RFC 7636 Deep Dive: How PKCE Kills Authorization Code Interception Attacks

Summary of RFC 7636: Proof Key for Code Exchange (PKCE)

RFC 7636, titled "Proof Key for Code Exchange by OAuth Public Clients," addresses a critical security vulnerability in the OAuth 2.0 protocol, particularly when using authorization codes with public clients like native mobile and desktop applications.

Problem Addressed

• Custom URI Schemes: Native apps often use custom URI schemes (e.g., myapp://callback) for redirecting back to the app after an authentication flow.
• Security Vulnerability: Any application can register a custom URI scheme, making it easy for attackers to intercept authorization codes and exploit them.

Solution: PKCE

PKCE introduces a mechanism to secure the delivery of authorization codes by using a mathematical secret (the code_verifier) that only the request initiator knows. This ensures that even if an attacker intercepts the authorization code, they cannot use it without knowing the corresponding code_verifier.

Key Components:

1. Code Verifier: A cryptographically strong random string generated by the client.
2. Code Challenge: The SHA-256 hash of the code_verifier, which is sent to the authorization server during

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