If your Spring Security config still uses the Implicit Grant or Resource Owner Password flow, you're running on borrowed time. This post breaks down why OAuth 2.1 makes PKCE mandatory, kills legacy flows, and shows the exact config difference between a legacy setup and a production-hardened one.

Sergio Lema

OAuth 2.1 removes the Implicit Grant and Resource Owner Password Credentials (ROPC) flows, mandating PKCE for all clients. The post explains why these legacy flows are security anti-patterns, how PKCE works to prevent authorization code interception, and provides a side-by-side Spring Security 6 code comparison between a legacy configuration and an OAuth 2.1-compliant setup. A migration checklist covers auditing clients, removing the password grant, upgrading to Spring Boot 3.x, and moving secrets to a dedicated provider.

Spring Security 6 OAuth 2.1: Replacing Implicit Grant and ROPC with PKCE

Modern Security Realities: The Lies Developers Tell Themselves

Why OAuth 2.1 is Mandatory for Modern Backend Architecture

The Kill List: Why Implicit Grant and ROPC are Security Anti-Patterns

Deep Dive: How PKCE Secures the Authorization Code Flow

Refactoring Spring Security 6 for OAuth 2.1 Compliance