Is CVE-2026-5389 exploited in the wild?

No. Junglewise Threat Intelligence has no report of CVE-2026-5389 being exploited in the wild.

How severe is CVE-2026-5389?

Medium severity, with a CVSS score of 6.1, published 23 August 2026.

Which products and versions are affected?

justhtml (PyPI) ECOSYSTEM:introduced=0,fixed=1.13.0.

Is there a fix for CVE-2026-5389?

Yes. justhtml (PyPI): upgrade to 1.13.0 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

How is CVE-2026-5389 attacked?

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

Am I affected by CVE-2026-5389?

Check whether you run justhtml (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-2026-5389: justhtml to_markdown() cross-site scripting via code fence breakout

CVE-2026-5389 · Severity: medium · CVSS 6.1 · Published 2026-08-23

justhtml is a Python library for sanitizing and converting HTML to Markdown. A flaw in the to_markdown() function allows attackers to inject backticks inside code blocks, breaking out of fixed-length fences and exposing raw HTML to downstream Markdown renderers. When the resulting Markdown is rendered, the attacker-controlled HTML can execute, leading to cross-site scripting attacks in applications that treat the output as safe.

Technologies: justhtml (PyPI). Vendors: PyPI.

Quick answers

Is CVE-2026-5389 exploited in the wild?: No. Junglewise Threat Intelligence has no report of CVE-2026-5389 being exploited in the wild.
How severe is CVE-2026-5389?: Medium severity, with a CVSS score of 6.1, published 23 August 2026.
Which products and versions are affected?: justhtml (PyPI) ECOSYSTEM:introduced=0,fixed=1.13.0.
Is there a fix for CVE-2026-5389?: Yes. justhtml (PyPI): upgrade to 1.13.0 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-5389 attacked?: It is attacked over the network, in a software library.
Am I affected by CVE-2026-5389?: Check whether you run justhtml (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

justhtml (PyPI): upgrade to 1.13.0 or later
```
pip install "justhtml>=1.13.0"
```

Technical details

The vulnerability is a cross-site scripting (XSS) bypass in the to_markdown() function's handling of HTML <pre> elements (CWE-80). When converting <pre> content to Markdown, the serializer wraps decoded text in fixed triple-backtick code fences (```) without checking if the content contains backtick sequences that could break the fence. An attacker can inject backtick runs (``` or longer) within sanitized <pre> elements to prematurely close the code block, allowing subsequent HTML-like text to appear outside the fence where it is treated as raw HTML by CommonMark/GFM renderers. No authentication is required; the attack is network-reachable and requires only user interaction (rendering the Markdown output). The vulnerability affects justhtml versions before 1.13.0, which patches the issue by dynamically selecting a fence length longer than the longest backtick run in the content.

Affected products

EmilStenstrom justhtml before 1.13.0

Identifiers

CVE-2026-5389
GHSA-5vp3-3cg6-2rq3

Timeline

2026-03-21: disclosed
2026-03-21: patched: Version 1.13.0 released
2026-08-23: advisory: CVE-2026-5389 published

References

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