Background

CILogon is a federated auth provider that allows users to authenticate themselves via a number of Identity Providers (IdP), focused primarily on educational and research institutions (such as Universities). More traditional and open IdPs such as GitHub, ORCID, Google, Microsoft, etc are also supported.

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.

Impact

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 <something>@berkeley.edu, and that will be treated exactly the same as someone logging in using the UC Berkeley official Identity Provider. This has two consequences:

1. Since GitHub (and most other providers we tested) only require you to verify your email once, a user can access a JupyterHub even if their access to the institution's IdP has been revoked or expired.
2. CILogon supports hundreds of identity providers - if even one of them allows users to set any email ids without verifying, that can be used to impersonate any user on any other identity provider! While CILogon itself has a stellar security record, this particular method of doing authorization means an attacker would only need to compromise a single identity provider to compromise all of CILogon

We currently do not know of any identity provider that provides unverified email addresses to CILogon, so this is not a severe known vulnerability. However, there are hundreds of IdPs, and we could not try them all.

Patches

This patch 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. So instead of berkeley.edu, you would specify urn:mace:incommon:berkeley.edu to allow logins from users currently with berkeley.edu accounts. GitHub users with a verified berkeley.edu email will no longer be allowed to log in.

For details on how to transition your CILogonOAuthenticator configuration to the patched version 15.0.0 or above, see the migration documentation.