Is CVE-2026-86258 exploited in the wild?

No. Junglewise Threat Intelligence has no report of CVE-2026-86258 being exploited in the wild.

How severe is CVE-2026-86258?

Medium severity, with a CVSS score of 5.9, published 6 September 2026.

Which products and versions are affected?

Jupyter Nbviewer through 1.0.1.

Is there a fix for CVE-2026-86258?

No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

How is CVE-2026-86258 attacked?

It is attacked over the network.

Am I affected by CVE-2026-86258?

Check whether you run Jupyter Nbviewer at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

CVE-2026-86258: Jupyter nbviewer path traversal in LocalFileHandler

CVE-2026-86258 · Severity: medium · CVSS 5.9 · Published 2026-09-06

nbviewer is a web application that renders Jupyter Notebooks as static web pages. A path traversal vulnerability in the LocalFileHandler component allows attackers to read files outside the configured root directory, potentially exposing sensitive notebooks and credentials stored on the same server.

Vendors: Jupyter.

Quick answers

Is CVE-2026-86258 exploited in the wild?: No. Junglewise Threat Intelligence has no report of CVE-2026-86258 being exploited in the wild.
How severe is CVE-2026-86258?: Medium severity, with a CVSS score of 5.9, published 6 September 2026.
Which products and versions are affected?: Jupyter Nbviewer through 1.0.1.
Is there a fix for CVE-2026-86258?: No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.
How is CVE-2026-86258 attacked?: It is attacked over the network.
Am I affected by CVE-2026-86258?: Check whether you run Jupyter Nbviewer at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

Technical details

The vulnerability exists in LocalFileHandler.can_show() which validates file paths using string-prefix comparison rather than proper path normalization and canonical path checking. An attacker can craft requests with paths that share the root directory as a textual prefix (e.g., if root is "/notebooks", requesting "/notebooks_sibling/file" bypasses validation). The flaw affects only the local file provider handler and requires network access to the nbviewer instance. An attacker can read arbitrary files accessible to the nbviewer process, including sensitive configuration and credential files. This is fixed in versions after 1.0.1.

Affected products

Jupyter nbviewer through 1.0.1

Timeline

2026-09-06: disclosed

References

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