# CVE-2026-44899: Mistune Image Directive CSS Injection Vulnerability

> An attacker can inject malicious CSS into image directives, potentially leading to cross-site scripting attacks in applications using the Mistune library.

- **Severity:** Medium 4.7 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:N/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** Yes
- **Published:** 26 May 2026
- **Updated:** 27 May 2026
- **Also known as:** AZL-88406, CLEANSTART-2026-OL63472, CLEANSTART-2026-SI19530, DEBIAN-CVE-2026-44899, GHSA-ccfx-mfmx-2fx9, PYSEC-2026-2209, UBUNTU-CVE-2026-44899

## What is CVE-2026-44899?

CVE-2026-44899 is a medium-severity cross-site scripting vulnerability in lepture mistune, published on 26 May 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

### Official description

Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.2.1, the Image directive plugin validates the :width: and :height: options with a regex compiled as _num_re = re.compile(r"^\d+(?:\.\d*)?"). When the validated value is not a plain integer, render_block_image() inserts it directly into a style="width:...;" or style="height:...;" attribute. Because the value was accepted by the prefix-only regex, any CSS after the leading digits reaches the style= attribute verbatim and without escaping. This vulnerability is fixed in 3.2.1.

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

## Which versions are affected by CVE-2026-44899?

3 open source packages ship an affected version, including mistune. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `mistune` | PyPI | All versions | 3.2.1 |
| `mistune` | PyPI | From 3.2.0 | 3.2.1 |
| `mistune` | PyPI | From 3.2.0 | 3.2.1 |

Products: Mistune Project Mistune.

## How do I fix CVE-2026-44899?

Upgrade mistune to 3.2.1 or later.

## Is CVE-2026-44899 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-44899 exists.

## Other vulnerabilities in lepture mistune

- [CVE-2026-59922](https://insights.sbomify.com/CVE-2026-59922/): Mistune plugins/formatting: quadratic-time parsing on long runs of `~~x~~`, `==x==`, and `^^x^^` markers (strikethrough / mark / insert) (high 7.5)
- [CVE-2026-59925](https://insights.sbomify.com/CVE-2026-59925/): inline_parser: quadratic-time parsing on long runs of `**x**` and `***x***` emphasis pairs (high 7.5)
- [CVE-2026-59928](https://insights.sbomify.com/CVE-2026-59928/): Mistune block_parser: quadratic-time parsing on long lists of repeated reference-link definitions (high 7.5)
- [CVE-2026-59923](https://insights.sbomify.com/CVE-2026-59923/): Mistune: XSS via percent-encoded javascript URI bypass in safe_url() (medium 6.1)
- [CVE-2026-59929](https://insights.sbomify.com/CVE-2026-59929/): Mistune renderers/html.safe_url: HARMFUL_PROTOCOLS list misses legacy and chained schemes that historically chain to `javascript:` execution (medium 6.1)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-44899 and fix it if it is affected.

What it is: CVE-2026-44899 is a medium-severity cross-site scripting vulnerability in lepture mistune, published on 26 May 2026. An attacker can exploit it over the internet without logging in, 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-2026-44899 exists.

Affected packages and fixed versions:
- mistune (PyPI): affected all versions; fixed in 3.2.1
- mistune (PyPI): affected from 3.2.0; fixed in 3.2.1
- mistune (PyPI): affected from 3.2.0; fixed in 3.2.1

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-2026-44899/ (Markdown: https://insights.sbomify.com/CVE-2026-44899/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-44899
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-44899
- OSV: https://osv.dev/vulnerability/CVE-2026-44899

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-44899/
Summaries are generated and may contain mistakes; the official description above is the record.