What is pkce

Understanding PKCE

PKCE, pronounced "pixie," stands for Proof Key for Public Exchange. It is an extension to the OAuth 2.0 authorization framework designed to secure the authorization code flow, particularly for mobile applications and single-page applications (SPAs). PKCE adds an extra layer of cryptographic verification that prevents attackers from intercepting authorization codes and exchanging them for access tokens, a vulnerability that existed in standard OAuth 2.0 flows.

The Problem PKCE Solves

In standard OAuth 2.0 flows, a user authorizes an application to access their data at a service provider. The authorization server issues an authorization code, which the application exchanges for an access token. However, in certain scenarios—particularly with mobile apps and SPAs—an attacker could intercept the authorization code before the legitimate application uses it. The attacker could then exchange the code for an access token, gaining unauthorized access. PKCE prevents this attack by requiring cryptographic proof that the application requesting the token is the same application that originally requested the authorization code.

How PKCE Works

PKCE operates through a carefully designed process. First, the application generates a random string called a code verifier. The application then creates a code challenge by hashing the code verifier using SHA-256 (or using the plain text in rare cases). During authorization, the application sends the code challenge to the authorization server. When the user grants authorization, the authorization server issues an authorization code. Later, when the application exchanges the authorization code for tokens, it must provide the original code verifier. The authorization server verifies that the code verifier hashes to the previously submitted code challenge, confirming the request's legitimacy.

PKCE Implementation and Standards

PKCE is defined in RFC 7636 and has become a standard security practice endorsed by major organizations including the OAuth Security Best Current Practice. Implementation is straightforward: applications generate cryptographically random verifiers (43-128 characters), create challenge strings, and include them in authorization requests. Authorization servers verify the hashes match when tokens are requested. Modern OAuth libraries and SDKs typically include PKCE support built-in, making implementation accessible to developers.

PKCE for Different Application Types

While originally developed for mobile applications, PKCE is now recommended for all OAuth 2.0 applications, including traditional web applications. SPAs benefit from PKCE protection since they run in browsers where authorization codes are visible. Native mobile apps, which cannot securely store client secrets, gain essential protection through PKCE. Even traditional server-side web applications with secure backend storage now use PKCE as a security best practice, as it provides defense-in-depth against various attack vectors.