Email Domain Discovery: Routing Logins to the Right IdP

Enterprise SaaS applications can route users from a single email-first login screen to the appropriate identity provider by matching verified email domains to tenant SSO configurations, while offering password login or signup when no enforced mapping exists. The central security requirement is domain ownership verification, typically through a high-entropy DNS TXT record tied to a specific tenant and domain, because an unverified mapping could allow attackers to direct employees to a malicious IdP and impersonate them. Routing data should be resolved server-side, enforce one tenant per domain, require a non-null verification timestamp, explicitly define subdomain handling, and block public consumer domains from tenant claims. The guidance distinguishes domain routing from tenant membership so invited contractors and partners can still access a tenant without matching its domain, and it recommends explicit resolution methods for organizations that legitimately share a domain across multiple tenants. OIDC WebFinger provides a standards-based discovery mechanism but is often impractical for B2B customers, while login_hint can improve the authentication experience but must never be trusted as identity evidence. Implementations should also validate OIDC issuer values exactly across discovery metadata and ID tokens, reverify domains periodically, audit verification changes, account for customer-enumeration risks through rate limiting and consistent responses, and treat DNS ownership, IdP token provenance, and verified user identity as separate security controls.

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