JupyterLab CVE-2026-42266

The allow-list of extensions that can be installed from PyPI Extension Manager (allowed_extensions_uris) is not correctly enforced by JupyterLab prior to 4.5.X. The PyPI Extension Manager was not contained to packages listed on the default PyPI index.

This has security implications for deployments that:

• have allow-listed specific extensions with aim to prevent users from installing packages
• have the kernel and terminals disabled or delegated to remote hosts (thus no access to install packages in the single-user server environment)
• have multi-tenant deployments that is not configured for untrusted users (as per documented on JupyterHub https://jupyterhub.readthedocs.io/en/5.2.1/explanation/websecurity.html)
• have the (default) PyPI Extension Manger enabled

Impact

An authenticated attacker - such as a student in a shared JupyterHub environment or a user in a multi-tenant JupyterLab deployment - can escalate their privileges. This might allow for data exfiltration, lateral movement within the network, and persistent compromise of the server infrastructure.

Patches

JupyterLab v4.5.7 contains the patch.

Users of applications that depend on JupyterLab, such as Notebook v7+, should update jupyterlab package too.

Workarounds

Switch to read-only extension manager by adding the following command line option:

--LabApp.extension_manager=readonly

or the following traitlet:

c.LabApp.extension_manager = 'readonly'

You can confirm that the read-only manager is in use from GUI:

<img width="293" height="293" alt="image" src="https://github.com/user-attachments/assets/8016c809-633e-4ed0-a5bc-6bc4793caa0f" />

Note: configuration of a PyPI proxy with allow-listed packages is not sufficient to protect from this vulnerability.

Resources

• allow-list https://jupyterlab.readthedocs.io/en/stable/user/extensions.html#listing-configuration
• https://jupyterhub.readthedocs.io/en/5.2.1/explanation/websecurity.html
• https://jupyterlab.readthedocs.io/en/latest/user/extensions.html#extension-manager-implementations