The OAuth spec [defines two types of
client](https://datatracker.ietf.org/doc/html/rfc6749#section-2.1),
confidential and public. Previously Gitea assumed all clients to be
confidential.
> OAuth defines two client types, based on their ability to authenticate
securely with the authorization server (i.e., ability to
> maintain the confidentiality of their client credentials):
>
> confidential
> Clients capable of maintaining the confidentiality of their
credentials (e.g., client implemented on a secure server with
> restricted access to the client credentials), or capable of secure
client authentication using other means.
>
> **public
> Clients incapable of maintaining the confidentiality of their
credentials (e.g., clients executing on the device used by the resource
owner, such as an installed native application or a web browser-based
application), and incapable of secure client authentication via any
other means.**
>
> The client type designation is based on the authorization server's
definition of secure authentication and its acceptable exposure levels
of client credentials. The authorization server SHOULD NOT make
assumptions about the client type.
https://datatracker.ietf.org/doc/html/rfc8252#section-8.4
> Authorization servers MUST record the client type in the client
registration details in order to identify and process requests
accordingly.
Require PKCE for public clients:
https://datatracker.ietf.org/doc/html/rfc8252#section-8.1
> Authorization servers SHOULD reject authorization requests from native
apps that don't use PKCE by returning an error message
Fixes #21299
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
This is "minimal" in the sense that only the Authorization Code Flow
from OpenID Connect Core is implemented. No discovery, no configuration
endpoint, and no user scope management.
OpenID Connect is an extension to the (already implemented) OAuth 2.0
protocol, and essentially an `id_token` JWT is added to the access token
endpoint response when using the Authorization Code Flow. I also added
support for the "nonce" field since it is required to be used in the
id_token if the client decides to include it in its initial request.
In order to enable this extension an OAuth 2.0 scope containing
"openid" is needed. Other OAuth 2.0 requests should not be impacted by
this change.
This minimal implementation is enough to enable single sign-on (SSO)
for other sites, e.g. by using something like `mod_auth_openidc` to
only allow access to a CI server if a user has logged into Gitea.
Fixes: #1310
Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: zeripath <art27@cantab.net>
