OAuth2 Authorization Code + PKCE Flow: Ensuring Secure…

What Is the OAuth2 Authorization Code Flow?

The OAuth 2.1 authorization code flow is a widely used protocol for granting third-party applications access to user accounts without sharing passwords. In this flow, an application redirects users to the authorization server where they grant permission. The server then returns an authorization code which the client exchanges for an access token.

This process ensures that sensitive information like tokens never leave the user's browser or device, enhancing security.

Why Does PKCE Matter?

Proof Key for Code Exchange (PKCE) is a security extension to OAuth 2.1 designed to protect against authorization code interception attacks. Without PKCE, an attacker could steal the authorization code and use it to obtain access tokens.

By generating a random code verifier on the client side and deriving a code challenge from it, PKCE ensures that even if the authorization code is intercepted, it cannot be used without the corresponding code verifier.

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How Does PKCE Work?

PKCE introduces two new parameters: `code_challenge` and `code_verifier`. The client generates a random string (the code verifier) and computes its SHA-256 hash to create the code challenge. This code challenge is sent to the authorization server.

Upon receiving the authorization code, the client must present both the original code verifier and the derived code challenge for validation before exchanging the code for an access token.

Real-World Examples

For instance, when a user logs into a web application using OAuth2 with PKCE, the application generates a random string. This string is hashed and sent to the authorization server along with the request for an authorization code.

If an attacker intercepts the authorization code, they cannot use it without knowing the original random string (code verifier), making such attacks significantly more difficult.

Frequently asked questions

What is PKCE and why is it important?

PKCE stands for Proof Key for Code Exchange. It's crucial because it prevents attackers from using intercepted authorization codes to gain unauthorized access, enhancing the security of OAuth2 flows.

How does PKCE protect against stolen authorization code attacks?

PKCE protects by requiring both a code challenge and a code verifier. The code challenge is derived from a random string (code verifier) that must be presented alongside the intercepted code to gain access, making it difficult for attackers to use the code without the original string.

Can PKCE be bypassed if an attacker has the code verifier?

No, because even with the code verifier, an attacker still needs to compute the exact same code challenge using the SHA-256 hash function. Without knowing this process or having access to the original random string, they cannot successfully exchange the authorization code for a token.

Is PKCE mandatory in all OAuth2 flows?

While not strictly required by the OAuth2 specification, it is highly recommended and often enforced by security best practices. Implementing PKCE significantly enhances the security of your application against various types of attacks.

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Everything above runs in your browser — open OAuth2 Authorization Code + PKCE Flow and change the parameters while it is running. Nothing is installed, nothing is uploaded, the whole model lives in one tab.

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OAuth2 Authorization Code + PKCE Flow: Ensuring Secure Authentication