Single Sign-On (SSO) Microsoft

Single Sign-On is the difference between an Office add-in that feels like part of Microsoft 365 and one that feels like a third-party tool stapled to it. When SSO is implemented correctly, the user opens the add-in and it already knows who they are: same account, same session, no extra login step.

We implement SSO for Office add-ins using the Office.js getAccessToken API combined with MSAL and an Azure AD app registration configured for the add-in context. The token the add-in receives can be used directly to identify the user or exchanged via the On-Behalf-Of flow for a Microsoft Graph token that accesses Microsoft 365 data.

The implementation details that trip up most teams: the app manifest must be configured correctly or getAccessToken returns errors that look unrelated to the manifest. The On-Behalf-Of exchange requires a server-side component, since it cannot happen in client-side JavaScript. Fallback auth must be implemented for Office versions and configurations where SSO is unavailable, which is still a significant share of enterprise environments.

We have implemented SSO for Outlook add-ins, Excel add-ins, Word add-ins, and PowerPoint add-ins, across single-tenant and multi-tenant scenarios. The auth layer is usually a 1 to 2 week engagement and removes the biggest friction point in the add-in user experience.

Three weeks into an enterprise pilot, a document management vendor watched their add-in stall for one reason: every user got a login popup, and the customer's IT policy blocked popups by default. The vendor's team had wired authentication with a standard web flow that works fine in a browser and poorly inside Office. We rebuilt it around getAccessToken so the add-in picks up the identity of the user already signed in to Microsoft 365, exchanged that token server-side for the Graph access the product needed, and kept a dialog fallback for the customer's shared kiosk machines where silent sign-in is unavailable. Support tickets about sign-in dropped to zero within the pilot group, and the customer's security team approved the rollout, largely because the token flow matched the pattern their own internal apps use.

This engagement makes sense when your add-in already exists and sign-in friction or auth errors are the blocker. If the add-in also talks to a non-Microsoft platform, the work usually pairs with an OAuth 2.0 build for that provider's tokens. And if there is no user identity involved at all, a background service with app-only permissions is simpler and cheaper than wiring silent sign-in you will never use.