Using Auth0 to secure a CLI
Authentication in CLI programs is straightforward if the identity provider supports sending credentials, like database connections, SMS passwordless and AD. If the identity provider requires a browser redirect, then the process is slightly more complicated.
Auth0 implements the Proof Key for Code Exchange by OAuth Public Clients. This flow makes it easy to add authentication to a CLI while keeping higher standards of security.
How PKCE works
Traditionally, public clients (e.g. mobile apps, SPAs and CLIs) have used the implicit flow to obtain a token. In this flow, there's no client authentication because there's no easy way of storing a
The PKCE flow (
pixy for friends), increases security by adding a cryptographic challenge in the token exchange. This prevents rogue apps to intercept the response from Auth0, and get hold of the token.
How to implement PKCE
The steps to follow to implement this grant are the following:
Create a Code Verifier. This is a randomly generated value that will be used to generate the
code_challenge(which will be sent in the authorization request).
Create a Code Challenge. A hashed (
SHA256) and base64Url encoded value, generated using the
Initiate the Authorization Request. The regular OAuth 2.0 authorization request, with the caveat that now it includes two parameters: the
code_challenge_methodwhich should be
S256. If the authorization is successful, then Auth0 will redirect the browser to the callback with a
- Exchange the Authorization Code for a Token. With the
code, the program then uses the /oauth/token endpoint to obtain a token. In this second step, the CLI program adds a
verifierparameter with the exact same random secret generated in step 1. Auth0 uses this to correlate and verify that the request originates from the same client. If successful the response is another JSON object, with an
access_token. Note that if the
verifierdoesn't match with what was sent in the /authorize endpoint, the request will fail.