GHSA-c9vm-hv86-f23r — justhtml
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.
justhtml includes multiple security fixes
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
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
justhtmlReal-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
justhtml1.15.0released 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(...)andclean_url_in_js_string(...)could accept URL values such asjavascript:..., which became activejavascript:URLs after HTML attribute parsing.- URL sanitization could treat values like
\\evil.example/xor/\\evil.example/xas safe relative URLs even though browsers could resolve them as remote requests. - Malformed bracketed hosts such as
https://[evil.example]/xcould 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.attributionsrcwas 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(...), orimage-set(...). - Mixed-case attribute names in custom transform pipelines could bypass or confuse security-related transforms such as
DropAttrs(...),DropUrlAttrs(...),AllowStyleAttrs(...), andMergeAttrs(...).
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=Truefor untrusted content - avoid preserving
<style>,<meta http-equiv="refresh">, and<base href>in custom policies - avoid allowing
ping,attributionsrc, orimagesrcsetunless you explicitly validate them - avoid serializing untrusted programmatic node names, attribute names, or comment data
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | justhtml | all versions | 1.15.0pip install --upgrade 'justhtml==1.15.0' |
Detection & mitigation playbook
Open-source dependencyDetect
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.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like GHSA-c9vm-hv86-f23r can be triaged on real exposure rather than presence alone.
Tailored to GHSA-c9vm-hv86-f23r. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is GHSA-c9vm-hv86-f23r in your dependencies?
O3 Security finds GHSA-c9vm-hv86-f23r across PyPI dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.