What is CVE-2026-6827?

justhtml before 1.17.0 contains multiple security issues in sanitization, serialization, and programmatic DOM handling. When custom policies preserve foreign namespaces (SVG/MathML), dangerous content such as HTML integration points (SVG , MathML ) and mutation-XSS parser-differential payloads could survive sanitization and become active HTML after reparse; SVG filter="url(...)" and preserved could leave resource-loading CSS (@import, background-image:url()). Programmatic script/style/Comment nodes could serialize into active markup.

How severe is CVE-2026-6827?

No CVSS score has been assigned to CVE-2026-6827 yet. Review the advisory details and affected package list to assess your exposure.

Which packages are affected by CVE-2026-6827?

CVE-2026-6827 affects the following packages: justhtml (PyPI). Ecosystems affected: PyPI.

How do I fix CVE-2026-6827?

Update justhtml to 1.17.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-6827 is resolved across your whole dependency graph.

How do I detect CVE-2026-6827 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-2026-6827 if there is no patch (or I can't update yet)?

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

How does O3 Security protect against CVE-2026-6827?

O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-6827 can be triaged on real exposure rather than presence alone.

Is CVE-2026-6827 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-6827 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-2026-6827?

CVE-2026-6827 has an EPSS (Exploit Prediction Scoring System) score of 0.2%, placing it in the 9th 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-2026-6827?

CVE-2026-6827 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-2026-6827 published, and has it been updated?

CVE-2026-6827 was published on August 23, 2026 and was last updated on August 31, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

CVE-2026-6827: justhtml XSS

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-2026-6827.

EPSS Exploitation Probability

via FIRST.org ↗

0.2%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs9th percentile — riskier than 9% of all scored CVEsHighest risk

EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.

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

justhtml before 1.17.0 contains multiple security issues in sanitization, serialization, and programmatic DOM handling. When custom policies preserve foreign namespaces (SVG/MathML), dangerous content such as HTML integration points (SVG <foreignObject>, MathML <annotation-xml encoding="text/html">) and mutation-XSS parser-differential payloads could survive sanitization and become active HTML after reparse; SVG filter="url(...)" and preserved <style> could leave resource-loading CSS (@import, background-image:url()). Programmatic script/style/Comment nodes could serialize into active markup. Additional hardening fixes address sanitize-pipeline cache mutation and DOM parent/child cycles that could cause infinite loops. Most issues affect advanced or custom configurations rather than the default JustHTML(..., sanitize=True) safe path.

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐍PyPI	`justhtml`	all versions	1.17.0`pip install --upgrade 'justhtml==1.17.0'`

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 1.17.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-6827 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-6827 can be triaged on real exposure rather than presence alone.

Tailored to CVE-2026-6827. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.

Frequently Asked Questions

justhtml before 1.17.0 contains multiple security issues in sanitization, serialization, and programmatic DOM handling. When custom policies preserve foreign namespaces (SVG/MathML), dangerous content such as HTML integration points (SVG <foreignObject>, MathML <annotation-xml encoding="text/html">) and mutation-XSS parser-differential payloads could survive sanitization and become active HTML after reparse; SVG filter="url(...)" and preserved <style> could leave resource-loading CSS (@import, background-image:url()). Programmatic script/style/Comment nodes could serialize into active markup.

O3 Security · Impact-Aware SCA

Is CVE-2026-6827 in your dependencies?

O3 Security finds CVE-2026-6827 across PyPI dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.

Scan my dependencies How O3 SCA works

CVE-2026-6827 — justhtml