SharePoint Online & Sign-In Acceleration – SSO for SPO

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SPO

For those using SharePoint Online, you may have noticed that it does not provide the same home realm discovery that Exchange Online does, with Home Realm discovery meaning the services knows what domain/upn is being used and leveraging that authentication type for the user (Managed/Online versus Federated/ADFS).  SharePoint Online support now supports Sign-In Acceleration, which allows SPO to understand whether a browsing user is a Federated user and send to https://login.microsoftonline.com, with a directive that instructs login.microsoftonline.com to then forward the authentication request on to their ADFS deployed endpoint (i.e. https://sts.contoso.com).

Note – To enable this for your Office 365 tenant, please log a support case for this request and they can fulfill your request!

Explanation

Once auto-acceleration is enabled, the SPO authentication process works as follows:

  1. The user navigates to https://contoso.sharepoint.com in their web browser.
  2. SharePoint receives the request and detects that auto-acceleration is enabled for this tenant by leveraging the domain name in the domain.sharepoint.com URL being used to access SPO.
  3. The user is then sent to login.microsoftonline.com with extra information in the URL (a whr tag). This tag indicates to AAD that it is safe to accelerate the user directly to the ADFS endpoint, login.contoso.com.
  4. Once there, the user may enter their credentials and sign-in. In the case of domain-joined machines, the user will be signed in immediately based on browser settings for SSO.

This effectively allows SPO to provide home realm discovery and SSO for users, like Exchange Online does today and will make your SPO users much more happy, reducing the authentication prompts and requests for UPNs/Passwords.

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