RFC 7636

PKCE Becomes Mandatory for OAuth Public Clients as RFC 9700 and OAuth 2.1 Close Authorization Code Interception Risks

PKCE, or Proof Key for Code Exchange, was introduced in RFC 7636 to prevent code interception attacks in OAuth 2.0 flows used by mobile and single-page applications. The mechanism generates a code_verifier and derives a code_challenge using S256 hashing to bind the authorization code to the original client session. Without PKCE, malicious apps on the same device can hijack custom URI schemes like myapp://callback and exchange stolen codes for access tokens. RFC 9700, published in January 2025, now mandates PKCE for public clients and recommends it for confidential ones while requiring S256 over the weaker plain method. The upcoming OAuth 2.1 draft further embeds PKCE into the core authorization code flow and removes implicit and resource owner password credentials grants. Major providers including Auth0, Okta, and Microsoft Entra ID show varying default support, highlighting the need for explicit S256 implementation. The standard also protects against code injection attacks even when client secrets are present.