How severe is CVE-2026-5751?

No CVSS score has been assigned to CVE-2026-5751 yet. Review the advisory details and affected package list to assess your exposure.

Which packages are affected by CVE-2026-5751?

CVE-2026-5751 affects the following packages: justhtml (PyPI). Ecosystems affected: PyPI.

How do I fix CVE-2026-5751?

Update justhtml to 1.14.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-5751 is resolved across your whole dependency graph.

How do I detect CVE-2026-5751 in my PyPI dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for justhtml, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2026-5751 if there is no patch (or I can't update yet)?

Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.

Is CVE-2026-5751 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-5751 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2026-5751?

CVE-2026-5751 has an EPSS (Exploit Prediction Scoring System) score of 0.3%, placing it in the 16th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-5751?

CVE-2026-5751 is classified as Cross-site Scripting (XSS) (CWE-79). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation. This is a high-impact weakness class that often enables remote code execution or data exposure.

When was CVE-2026-5751 published, and has it been updated?

CVE-2026-5751 was published on August 23, 2026 and was last updated on August 28, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐍 PyPI

Not in CISA KEV

CVE-2026-5751 — justhtml

Fix: EmilStenstrom/justhtml@8adc4df

CVE-2026-5751 is a Cross-site Scripting (XSS) vulnerability in justhtml. A fix is available for justhtml — see the affected versions and patch details below.

justhtml before 1.14.0 Mutation XSS via custom sanitization policies

Also known asGHSA-r758-8hxw-4845

Published

Aug 23, 2026

Updated

Aug 28, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Sep 26, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-5751.

EPSS Exploitation Probability

via FIRST.org ↗

0.3%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs16th percentile — riskier than 16% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

Real-World Exposure

1 pkg affected

🐍justhtml

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.

Description

Summary

A parser-differential / mutation XSS issue was found in justhtml when using a custom sanitization policy that preserves foreign namespaces such as SVG or MathML.

Under these custom settings, specially crafted input could sanitize into HTML that looked safe at first, but became unsafe when parsed again by a browser or another HTML parser.

Impact

This issue does not affect the default safe configuration.

You may be affected if you use a custom SanitizationPolicy with settings like:

drop_foreign_namespaces=False
allowlisted foreign elements such as MathML or SVG
allowlisted raw-text containers such as <style>

In that case, an attacker could inject markup that survives sanitization and turns into active HTML after re-parsing.

Affected versions

justhtml <= 1.13.0

Fixed version

Fixed in 1.14.0

Workarounds

Until you upgrade:

keep drop_foreign_namespaces=True
avoid allowlisting foreign namespaces for untrusted input
avoid allowlisting raw-text containers such as <style> in custom policies

Notes

The default JustHTML(..., sanitize=True) behavior was not found to be vulnerable in this issue.

Credit

Discovered by JustHTML author during a LLM-based security review of justhtml.

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐍PyPI	`justhtml`	≥ 1.13.0&&< 1.14.0	1.14.0`pip install --upgrade 'justhtml==1.14.0'`

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for justhtml, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update justhtml to 1.14.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-5751 is resolved across your whole dependency graph.
Workarounds
Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.

Frequently Asked Questions

## Summary A parser-differential / mutation XSS issue was found in `justhtml` when using a **custom sanitization policy** that preserves foreign namespaces such as SVG or MathML. Under these custom settings, specially crafted input could sanitize into HTML that looked safe at first, but became unsafe when parsed again by a browser or another HTML parser. ## Impact This issue does **not** affect the default safe configuration. You may be affected if you use a custom `SanitizationPolicy` with settings like: - `drop_foreign_namespaces=False` - allowlisted foreign elements such as MathML or

O3 Security · Impact-Aware SCA

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