OIDC Authentication: OAuth 2.0 Flows, Tokens, and Security Pitfalls

Part of the Cybersecurity Skills Guide — This article is one deep-dive in our complete guide series.

By HADESS Team | February 28, 2026 | Updated: February 28, 2026 | 5 min read

OpenID Connect (OIDC) adds an identity layer on top of OAuth 2.0. OAuth handles authorization — what you can access. OIDC handles authentication — who you are. Confusing the two leads to broken implementations. Understanding the protocol mechanics is necessary for building secure authentication and for finding vulnerabilities in existing systems.

OAuth 2.0 Flows

Authorization Code Flow is the standard for server-side applications. The client redirects the user to the authorization server, the user authenticates, the authorization server redirects back with a code, and the client exchanges the code for tokens on a back channel. The code-for-token exchange happens server-to-server, keeping tokens out of the browser.

Authorization Code Flow with PKCE (Proof Key for Code Exchange) extends this for public clients — SPAs and mobile apps that cannot securely store a client secret. The client generates a random code_verifier, sends a code_challenge (SHA-256 hash of the verifier) with the authorization request, and sends the original verifier with the token exchange. This prevents authorization code interception attacks.

Use PKCE for all clients, not just public ones. It adds protection with no downside.

Client Credentials Flow is for machine-to-machine authentication. No user is involved. The client authenticates directly with its credentials and receives an access token. Use this for backend services calling other backend services.

Implicit Flow is deprecated. It returned tokens directly in URL fragments, exposing them in browser history and referer headers. If you have existing implicit flow implementations, migrate to authorization code with PKCE.

Token Management

ID Tokens are JWTs that contain claims about the user — sub, email, name, issuer, audience, expiration. Always validate: issuer matches your expected provider, audience matches your client ID, the token is not expired, and the signature is valid.

Access Tokens authorize API requests. They can be JWTs or opaque strings. Keep access token lifetimes short (5-15 minutes). Use refresh tokens to obtain new access tokens without re-authentication.

Refresh Tokens must be stored securely — server-side sessions or encrypted, HttpOnly cookies. Implement refresh token rotation: each use issues a new refresh token and invalidates the old one. Detect token reuse as a potential compromise indicator.

Never store tokens in localStorage. It is accessible to any JavaScript on the page, including XSS payloads. Use HttpOnly cookies or server-side sessions.

Provider Configuration

OIDC providers publish a discovery document at /.well-known/openid-configuration. This contains endpoints, supported flows, signing algorithms, and available scopes. Fetch this document programmatically rather than hardcoding endpoints.

Configure your client to request only the scopes you need. openid is required. profile and email are common. Avoid requesting broad scopes that grant unnecessary access.

Register redirect URIs exactly. Do not use wildcards. An open redirect in your redirect URI handling lets attackers steal authorization codes.

Security Pitfalls

State parameter omission enables CSRF attacks on the OAuth flow. Always generate a random state value, store it in the session, and validate it when the callback arrives.

Token validation shortcuts — skipping signature verification, accepting expired tokens, or not checking the audience claim. Each shortcut is a vulnerability.

Mixing up access tokens and ID tokens — using ID tokens for API authorization or access tokens for user identity. They serve different purposes and have different security properties.

Open redirect in callback URIs — if your callback endpoint redirects to a user-supplied URL, attackers can steal authorization codes.

Related Career Paths

OIDC and OAuth expertise maps to Application Security Expert and Security Engineer career paths. Both roles encounter authentication protocols in code reviews, architecture assessments, and penetration tests.

Next Steps

• Evaluate your authentication protocol knowledge with the skills assessment
• Explore the skills library for identity and access management topics
• Use the salary calculator to benchmark identity security roles

Related Guides in This Series

• Agile Security: Embedding Security in Sprints and Development Cycles — HADESS | 2026
• AI/ML Security: Adversarial Attacks, Model Poisoning, and Prompt Injection — HADESS | 2026
• Compliance Management: SOC 2, ISO 27001, PCI DSS, and HIPAA — HADESS | 2026

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