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.

Your users belong in your own database - Authorizer, an open-source authentication and authorization server shipping as a single Go binary, keeps them there. It connects to 13+ backends - PostgreSQL, MySQL, SQLite, SQL Server, MariaDB, MongoDB, Cassandra, ScyllaDB, ArangoDB, DynamoDB, Couchbase, YugabyteDB, PlanetScale, and CockroachDB - so identity data lives beside the application it protects instead of in an auth vendor's cloud. The server is fully OAuth 2.0 and OpenID Connect compliant, including authorization code flow with PKCE, a JWKS endpoint, token revocation, and nine JWT signing algorithms. Login options cover email/password, magic links, TOTP multi-factor, SMS OTP via Twilio, and social providers including Google, GitHub, Apple, Microsoft, and Discord. Authorization goes beyond roles: an embedded OpenFGA engine provides Zanzibar-style relationship-based permission checks in process. APIs are exposed over GraphQL, REST, and gRPC, with SDKs for JavaScript, React, Go, and Python, plus themeable built-in login pages and an admin dashboard. Apache 2.0 licensed.

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.

Backed by 9,000+ GitHub stars and developed by a Germany-based team with deep FIDO2 expertise, Hanko delivers the open-source authentication platform that replaces Auth0, Clerk, and Firebase Auth with a passkey-first architecture built on phishing-resistant WebAuthn credentials. The Go backend exposes both public and administrative HTTP APIs handling passkey registration and login, password authentication, email passcodes, TOTP-based MFA, security key verification, server-side sessions with remote revocation, and JWT issuing with configurable expiry and refresh policies. OAuth SSO connects Apple, Google, GitHub, Microsoft, and custom OIDC providers, while SAML Enterprise SSO integrates corporate identity providers for single sign-on across the organization. Hanko Elements provides framework-agnostic web components that embed complete onboarding, login, and user profile flows into any application with two lines of code, fully customizable via CSS variables and supporting i18n with custom translations. The JavaScript frontend SDK handles API communication, credential management, and session state for teams building custom authentication UIs without the pre-built components. Webhooks notify external services of authentication events including user creation, login, email verification, and password changes. Privacy-first design principles enforce data minimalism with user-deletable passwords and configurable identifier strategies supporting email-only, username-only, or combined approaches. Docker deployment starts the backend with PostgreSQL or MySQL, serving both APIs on configurable ports. Running on a dedicated VPS on RepoCloud with guaranteed CPU, RAM, and SSD, full root SSH access, and a browser serial console. AGPL-3.0 (backend) / MIT (frontend) licensed.

Powering authentication for over 15,000 GitHub stargazers and production deployments across startups and enterprises alike, SuperTokens delivers the complete login infrastructure that Auth0, Firebase Auth, and AWS Cognito charge premium prices for — entirely open source under Apache 2.0 with no per-user limits. The Java-based core service runs as a lightweight HTTP microservice that connects to PostgreSQL or MySQL and handles email-password login, passwordless magic links and OTP via email or SMS, social OAuth with Google, GitHub, Facebook, Apple, and any custom OIDC provider, phone-password authentication, and time-based one-time password multi-factor authentication. Session management uses rotating refresh tokens with anti-CSRF and anti-session-fixation protections, verifying tokens locally in your backend SDK without network round-trips to the core. Frontend SDKs for React, React Native, Angular, Vue, and vanilla JavaScript provide pre-built UI components for login, signup, email verification, and password reset flows, all customizable through an override system that lets you add pre- and post-API logic, custom hooks, and user event handlers. The built-in user management dashboard enables administrators to view users, reset passwords, revoke sessions, modify roles, and manage email verification status. Multi-tenancy and organization support powers B2B SaaS applications with tenant-level authentication configuration and Enterprise SSO via SAML and OIDC. Migration tools support bulk importing millions of existing password hashes, TOTP secrets, and role mappings. 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.

With 13,800+ GitHub stars and active releases through 2026, Casdoor is the open-source identity platform that ships OAuth 2.0, OIDC, SAML 2.0, CAS, LDAP, SCIM 2.0, WebAuthn passkeys, TOTP multi-factor authentication, and Face ID biometric login in a single Go binary with a React web console — covering every authentication protocol your organization will ever need. Social login connects 50+ identity providers including Google, GitHub, Microsoft Azure AD, Facebook, WeChat, DingTalk, Apple, Twitter, and LinkedIn without custom code. The AI-first architecture includes a Model Context Protocol gateway for AI agent authentication and Agent-to-Agent protocol support for securing LLM-powered workflows. Built-in Casbin integration delivers authorization primitives beyond basic RBAC: attribute-based access control, relationship-based access control, and custom policy models through a unified engine. The organization-scoped multi-tenancy model isolates users, applications, and identity providers per tenant with customizable login pages, branding, and SMTP configuration. SCIM 2.0 endpoints automate user provisioning from HR systems and Active Directory. Official SDKs for Go, Java, Node.js, Python, and PHP accelerate application integration. The RESTful API exposes every resource for programmatic management. Comprehensive audit logs record authentication events and administrative actions. Deploy via Docker Compose with MySQL or PostgreSQL, the all-in-one SQLite Docker image for quick trials, or the official Kubernetes Helm chart for production clusters. 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.

Authentication that lives inside your application rather than behind a redirect to an external identity provider - SuperTokens takes a fundamentally different architecture from Auth0 and AWS Cognito. Three tiers make that work - frontend SDKs (React, Angular, Vue, vanilla JS, React Native) render overridable login UI and manage tokens; backend SDKs (Node.js, Python, Go) expose auth endpoints on your own API domain; and SuperTokens Core, the piece you host here, is the stateless HTTP service handling core auth logic, password hashing, token signing, and database operations against PostgreSQL. The recipe system keeps features decoupled: use email/password, social login, passwordless (magic links, OTP), phone-password, multi-factor authentication (TOTP, WebAuthn), user roles, and microservice auth - individually or combined; you can even use SuperTokens purely for session management alongside another login provider. Sessions are where it shines: rotating refresh tokens with theft detection, automatic access-token refresh, CSRF protection, and secure cookie handling out of the box - the details that become vulnerabilities when hand-rolled. Verification happens locally in your backend via cached JWT signing keys, so the Core stays off the hot path. Self-hosted means no user limits, free forever, with all user data in your database. Apache-licensed.