CVE-2020-26250

OAuthenticator is an OAuth login mechanism for JupyterHub. In oauthenticator from version 0.12.0 and before 0.12.2, the deprecated (in jupyterhub 1.2) configuration Authenticator.whitelist, which should be transparently mapped to Authenticator.allowed_users with a warning, is instead ignored by OAuthenticator classes, resulting in the same behavior as if this configuration has not been set. If this is the only mechanism of authorization restriction (i.e. no group or team restrictions in configuration) then all authenticated users will be allowed. Provider-based restrictions, including deprecated values such as GitHubOAuthenticator.org_whitelist are not affected. All users of OAuthenticator 0.12.0 and 0.12.1 with JupyterHub 1.2 (JupyterHub Helm chart 0.10.0-0.10.5) who use the admin.whitelist.users configuration in the jupyterhub helm chart or the c.Authenticator.whitelist configuration directly. Users of other deprecated configuration, e.g. c.GitHubOAuthenticator.team_whitelist are not affected. If you see a log line like this and expect a specific list of allowed usernames: [I 2020-11-27 16:51:54.528 JupyterHub app:1717] Not using allowed_users. Any authenticated user will be allowed. you are likely affected. Updating oauthenticator to 0.12.2 is recommended. A workaround is to replace the deprecated c.Authenticator.whitelist = ... with c.Authenticator.allowed_users = .... If any users have been authorized during this time who should not have been, they must be deleted via the API or admin interface, per the referenced documentation.

Weakness

The product performs an authorization check when an actor attempts to access a resource or perform an action, but it does not correctly perform the check. This allows attackers to bypass intended access restrictions.

Affected Software

Extended Description

Assuming a user with a given identity, authorization is the process of determining whether that user can access a given resource, based on the user’s privileges and any permissions or other access-control specifications that apply to the resource. When access control checks are incorrectly applied, users are able to access data or perform actions that they should not be allowed to perform. This can lead to a wide range of problems, including information exposures, denial of service, and arbitrary code execution.

Potential Mitigations

• Divide the product into anonymous, normal, privileged, and administrative areas. Reduce the attack surface by carefully mapping roles with data and functionality. Use role-based access control (RBAC) [REF-229] to enforce the roles at the appropriate boundaries.
• Note that this approach may not protect against horizontal authorization, i.e., it will not protect a user from attacking others with the same role.
• Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.
• For example, consider using authorization frameworks such as the JAAS Authorization Framework [REF-233] and the OWASP ESAPI Access Control feature [REF-45].
• For web applications, make sure that the access control mechanism is enforced correctly at the server side on every page. Users should not be able to access any unauthorized functionality or information by simply requesting direct access to that page.
• One way to do this is to ensure that all pages containing sensitive information are not cached, and that all such pages restrict access to requests that are accompanied by an active and authenticated session token associated with a user who has the required permissions to access that page.

References

• https://github.com/jupyterhub/oauthenticator/blob/master/docs/source/changelog.md#0122---2020-11-30
• https://github.com/jupyterhub/oauthenticator/commit/a4aac191c16cf6281f3d346615aefa75702b02d7
• https://github.com/jupyterhub/oauthenticator/security/advisories/GHSA-384w-5v3f-q499
• https://jupyterhub.readthedocs.io/en/1.2.2/getting-started/authenticators-users-basics.html#add-or-remove-users-from-the-hub