Support Client Credentials Flow

[cbaenziger]

This provides support for the OAuth 2.0 Client Credentials authentication login flow.

The Client Credentials flow has been asked about in #931. I have been working with a piece of software which only accepts a K8s configuration for connection setup and authentication, so client credentials works great for me for machine-to-machine communications. I've used this code against a private IdP successfully.

[cbaenziger]

It looks like the docker tests are expected to fail? I see the following which seems reasonable:

#25 ERROR: failed to push ghcr.io/int128/kubelogin:pr-1231: unexpected status from POST request to https://ghcr.io/v2/int128/kubelogin/blobs/uploads/: 403 Forbidden

[int128]

docker/build-push-action was failing on forked pull request. I have fixed the problem as #1233.

[int128]

I think it needs to verify the retrieved token.
For example,

kubelogin/pkg/oidc/client/client.go

Line 165 in 9263439

return c.verifyToken(ctx, token, "")

[cbaenziger]

I think I've implemented this now.

[cbaenziger]

Unfortunately in my extended testing, verifyToken fails to accept my valid OAuth 2.0 token. Perhaps you can advise which path you prefer for me to follow?

Per the OAuth2 RFC, where the Client Credential flow is defined, it looks like access_token is a required response per the RFC8693 Successful Response definition but no longer is id_token mentioned. (Similarly Auth0 and Okta have diagrams pointing out an access_token is returned for Client Credentials.)

Meanwhile, I do see in the OIDC Core Spec, id_token is required it its Token Response section.

Using verifyToken my exchange works but I fail to accept the token:

./kubelogin get-token \
    --grant-type=client-credentials \
    --oidc-issuer-url=https://<elided> \
    --oidc-client-id=<elided> \
    --oidc-client-secret=<elided> \
    --no-keyring
error: get-token: authentication error: client-credentials error: authorization error: id_token is missing in the token response: &oauth2.Token{AccessToken:"... elided ...", TokenType:"Bearer", RefreshToken:"", Expiry:time.Date(2025, time.January, 16, 23, 31, 55, 191375292, time.Local), ExpiresIn:0, raw:map[string]interface {}{"access_token":"... elided ...", "expires_in":59, "token_type":"Bearer"}, expiryDelta:0}

The code does work for me, as originally implemented, returning the access_token in my client.authentication.k8s.io/v1beta1 JSON payload as token which my API server accepts.

[int128]

How about using this feature with --use-access-token flag? It will use the access_token as the token response for Kubernetes API server. See also #1083.

[cbaenziger]

Unfortunately, the structure of the token is different than what VerifyToken is expecting.

The Client Credentials RFC token specification does not return an OIDC IDToken? (Specifically the response specification in RFC 6749 does not include even an optional IDToken field.)

As such, at least as configured by my IdP, there is no IDToken envelope to pull the AccessToken out of; the AccessToken is all I receive:

&oauth2.Token{AccessToken:"[...]", TokenType:"Bearer", RefreshToken:"", Expiry:time.Date(2025, time.January, 26, 9, 43, 35, 530935198, time.Local)

Questions to confirm:

1. I don't think it will be possible to use --grant-type=client-credentials without --use-access-token as I've implemented?
2. VerifyToken would need to parse a different token type to still be used but I also do not believe we would have the need for verification like with the other grant types (the token could be fully opaque per the specification)?

[cbaenziger]

@int128 I think I understand questions 1 and 2 now. I think this code should be good.

Does it look okay to you?