Design cross-tenant access settings before B2B collaboration scales

Source facts: cross-tenant access has inbound, outbound, and trust decisions

Microsoft’s B2B cross-tenant access documentation explains that settings control how users in external Microsoft Entra organizations access applications in your tenant and how your users access applications in external organizations. Administrators can configure defaults and organization-specific inbound and outbound settings and can decide whether to trust MFA, compliant-device, or hybrid-joined-device claims from another Entra organization.

Microsoft’s governed B2B collaboration architecture recommends identifying collaboration partners and using controls including external collaboration settings, cross-tenant access settings, and entitlement management. It notes that one partner can have multiple domains and domains can change, which makes a simple email-domain list an incomplete model of the relationship.

Cross-tenant settings do not replace application authorization, Conditional Access, guest lifecycle, data-sharing controls, terms of use, or contractual review. Some collaboration modes require reciprocal configuration, and behavior varies by tenant type, Microsoft cloud, application, and license. Trusting an external claim means relying on another tenant’s relevant control; it is not Microsoft attesting that the partner’s whole security program meets yours.

DSE recommendation: make each tenant relationship an owned security object

Inventory current B2B reality before changing the defaults. Identify external tenant IDs, guest objects, invitations, sign-ins, resources accessed, Teams shared channels, SharePoint sharing, cross-tenant synchronization, access packages, partner owners, internal sponsors, and contractual purpose. Separate verified tenant relationships from unredeemed, orphaned, consumer, and unknown identities.

Set a deliberate default. Decide what any organization without a specific rule may do inbound and what your users may do outbound. Test the business effect before tightening; default changes can affect many relationships that nobody documented.

Identify the partner by tenant. Verify the tenant ID through an approved business contact and record legal entity, sponsor, purpose, users or groups, applications, data classes, start, review, and end dates. Do not infer trust solely from a familiar domain or display name.

Scope both directions. Select only required external users, groups, and applications for inbound access and required internal users, groups, and external applications for outbound access. A narrowly scoped collaboration project should not silently become tenant-wide access.

Decide claim trust explicitly. If accepting a partner’s MFA or device claims, document why its assurance and lifecycle are sufficient, how exceptions are handled, and who reviews the decision. Otherwise require controls in your own resource tenant as supported.

Test the complete journey. Exercise invitation, redemption, sign-in, Conditional Access, resource authorization, sharing, mobile and browser access, access review, revocation, and re-invitation. Include a user who changes employer or home-tenant identity and an application outside the approved scope.

Monitor and exit. Alert on organization-setting changes, trust changes, unusual guest sign-ins, new high-value resource access, and stale guests. At relationship end, remove access, synchronization and invitations as appropriate, preserve required records, and verify the resource no longer grants access.

Maintain separate owners for the business relationship, external identity configuration, application, and shared data. Require coordinated review when either tenant merges, rebrands, changes domains, changes its authentication program, or transfers the application. Reciprocal settings should be compared, not assumed to match.

Document effective defaults and organization-specific exceptions in plain language so a reviewer can answer who trusts whom, for what, and until when. The goal is not to block collaboration; it is to keep collaboration from expanding beyond an identified partner, approved resource, sufficient proof, and managed lifecycle.

Use release criteria for each relationship: verified tenant ID, named sponsors on both sides, least-privilege user and application scope, an explicit claim-trust decision, successful revocation test, and a dated exit owner. Stop deployment if the partner cannot verify its tenant, required access cannot be scoped, or test users reach an unapproved resource. Business urgency does not convert an unidentified trust boundary into an acceptable one.

Official references

Microsoft, Manage cross-tenant access settings for B2B collaboration.

Microsoft, Transition to governed collaboration with Microsoft Entra B2B collaboration.