Is CVE-2026-44899 exploited in the wild?

No. Junglewise Threat Intelligence has no report of CVE-2026-44899 being exploited in the wild.

How severe is CVE-2026-44899?

Medium severity, with a CVSS score of 4.7, published 26 May 2026.

Which products and versions are affected?

mistune (PyPI) = 3.2.0, ECOSYSTEM:introduced=0,fixed=3.2.1, ECOSYSTEM:introduced=3.2.0,fixed=3.2.1.

Is there a fix for CVE-2026-44899?

Yes. mistune (PyPI): upgrade to 3.2.1 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

How is CVE-2026-44899 attacked?

It is attacked over the network, in a software library.

Am I affected by CVE-2026-44899?

Check whether you run mistune (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.

CVE-2026-44899: lepture Mistune CSS injection in Image directive plugin

CVE-2026-44899 · Severity: medium · CVSS 4.7 · Published 2026-05-26

Mistune is a Python library used to convert Markdown text into HTML. A vulnerability in its image processing plugin allows an attacker to inject malicious CSS code into a web page. This could be used to create deceptive overlays, such as fake login forms or phishing screens, that cover the legitimate content of a website.

Technologies: mistune (PyPI). Vendors: PyPI.

Quick answers

Is CVE-2026-44899 exploited in the wild?: No. Junglewise Threat Intelligence has no report of CVE-2026-44899 being exploited in the wild.
How severe is CVE-2026-44899?: Medium severity, with a CVSS score of 4.7, published 26 May 2026.
Which products and versions are affected?: mistune (PyPI) = 3.2.0, ECOSYSTEM:introduced=0,fixed=3.2.1, ECOSYSTEM:introduced=3.2.0,fixed=3.2.1.
Is there a fix for CVE-2026-44899?: Yes. mistune (PyPI): upgrade to 3.2.1 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-44899 attacked?: It is attacked over the network, in a software library.
Am I affected by CVE-2026-44899?: Check whether you run mistune (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.

How to fix

mistune (PyPI): upgrade to 3.2.1 or later
```
pip install "mistune>=3.2.1"
```

Technical details

The Image directive plugin in Mistune uses a regular expression (`_num_re = re.compile(r"^\d+(?:\.\d*)?")`) that lacks an end-of-string anchor ($). Because validation is performed using `re.match()`, it only checks if the input starts with digits. If the input is not a plain integer (e.g., containing CSS units or malicious payloads), `render_block_image()` inserts the value directly into a `style` attribute without escaping. An attacker can provide a crafted `:width:` or `:height:` value containing a chain of CSS properties (e.g., `position:fixed; z-index:9999`). This allows for full-page UI redressing, phishing overlays, or other CSS-based attacks. The issue is fixed in version 3.2.1 by using a stricter regex.

Affected products

lepture mistune < 3.2.1

Identifiers

CVE-2026-44899
GHSA-ccfx-mfmx-2fx9
PYSEC-2026-2209

Timeline

2026-05-03: patched: Version 3.2.1 released on GitHub.
2026-05-12: advisory: GitHub Security Advisory GHSA-ccfx-mfmx-2fx9 published.
2026-05-26: disclosed: CVE-2026-44899 published to NVD.

References

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