OAuth 2.1 is the latest version of the OAuth authorization framework, which aims to improve security and usability while simplifying the specification. It builds upon OAuth 2.0, consolidating key security improvements from multiple RFCs, deprecates legacy or unsafe flows, and provides clearer guidance for developers and implementers. The new specification introduces mandatory Proof Key for Code Exchange (PKCE) for all authorization code flows, removes implicit flow, formalizes how single-page apps can securely use refresh tokens, and emphasizes secure transport, scope minimization, and token storage to mitigate security risks associated with bearer tokens. The update also includes changes to redirect URIs, password grant, and implementation guidance for different app types. To ensure a smooth transition, OAuth 2.1 provides a migration checklist that outlines steps to take when updating existing implementations or building new ones.