Quick answers
- Is CVE-2026-6658 exploited in the wild?
- No. Junglewise Threat Intelligence has no report of CVE-2026-6658 being exploited in the wild.
- How severe is CVE-2026-6658?
- Medium severity, with a CVSS score of 5.4, published 26 June 2026.
- Which products and versions are affected?
- nbconvert (PyPI) <= 7.17.0, ECOSYSTEM:introduced=0,last_affected=7.17.0.
- Is there a fix for CVE-2026-6658?
- 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-2026-6658 attacked?
- It is attacked over the network, in a software library.
- Am I affected by CVE-2026-6658?
- Check whether you run nbconvert (PyPI) 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
A stored Cross-site Scripting (XSS) vulnerability exists in jupyter/nbconvert versions up to and including 7.17.0. The root cause is located in the `data_mermaid` block within the `share/templates/lab/base.html.j2` template, which fails to properly escape `text/vnd.mermaid` cell output. An attacker with the ability to provide or modify a notebook can craft a malicious Mermaid diagram that breaks out of the `<pre>` tag to inject arbitrary HTML or JavaScript. This code executes in the security context of any user viewing the exported HTML file. The vulnerability requires the victim to view the rendered output and typically requires the attacker to have low-level privileges to provide the notebook content.
Affected products
- Jupyter nbconvert <= 7.17.0
Identifiers
- CVE-2026-6658
- GHSA-9754-6rhw-gj3h
Timeline
- 2026-06-26: disclosed
- 2026-06-26: advisory