What is pkce in oauth

Overview

PKCE is an extension to the OAuth 2.0 authorization protocol designed to add an additional layer of security, particularly for public clients that cannot securely store secrets. Public clients include mobile applications, single-page applications (SPAs), and desktop applications where client secrets cannot be reliably protected. PKCE addresses a fundamental vulnerability in the standard OAuth 2.0 authorization code flow.

How PKCE Works

PKCE introduces two additional parameters to the authorization flow: code_verifier and code_challenge. The client generates a random code_verifier, which is a string of 43-128 characters. This verifier is then hashed using SHA-256 to create a code_challenge, which is sent to the authorization server. When the authorization server provides an authorization code, the client must prove ownership by sending the original code_verifier during the token exchange. The server verifies that the hash of the provided verifier matches the original challenge, confirming the request came from the legitimate client.

Security Benefits

PKCE prevents authorization code interception attacks where malicious applications could intercept authorization codes and exchange them for access tokens. Without PKCE, an attacker could potentially use an intercepted authorization code to gain unauthorized access. By requiring the code_verifier to match the code_challenge, PKCE ensures that only the original application that initiated the authorization request can complete the token exchange.

Implementation Requirements

To implement PKCE, developers must:

• Generate a random code_verifier for each authorization request
• Create a code_challenge by hashing the verifier with SHA-256 or plain text
• Include the code_challenge in the authorization request
• Send the original code_verifier during the token exchange step
• Handle the verification response from the authorization server

Current Standards

PKCE has become a standard requirement for OAuth 2.0 implementations. Major identity providers including Google, Microsoft, and Apple now mandate PKCE for public client applications. Industry best practices recommend PKCE for all OAuth 2.0 implementations, not just public clients, as it provides additional security without significant complexity.