OAuth2 Authorization Code Injection Prevention — Protecting Against Code Interception Attacks

In this tutorial, you will learn about OAuth2 Authorization Code Injection Prevention. We cover key concepts, practical examples, and best practices to help you master this topic.

OAuth2 authorization code injection prevention protects against attacks where an intercepted authorization code is used by an attacker to obtain tokens, using PKCE, state validation, nonce checking, and strict redirect URI matching.

What You'll Learn

• Authorization code interception attack
• PKCE as the primary defense
• State parameter anti-CSRF protection
• Nonce verification for ID tokens
• Redirect URI matching strategies

sequenceDiagram
    participant App as Legitimate App
    participant Browser
    participant Auth as Auth Server
    participant Attacker

    App->>Browser: Authorization Request + code_challenge + state
    Browser->>Auth: Redirect with params

    Note over Attacker: Intercepts authorization code
    Attacker->>Auth: Token request with stolen code
    Auth->>Auth: Code requires PKCE verifier
    Attacker->>Auth: Sends wrong verifier (doesn't know challenge)
    Auth-->>Attacker: Invalid grant (code_verifier mismatch)

    Browser->>Auth: User approves
    Auth-->>Browser: Authorization code
    Browser->>App: Code (via redirect with state)
    App->>Auth: Token request + code_verifier (correct!)
    Auth-->>App: Tokens

What's Next

Implement protections in your {{< ilink "OAuth" "OAuth2 Authorization Server" }} and review {{< ilink "OAuth" "OAuth2 Security Best Practices" }}.