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"