CVE-2026-33175: oauthenticator: auth bypass enables JupyterHub account takeover

### Summary An authentication bypass vulnerability in `oauthenticator` allows an attacker with an unverified email address on an Auth0 tenant to login to JupyterHub. When `email` is used as the usrname_claim, this gives users control over their username and the possibility of account takeover. ### Impact This is an **Authentication Bypass Vulnerability**. Any Auth0 tenant leveraging the `Auth0OAuthenticator` mapping the `email` claim to the JupyterHub username is impacted. By default, Auth0 handles email verification as a user flag, not a hard block to authentication streams. If an attacker can register an account with the Auth0 tenant with an unverified email and knows the email of an existing user on the system, they can authenticate as that user. ### Patches - Upgrade oauthenticator to 17.4 ### Workarounds - Check `email_verified` field in an `Authenticator.post_auth_hook` function - Do not use `email` as the username claim - [Enforce email verification in auth0](https://support.auth0.com/center/s/article/Enforce-Email-Verification-With-Sending-Email-After-Each-Denied-Access)

An attacker targets a data scientist at a company using JupyterHub with Auth0 SSO. The attacker obtains the target's corporate email from LinkedIn, a conference talk bio, or a GitHub commit. They register a new account on the same Auth0 tenant — possible if the tenant allows self-service registration or if the attacker has any legitimate access — using the target's email without completing email verification. Auth0 allows authentication for unverified accounts by default. The attacker authenticates to JupyterHub, which maps the unverified email claim directly to the JupyterHub username without checking email_verified, granting full access to the victim's environment including notebooks with proprietary model code, mounted datasets, and stored API keys for OpenAI, AWS, or other cloud services.