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 4.5.7. 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 Manager enabled

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:

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

References

• 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

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.

The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths. Typical impact: varies by context: data corruption, logic bypass, or denial of service.

CVE-2026-42266 has a CVSS score of 8.8 (High). The vector is network-reachable, low privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (4.5.7); upgrading removes the vulnerable code path.

Kodem intelligence

JupyterLab v4.5.7 contains the patch.

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

1. What is CVE-2026-42266? CVE-2026-42266 is a high-severity improper input validation vulnerability in jupyterlab (pip), affecting versions >= 4.0.0, <= 4.5.6. It is fixed in 4.5.7. The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths.
2. How severe is CVE-2026-42266? CVE-2026-42266 has a CVSS score of 8.8 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of jupyterlab are affected by CVE-2026-42266? jupyterlab (pip) versions >= 4.0.0, <= 4.5.6 is affected.
4. Is there a fix for CVE-2026-42266? Yes. CVE-2026-42266 is fixed in 4.5.7. Upgrade to this version or later.
5. Is CVE-2026-42266 exploitable, and should I be worried? Whether CVE-2026-42266 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-42266 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-42266? Upgrade jupyterlab to 4.5.7 or later.