OAuth 2.1 Is Here: What Changed, What's Deprecated, and How to Migrate Your App

If you shipped a single-page application before 2024, your OAuth implementation is probably insecure. Not "theoretically vulnerable" insecure — actually exploitable insecure. The Implicit Grant flow that every React tutorial taught you to use? Removed in OAuth 2.1. The Resource Owner Password Credentials (ROPC) flow your mobile app relies on? Also removed. Bearer tokens in URL query strings? Banned. OAuth 2.1 isn't a minor version bump. It's a decade of security lessons codified into spec, and it breaks real production code. Libraries you depend on are already shipping OAuth 2.1 defaults. Identity providers are deprecating legacy endpoints. If you haven't migrated yet, you're running on borrowed time. This guide covers every breaking change in OAuth 2.1, explains why each decision was made (so you know it's not arbitrary bureaucracy), and provides production-ready TypeScript code to migrate your existing implementations. What is OAuth 2.1, exactly? OAuth 2.1 is not a new protocol. It's