How severe is GHSA-c9vm-hv86-f23r?

No CVSS score has been assigned to GHSA-c9vm-hv86-f23r yet. Review the advisory details and affected package list to assess your exposure.

Which packages are affected by GHSA-c9vm-hv86-f23r?

GHSA-c9vm-hv86-f23r affects the following packages: justhtml (PyPI). Ecosystems affected: PyPI.

How do I fix GHSA-c9vm-hv86-f23r?

Update justhtml to 1.15.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-c9vm-hv86-f23r is resolved across your whole dependency graph.

How do I detect GHSA-c9vm-hv86-f23r 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 GHSA-c9vm-hv86-f23r 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.

Is GHSA-c9vm-hv86-f23r actively exploited in the wild?

No public exploit code has been indexed for GHSA-c9vm-hv86-f23r 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 GHSA-c9vm-hv86-f23r?

GHSA-c9vm-hv86-f23r has an EPSS (Exploit Prediction Scoring System) score of 0.6%, placing it in the 48th 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 GHSA-c9vm-hv86-f23r?

GHSA-c9vm-hv86-f23r is classified as Improper Input Validation (CWE-20). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was GHSA-c9vm-hv86-f23r published, and has it been updated?

GHSA-c9vm-hv86-f23r was published on April 10, 2026 and was last updated on August 24, 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

justhtml includes multiple security fixesGHSA-c9vm-hv86-f23r

GHSA-c9vm-hv86-f23r is a Improper Input Validation vulnerability in justhtml. A fix is available for justhtml — see the affected versions and patch details below.

Also known asCVE-2026-5388

Published

Apr 10, 2026

Updated

Aug 24, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Nov 2, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
A successful exploit gives an attacker total control of the affected component, not partial access.
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 GHSA-c9vm-hv86-f23r.

EPSS Exploitation Probability

via FIRST.org ↗

0.6%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs48th percentile — riskier than 48% 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

justhtml 1.15.0 includes multiple security fixes affecting URL sanitization helpers, HTML serialization, Markdown passthrough, and several custom sanitization-policy edge cases.

These issues have different impact levels and do not all affect the default configuration in the same way.

Affected versions

justhtml <= 1.14.0

Fixed version

justhtml 1.15.0 released on April 9, 2026

Impact overview

Helper and serialization issues

These issues could affect applications using JustHTML helpers or programmatic DOM construction, even outside the default HTML sanitization path.

JustHTML.clean_url_value(...) and clean_url_in_js_string(...) could accept URL values such as javascript&#58..., which became active javascript: URLs after HTML attribute parsing.
URL sanitization could treat values like \\evil.example/x or /\\evil.example/x as safe relative URLs even though browsers could resolve them as remote requests.
Malformed bracketed hosts such as https://[evil.example]/x could raise exceptions and crash sanitization when host allowlists were used.
Programmatic element or attribute names containing markup-breaking characters could be serialized into active HTML.
Programmatic HTML comments containing --> could break out of the comment and inject live markup.

Markdown passthrough issue

to_markdown(html_passthrough=True) could reintroduce active HTML from sanitized <textarea> content by emitting a raw closing </textarea> sequence.

Custom policy issues

These issues affected custom policies more than the default safe configuration.

a[ping] was handled as a single URL even though browsers interpret it as a space-separated URL list.
attributionsrc was not treated as URL-bearing and could preserve attacker-controlled reporting endpoints.
link[imagesrcset] was not treated as URL-bearing and could preserve attacker-controlled image candidates.
Preserved <meta http-equiv="refresh"> tags could keep redirect targets without URL-policy enforcement.
Preserved <base href> tags could rewrite how later relative URLs resolved in the browser.
Preserved <style> blocks could keep resource-loading CSS such as @import, url(...), or image-set(...).
Mixed-case attribute names in custom transform pipelines could bypass or confuse security-related transforms such as DropAttrs(...), DropUrlAttrs(...), AllowStyleAttrs(...), and MergeAttrs(...).

Default configuration

Most of the custom-policy issues above did not affect the default JustHTML(..., sanitize=True) behavior.

The main exceptions were:

helper APIs such as clean_url_value(...)
programmatic DOM / serializer usage
applications explicitly using html_passthrough=True
applications using custom policies or custom transform pipelines

Recommended action

Upgrade to justhtml 1.15.0.

If you cannot upgrade immediately:

avoid html_passthrough=True for untrusted content
avoid preserving <style>, <meta http-equiv="refresh">, and <base href> in custom policies
avoid allowing ping, attributionsrc, or imagesrcset unless you explicitly validate them
avoid serializing untrusted programmatic node names, attribute names, or comment data

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐍PyPI	`justhtml`	all versions	1.15.0`pip install --upgrade 'justhtml==1.15.0'`

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 1.15.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-c9vm-hv86-f23r 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.

Frequently Asked Questions

## Summary `justhtml` `1.15.0` includes multiple security fixes affecting URL sanitization helpers, HTML serialization, Markdown passthrough, and several custom sanitization-policy edge cases. These issues have different impact levels and do not all affect the default configuration in the same way. ## Affected versions - `justhtml` `<= 1.14.0` ## Fixed version - `justhtml` `1.15.0` released on April 9, 2026 ## Impact overview ### Helper and serialization issues These issues could affect applications using JustHTML helpers or programmatic DOM construction, even outside the default HTML

O3 Security · Impact-Aware SCA

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