With 13,800 GitHub stars, adoption by OpenAI for ChatGPT's authentication infrastructure serving 800 million weekly active users, and governance under the Ory open-source ecosystem, Ory Kratos provides enterprise-grade identity management as a headless API server that separates authentication logic from user interface — enabling any frontend framework to consume standardized self-service flows for registration, login, recovery, verification, and profile management. Multi-factor authentication supports passkeys, WebAuthn security keys, TOTP authenticator apps, SMS one-time codes, email magic links, and backup recovery codes with configurable Authenticator Assurance Levels enforcing step-up authentication for sensitive operations. Social sign-in connects to any OpenID Connect provider including Google, GitHub, Apple, and Microsoft, plus enterprise SAML 2.0 federation for B2B single sign-on. Customizable identity schemas using JSON Schema define user profiles with flexible trait structures, while pre- and post-flow webhooks enable integration with CRMs, analytics, and compliance systems. The courier system handles email and SMS delivery for verification codes and password recovery links. Session management provides device tracking, forced logout, and configurable lifespans with privileged session elevation. Deploy alongside Ory Hydra for OAuth2/OIDC token issuance and Ory Keto for fine-grained permissions to build a complete identity platform. Docker Compose deployment with PostgreSQL. Running on a dedicated VPS on RepoCloud with guaranteed CPU, RAM, and SSD, full root SSH access, and a browser serial console. Apache 2.0 licensed.

Securing a SaaS product, running B2B onboarding, or replacing Auth0 and Keycloak with a stack they own - teams needing more than basic auth reach for ZITADEL, an open-source identity and access management platform built in Go. Its multi-tenancy model is the differentiator: a strict Instance, Organization, Project hierarchy isolates data and scopes policy at each level, with identity brokering (pre-built templates for Google, GitHub, Microsoft, Apple, plus generic OIDC, OAuth, SAML, and LDAP), domain discovery that routes users to the right organization by email domain, and delegated management so customers administer their own users and roles. Authentication covers OpenID Connect (certified, including device authorization and token exchange), SAML 2.0 as both IdP and SP, SCIM, FIDO2 passkeys for phishing-resistant passwordless login, and MFA via OTP, email, SMS, and U2F; machine-to-machine flows support JWT profile, PATs, and client credentials. The architecture is event-sourced - every mutation is an immutable event, yielding a complete audit trail - with relational projections for queries and no external session store, so it scales horizontally. API-first with gRPC and REST, extensible via Actions webhooks, and the same codebase self-hosted (Docker Compose or Helm on PostgreSQL) as in the cloud.