Linking multiple providers in Firebase - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

What happens when you link multiple providers to a Firebase user?

In Firebase Authentication, what is the main effect of linking multiple providers (like Google and Facebook) to a single user account?

A Each provider creates a separate user account with isolated data.

B The user can sign in using any linked provider and access the same account data.

C Linking providers merges user data from all providers automatically without user consent.

D Linking providers disables sign-in from the original provider.

💡 Hint

Think about how linking providers affects user sign-in options.

Linking multiple providers allows a user to sign in with any linked provider but access the same Firebase user account and data.

❓ Configuration

intermediate

Which Firebase method links a new provider to an existing user?

Given an authenticated Firebase user, which method correctly links a new OAuth provider credential to their account?

Firebase

const credential = firebase.auth.GoogleAuthProvider.credential(idToken);
// User is already signed in
// Which method links the credential to the user?

A firebase.auth().currentUser.linkWithCredential(credential)

B firebase.auth().linkUserWithCredential(credential)

C firebase.auth().currentUser.signInWithCredential(credential)

D firebase.auth().linkCredentialToUser(credential)

💡 Hint

Check the Firebase user object methods for linking credentials.

The method linkWithCredential on the current user links a new provider credential to that user.

❓ Architecture

advanced

How to design user data storage for multiple linked providers?

You want to store user profile data in Firestore for users who sign in with multiple linked providers. What is the best practice for structuring this data?

A Store one user document per Firebase UID with a nested field listing all linked providers.

B Create separate user documents for each provider linked to the same email.

C Store user data only under the first provider used to sign up.

D Duplicate user data in multiple collections, one per provider.

💡 Hint

Consider how Firebase UID relates to user identity across providers.

Firebase UID is unique per user regardless of linked providers. Storing one document per UID with provider info nested avoids duplication and keeps data consistent.

❓ security

advanced

What security risk arises if you do not verify provider emails before linking?

When linking multiple providers to a Firebase user, what risk occurs if you do not verify that the email addresses from providers match and are verified?

A Users will be unable to sign in with any provider after linking.

B Firebase automatically prevents any security risks without verification.

C An attacker could link their provider account to another user's Firebase account and gain access.

D The linked providers will lose their authentication tokens.

💡 Hint

Think about what happens if two accounts share the same email but are not verified.

If emails are not verified before linking, an attacker could link their account with the victim's email and access the victim's Firebase user data.

❓ service_behavior

expert

What is the Firebase user state after unlinking a provider?

After unlinking a provider from a Firebase user who has multiple linked providers, what is the state of the user account?

A The user loses access to all linked providers until re-linked.

B The user is signed out immediately and must sign in again.

C The user account is deleted if any provider is unlinked.

D The user remains signed in and can sign in with remaining linked providers.

💡 Hint

Consider what unlinking a provider means for the user's authentication options.

Unlinking removes one provider but the user account remains active and signed in if other providers are linked.