Authorization Bypass Through User-Controlled Key when using CILogonOAuthenticator in oauthenticator

OAuthenticator is an OAuth token library for the JupyerHub login handler. CILogonOAuthenticator is provided by the OAuthenticator package, and lets users log in to a JupyterHub via CILogon. This is primarily used to restrict a JupyterHub only to users of a given institute. The allowed_idps configuration trait of CILogonOAuthenticator is documented to be a list of domains that indicate the institutions whose users are authorized to access this JupyterHub. This authorization is validated by ensuring that the email field provided to us by CILogon has a domain that matches one of the domains listed in allowed_idps.If allowed_idps contains berkeley.edu, you might expect only users with valid current credentials provided by University of California, Berkeley to be able to access the JupyterHub. However, CILogonOAuthenticator does not verify which provider is used by the user to login, only the email address provided. So a user can login with a GitHub account that has email set to @berkeley.edu, and that will be treated exactly the same as someone logging in using the UC Berkeley official Identity Provider. The patch fixing this issue makes a breaking change in how allowed_idps is interpreted. It's no longer a list of domains, but configuration representing the EntityID of the IdPs that are allowed, picked from the list maintained by CILogon. Users are advised to upgrade.