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Overview

Account linking allows users to authenticate with multiple SSO providers to the same user. This is automatically enabled in Nile Auth, and will be used if you have multiple SSO providers configured or if you choose to support both email/password and SSO authentication.
Combining SSO and email/password authentication requires the email address to be verified. You can set up email provider and templates in Nile console.

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How it works

When a user first signs up, no matter what authentication method they use, they are assigned a unique user ID that is linked to their email address. You can see this in the users.users table:
If the user then signs in with a different SSO provider, and the email address is the same, the new authentication method will be linked to the same user ID. You can see that the user ID has multiple credentials in the users.credentials table:
The user details in the users.users table are not affected by account linking. This means that the user will still have the same user ID, the same email address, and the same user details such as first_name, last_name, and picture. The only thing that changes is the addition of the new authentication method in the users.credentials table.

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Understanding Account Linking Scenarios

There are three scenarios that can occur when a user signs up with multiple authentication methods, and in order to maintain the security of the system, the behavior is slightly different for each scenario:

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User signs up with multiple SSO providers

If you have multiple SSO providers configured, and a user has the same email address across all of them, the user will be linked to the same user ID. This is the simplest and most secure scenario.

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User signs up with email/password and then signs in with SSO

If a user signs up with email/password and then signs in with a SSO provider, the SSO provider will be linked to the same user ID. However, unless the email address is verified in Nile Auth, the user will not be able to sign in to their email/password account after linking. Why? Because the email address is not verified, it is possible that a malicious actor created the same email address, and could potentially hijack the account. In order to prevent this, we require the email address to be verified before the user can sign in to their email/password account after linking. You can use our email verification feature to automatically send developers an email to verify their email address when they sign up. Or alternatively, you can manually verify any email address in the Nile console. For this reason, we strongly recommend that you verify email addresses in your production application. We support unverified email addresses, but we recommend limiting this to development environments.

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User signs up with SSO and then signs in with email/password

If a user signs up with a SSO provider and then wants to add email/password authentication to their account, they can do this by first signing in with their SSO provider, and then use the reset-password form to add email/password authentication to their account. This will create a new email/password credential for the user, and link it to the same user ID.