How to migrate from a custom auth system to a third-party provider

Authentication migrations should begin with a full inventory of credentials, sessions, password recovery, social login, MFA, enterprise SSO, authorization data, and audit trails, rather than focusing only on a users table. Existing password hashes are often portable when their precise algorithms, parameters, salts, and formatting are known, allowing bulk imports that preserve user passwords and avoid disruptive resets; however, insecure hashes such as MD5 or unsalted SHA variants require resets. The main migration options are bulk import for portable hashes, lazy migration when hashes cannot be exported, and temporary dual-writing to prevent gaps during an active signup period, with many projects using a combination. Sessions generally require expiration or a transition period with both token validators, social login configurations can often be reused, duplicate accounts need deliberate resolution, and MFA commonly requires users to enroll again. Enterprise SSO can determine the overall timeline because customer identity-provider settings may need updating, although existing callback handlers and organization-level routing can enable gradual, reversible migrations for many connections. A safe rollout imports and tests data before switching traffic, moves cohorts through provider-backed authentication using feature flags, monitors sign-in success and reset volume, and retains the old path until traffic has ceased for weeks. Clear rules for inactive users, careful decommissioning criteria, and provider evaluation based on hash compatibility, SSO portability, tenant-level rollout controls, import scale, and authorization boundaries help ensure the legacy system can ultimately be retired.

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