CVE-2026-42266: JupyterLab Extension Manager API Policy Discrepancy Leading to Malicious Extension Installation
Summary
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. Affects jupyterlab versions >= 4.0.0, < 4.5.7. Fixed in 4.5.7. How to fix ↓
Also tracked for
- jupyterlab · Bitnami
The same CVE-2026-42266 is tracked separately for each ecosystem and vendor. Its fix information is the same; the package and affected versions differ.
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Background
JupyterLab is an open-source web-based interactive development environment for Jupyter notebooks, code, and data. It allows users to create and share documents that contain live code, equations, visualizations, and narrative text. The platform supports various extensions to enhance its functionality. However, a recent vulnerability has been identified in the Extension Manager API, which could allow unauthorized installation of third-party extensions, posing significant security risks.
Vulnerability Detail
The vulnerability arises from a failure in enforcing the allow-list of extensions that can be installed from the PyPI Extension Manager. Specifically, the allowed_extensions_uris setting is not properly enforced in JupyterLab versions prior to 4.5.X. This oversight allows authenticated attackers, such as students in shared environments or users in multi-tenant deployments, to install malicious extensions via POST requests. The implications of this vulnerability include potential privilege escalation, data exfiltration, lateral movement within the network, and persistent compromise of server infrastructure.
How To Fix
Latest published version: 4.6.4 as of 22 Oct 2026
pip install --upgrade "jupyterlab==4.6.4"Upgrading to it may include breaking changes; the version below is the minimum fixed release for this vulnerability.
If you need the minimum change
4.5.7
This is the first release that contains the fix.
pip install --upgrade "jupyterlab==4.5.7"Tip — Not sure which version your app runs? Vulert can match this fix to it. Scan free →
Workaround
As a temporary workaround, you can switch to a read-only extension manager by adding the following command line option when starting JupyterLab: --LabApp.extension_manager=readonly. Alternatively, you can set the following traitlet in your configuration: c.LabApp.extension_manager = 'readonly'. This will prevent users from installing any extensions until the vulnerability is patched.
Conclusion
CVE-2026-42266 represents a significant security risk for JupyterLab users, particularly in shared or multi-tenant environments. It is crucial to update to the patched version 4.5.7 or implement the recommended workarounds to mitigate potential threats.
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FAQ
CVE-2026-42266 is a vulnerability in JupyterLab that allows authenticated users to install malicious extensions via the Extension Manager API due to improper enforcement of the allow-list.
Any deployment of JupyterLab prior to version 4.5.7, especially in multi-tenant environments or where kernel access is restricted, is at risk.
The risks include privilege escalation, data exfiltration, lateral movement within the network, and persistent compromise of server infrastructure.
You can use Vulert, which monitors and alerts for open-source vulnerabilities in real-time. You can check your application against this vulnerability using the Vulert playground without signing up.
The CVSS score for CVE-2026-42266 is 8.8, indicating a high severity level.
References
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