# CVE-2026-6658: Cross-site Scripting (XSS) in jupyter/nbconvert

> Attackers can inject arbitrary JavaScript into HTML exports via unsanitized Mermaid output, executing code in the viewer's browser context.

- **Severity:** Medium 5.4 (`CVSS:3.0/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** Yes
- **Published:** 26 Jun 2026
- **Updated:** 26 Jun 2026
- **Also known as:** DEBIAN-CVE-2026-6658, GHSA-9754-6rhw-gj3h, UBUNTU-CVE-2026-6658, openSUSE-SU-2026:11725-1

## What is CVE-2026-6658?

CVE-2026-6658 is a medium-severity cross-site scripting vulnerability in jupyter/jupyter, published on 26 Jun 2026. Any signed-in user can exploit it over the network, and only if a user opens a file or follows a link.

### Official description

A vulnerability in jupyter/nbconvert versions <= 7.17.0 allows for Cross-site Scripting (XSS) via unsanitized `text/vnd.mermaid` output in HTML exports. The `data_mermaid` block in `share/templates/lab/base.html.j2` renders `text/vnd.mermaid` cell output directly into HTML without escaping, enabling attackers to inject arbitrary HTML/JavaScript by breaking out of the `<pre>` tag. This vulnerability impacts any server using nbconvert to render notebooks as HTML, allowing attackers to execute arbitrary JavaScript in the context of users viewing the HTML export.

Weakness: CWE-79 (Improper Neutralization of Input During Web Page Generation)

## Which versions are affected by CVE-2026-6658?


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `python-nbconvert` | openSUSE:Tumbleweed | All versions | 7.17.1-2.1 |

## How do I fix CVE-2026-6658?

Upgrade to a fixed version.

## Is CVE-2026-6658 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-6658 exists.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-6658 and fix it if it is affected.

What it is: CVE-2026-6658 is a medium-severity cross-site scripting vulnerability in jupyter/jupyter, published on 26 Jun 2026. Any signed-in user can exploit it over the network, and only if a user opens a file or follows a link.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-6658 exists.

Affected product: jupyter/jupyter. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-6658/ (Markdown: https://insights.sbomify.com/CVE-2026-6658/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-6658
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-6658
- OSV: https://osv.dev/vulnerability/CVE-2026-6658

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-6658/
Summaries are generated and may contain mistakes; the official description above is the record.