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OAuth 2.0 and OIDC Implementation Guide 2026: Complete Developer Walkthrough

OAuth 2.0 and OpenID Connect (OIDC) are the foundation of modern authentication — every "Sign in with Google/GitHub/Apple" button uses them. But implementing OAuth 2.0 correctly is notoriously tricky: the spec is 80+ pages, and getting it wrong means security vulnerabilities. This guide walks through a complete implementation, from the authorization code flow to PKCE, token storage, and session management.

OAuth 2.0 Grant Types: When to Use Each

Grant Type Use Case Security Level Requires Client Secret?

Authorization Code + PKCE Single-page apps, mobile apps, all modern web apps Highest No (PKCE replaces the secret)

Authorization Code (classic) Server-rendered web apps (backend can keep a secret) High Yes

Client Credentials Machine-to-machine, service accounts, API…