PYSEC-2026-1480

### Impact

Affected configurations:

- Single-origin JupyterHub deployments - JupyterHub deployments with user-controlled applications running on subdomains or peer subdomains of either the Hub or a single-user server.

By tricking a user into visiting a malicious subdomain, the attacker can achieve an XSS directly affecting the former's session. More precisely, in the context of JupyterHub, this XSS could achieve the following:

- Full access to JupyterHub API and user's single-user server, e.g. - Create and exfiltrate an API Token - Exfiltrate all files hosted on the user's single-user server: notebooks, images, etc. - Install malicious extensions. They can be used as a backdoor to silently regain access to victim's session anytime.

### Patches

To prevent cookie-tossing:

- Upgrade to JupyterHub 4.1 (both hub and user environment) - enable per-user domains via `c.JupyterHub.subdomain_host = "https://mydomain.example.org"` - set `c.JupyterHub.cookie_host_prefix_enabled = True` to enable domain-locked cookies

or, if available (applies to earlier JupyterHub versions):

- deploy jupyterhub on its own domain, not shared with any other services - enable per-user domains via `c.JupyterHub.subdomain_host = "https://mydomain.example.org"`