-in using [[FIRAuth auth] signInWithCustomToken:token completion:...].
4. In the completion handler, observe that authResult.credential is nil.
5. Call the FirebaseUI completion block with this nil credential:

   // authResult.credential is nil here
   completion(authResult.credential, nil, nil, nil);

6. The app crashes immediately.

Code Sample

// Inside a custom FUIAuthProvider implementation
- (void)signInWithDefaultValue:(nullable NSString *)defaultValue
      presentingViewController:(nullable UIViewController *)presentingViewController
                    completion:(nullable FUIAuthProviderSignInCompletionBlock)completion {

    // ... logic to get custom token ...

    [[FIRAuth auth] signInWithCustomToken:customToken completion:^(FIRAuthDataResult * _Nullable authResult, NSError * _Nullable error) {
        if (error) {
            completion(nil, error, nil, nil);
            return;
        }

        // authResult.credential is NIL for custom tokens (Design behavior of the main SDK).
        // Passing this nil credential causes FUIAuth to crash.
        completion(authResult.credential, nil, nil, nil);
    }];
}

Step 3: Expected behavior

FirebaseUI should gracefully handle the case where a custom provider signs in successfully via a Custom Token (where no OAuth credential exists).

Since signInWithCustomToken is a valid way to authenticate, FUIAuth should check if [FIRAuth auth].currentUser is set, or allow a nil credential in the completion block without crashing, and proceed to the authUI:didSignInWith:error: delegate method.

Step 4: Actual behavior

The app crashes with EXC_BAD_ACCESS.

Crash Log snippet:

Task 55: EXC_BAD_ACCESS (code=1, address=0x8)
...
[VerifyAssertionRequest ...] 
...

(It seems to be trying to access a property, likely provider, on the nil credential object).

Step 5: Environment details

• Xcode version: [26]
• FirebaseSDK version: [last version]
• FirebaseUI version: [15.1.0],
• Installation method: CocoaPods

Workaround

To prevent the crash, I currently have to "trick" FirebaseUI by reporting a fake "User Cancelled" error when the custom token login succeeds. This stops FirebaseUI from processing the nil credential, and I manually handle the successful state in the FUIAuthDelegate.

// Current workaround
if (authResult.credential == nil) {
    // Manually dismiss the view controller on main thread
    dispatch_async(dispatch_get_main_queue(), ^{
        [self.presentingViewController dismissViewControllerAnimated:YES completion:nil];
    });

    // Send a fake cancel error to stop FUIAuth internal processing
    NSError *cancelError = [NSError errorWithDomain:NSCocoaErrorDomain code:NSUserCancelledError userInfo:nil];
    completion(nil, cancelError, nil, nil);
}

[russellwheatley]

Hey @fangyuxi - This issue relates to the old FirebaseUI which has now been removed from the repository. The latest FirebaseUI has been written in SwiftUI, here is a link to the documentation on installation if you would like to give it a try: https://github.com/firebase/FirebaseUI-iOS/blob/main/FirebaseSwiftUI/README.md