Mastering OAuth 2.0 & Microservice Security for Senior Interviews

Are you preparing for a senior security or backend engineering interview and struggling to articulate how to secure microservices in a zero-trust environment? In this deep dive, we break down the definitive guide to OAuth 2.0, OpenID Connect, and advanced token security to help you move beyond textbook definitions and start designing banking-grade architectures.Whether you are designing a Backend-For-Frontend (BFF) or securing a massive microservice mesh, this episode is your ultimate cheat sheet!What We Cover in This Episode:The "Hotel Keycard" Analogy (AuthN vs. AuthZ): We clarify the critical difference between OpenID Connect (verifying your identity at the front desk) and OAuth 2.0 (the keycard that tells the lock what you can access).The "Secret Handshake" (PKCE): Discover why the Proof Key for Code Exchange (PKCE) is now mandatory for public clients to prevent authorisation code interception attacks.The "Clear Backpack" Trap: We reveal why storing tokens in browser localStorage is a major interview red flag, and how the Backend-For-Frontend (BFF) pattern keeps tokens securely on the server.Defeating the "Forged Badge" (JWT Vulnerabilities): We unpack the notorious alg:none vulnerability and exactly what steps a Resource Server must take to validate a JWT signature safely.Zero-Trust Microservices & Token Exchange: Learn how to move past weak shared secrets. We explain how to use private_key_jwt (RFC 7523) for strong service identity, and why you should use Token Exchange (RFC 8693) to maintain a secure chain of custody across microservices.Banking-Grade Security (DPoP & Token Rotation): We dive into the ultimate defenses against token theft: Refresh Token Rotation, which acts as a tripwire to invalidate compromised token families, and DPoP (Sender-Constrained Tokens, RFC 9449), which mathematically binds a token to the client's private key.