oauth21-pkce

To implement PKCE for OAuth 2.1 public clients, generate a high-entropy code_verifier and compute its S256 code_challenge using SHA-256 for every authorization request. This binds the authorization code to the client, preventing code interception and injection attacks.

You need PKCE for single-page and mobile applications because these public clients cannot securely store client secrets. PKCE protects the authorization code flow by ensuring the code is bound to the initial request, preventing interception even if the code is stolen.

The S256 method derives the code_challenge by hashing the code_verifier with SHA-256, while the plain method sends the verifier directly. OAuth 2.1 PKCE validation uses S256 to ensure the code_verifier remains confidential throughout the authorization flow.

Yes, PKCE exemptions can be considered for confidential clients when appropriate, as they can securely store secrets. However, enforcing PKCE validation provides an extra layer of security against code injection, and clear rejection rules apply for missing or unsupported parameters.

If the code_verifier is missing or mismatched during a token request, the OAuth 2.1 flow will reject the authorization. The validation process enforces clear rejection rules for missing parameters, mismatched values, and unsupported challenge methods to ensure secure authorization.

An authorization server validates a PKCE code_challenge by comparing the stored challenge against the received code_verifier. It properly handles plain versus S256 methods, rejecting requests with missing or mismatched parameters to ensure secure token issuance.