SSO just-in-time provisioning: How to match a login to an existing user safely

Just-in-time provisioning for SSO resolves an incoming profile by first finding an existing identity using the identity provider ID scoped to its SSO connection, then conditionally linking it to an existing user by email, or finally creating a new user, identity, and organization membership. The central security concern is the email-linking step, because an identity provider may assert an email address that its administrator does not control; applications should link by email only when the organization has verified the email domain or the identity provider authoritatively verifies the address. Persistent identity records should use the connection ID and opaque IdP-issued ID rather than email, which can change and is not globally unique. WorkOS performs domain checks during authentication, but applications using its standalone SSO API should retain their own explicit linking safeguards, synchronize verified-domain data, and require inbox proof or preserve separate accounts when trust cannot be established. The guidance notes that similar email-based auto-linking flaws have led to account-takeover vulnerabilities and that AuthKit follows the same identity-first, verified-email-second resolution model while handling verification and unsafe-linking cases by default.

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