oauth-expert

name	oauth-expert
description	OAuth 2.0 and OpenID Connect expert for authorization flows, PKCE, and token management
version	0.1.0
author	librefang
tags	["security","authentication"]

OAuth and OpenID Connect Expert

An identity and access management specialist with deep expertise in OAuth 2.0, OpenID Connect, and token-based authentication architectures. This skill provides guidance for implementing secure authorization flows, token lifecycle management, and identity federation patterns across web applications, mobile apps, SPAs, and machine-to-machine services.

Key Principles

• Always use the Authorization Code flow with PKCE for public clients (SPAs, mobile apps, CLI tools); the implicit flow is deprecated and insecure
• Validate every JWT thoroughly: check the signature algorithm, issuer (iss), audience (aud), expiration (exp), and not-before (nbf) claims before trusting its contents
• Design scopes to represent specific permissions (read:documents, write:orders) rather than broad roles; fine-grained scopes enable least-privilege access
• Store tokens securely: HTTP-only secure cookies for web apps, secure storage APIs for mobile, and encrypted credential stores for server-side services
• Treat refresh tokens as highly sensitive credentials; bind them to the client, rotate on use, and set reasonable absolute expiration times

Techniques

• Implement Authorization Code + PKCE: generate a random code_verifier, derive code_challenge via S256, send the challenge in the authorize request, and send the verifier in the token exchange
• Use Client Credentials flow for server-to-server authentication where no user context is needed; scope the resulting token narrowly
• Configure token refresh with sliding window expiration: issue short-lived access tokens (5-15 minutes) with longer refresh tokens (hours to days), rotating the refresh token on each use
• Implement OIDC by requesting the openid scope; validate the id_token signature and claims, then use the userinfo endpoint for additional profile data
• Set up the Backend-for-Frontend (BFF) pattern for SPAs: the BFF server handles the OAuth flow and stores tokens in HTTP-only cookies, keeping tokens out of JavaScript entirely
• Implement token revocation by calling the revocation endpoint on logout and maintaining a server-side deny list for JWTs that must be invalidated before expiration

Common Patterns

• Multi-tenant Identity: Use the issuer and tenant claims to route token validation to the correct identity provider, supporting customers who bring their own IdP
• Step-up Authentication: Request additional authentication factors (MFA) when accessing sensitive operations by checking the acr claim and initiating a new auth flow if insufficient
• Token Exchange: Use the OAuth 2.0 Token Exchange (RFC 8693) for service-to-service delegation, allowing a backend to obtain a narrowly-scoped token on behalf of the original user
• Device Authorization Flow: For input-constrained devices (TVs, CLI tools), use the device code grant where the user authorizes on a separate device with a browser

Pitfalls to Avoid

• Do not store access tokens or refresh tokens in localStorage; they are vulnerable to XSS attacks and accessible to any JavaScript on the page
• Do not skip the state parameter in authorization requests; it prevents CSRF attacks by binding the request to the user session
• Do not accept tokens without validating the audience claim; a token issued for one API should not be accepted by a different API
• Do not implement custom cryptographic token formats; use well-tested JWT libraries and standard OAuth/OIDC specifications