How severe is GHSA-4p64-v8f5-r2gx?

No CVSS score has been assigned to GHSA-4p64-v8f5-r2gx yet. Review the advisory details and affected package list to assess your exposure.

Which packages are affected by GHSA-4p64-v8f5-r2gx?

GHSA-4p64-v8f5-r2gx affects the following packages: justhtml (PyPI). Ecosystems affected: PyPI.

How do I fix GHSA-4p64-v8f5-r2gx?

Update justhtml to 1.16.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-4p64-v8f5-r2gx is resolved across your whole dependency graph.

How do I detect GHSA-4p64-v8f5-r2gx in my PyPI dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for justhtml. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.

How do I mitigate GHSA-4p64-v8f5-r2gx 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 GHSA-4p64-v8f5-r2gx?

O3 pinpoints whether GHSA-4p64-v8f5-r2gx is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.

Is GHSA-4p64-v8f5-r2gx actively exploited in the wild?

No public exploit code has been indexed for GHSA-4p64-v8f5-r2gx 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.

When was GHSA-4p64-v8f5-r2gx published, and has it been updated?

GHSA-4p64-v8f5-r2gx was published on April 14, 2026 and was last updated on April 14, 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

GHSA-4p64-v8f5-r2gx

Multiple security fixes in justhtml

Published

Apr 14, 2026

Updated

Apr 14, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Blast Radius

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 1.16.0 fixes multiple security issues in sanitization, serialization, and programmatic DOM handling.

Most of these issues affected one of these advanced paths rather than ordinary parsed HTML with the default safe settings:

programmatic DOM input to sanitize() or sanitize_dom()
reused or mutated sanitization policy objects
custom policies that preserve foreign namespaces such as SVG or MathML

Affected versions

justhtml <= 1.15.0

Fixed version

justhtml 1.16.0 released on April 12, 2026

Impact

Policy reuse and mutation

Nested mutation of sanitization policy internals could weaken later sanitization by leaving stale compiled sanitizers active, or by mutating exported default policy internals process-wide.

In-memory sanitization gaps

Programmatic DOM sanitization could miss dangerous mixed-case tag names such as ScRiPt or StYlE, and custom drop_content_tags values such as {"SCRIPT"} could silently fail to drop dangerous subtrees.

Serialization injection

Crafted programmatic doctype names could serialize into active markup before the document body.

Foreign-namespace policy bypasses

Custom policies that preserve SVG or MathML could allow active SVG features to survive sanitization, including:

animation elements such as <set> and <animate> that mutate already-sanitized attributes after sanitization
presentation attributes such as fill, clip-path, mask, marker-start, and cursor containing external url(...) references
programmatic DOM trees that claim namespace="html" but serialize as <svg> or <math>, bypassing foreign-content checks

Rawtext hardening gap

Mixed-case programmatic style or script nodes could bypass rawtext hardening and preserve active stylesheet content such as remote @import rules.

Default configuration

Most of these issues did not affect the normal JustHTML(..., sanitize=True) path for ordinary parsed HTML.

The main exceptions were policy-mutation issues, which could weaken later sanitization if code mutated nested state on reused policy objects or exported defaults.

Recommended action

Upgrade to justhtml 1.16.0.

If you cannot upgrade immediately:

do not mutate DEFAULT_POLICY, DEFAULT_DOCUMENT_POLICY, or nested policy internals
avoid reusing policy objects after mutating nested state
avoid preserving SVG or MathML for untrusted input
avoid preserving style or script in custom policies for untrusted input
avoid serializing untrusted programmatic doctypes or DOM trees

Credit

Discovered during an internal security review of justhtml.

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐍PyPI	`justhtml`	all versions	1.16.0

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for justhtml. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update justhtml to 1.16.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-4p64-v8f5-r2gx 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 pinpoints whether GHSA-4p64-v8f5-r2gx is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.

Tailored to GHSA-4p64-v8f5-r2gx. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.

Frequently Asked Questions

## Summary `justhtml` `1.16.0` fixes multiple security issues in sanitization, serialization, and programmatic DOM handling. Most of these issues affected one of these advanced paths rather than ordinary parsed HTML with the default safe settings: - programmatic DOM input to `sanitize()` or `sanitize_dom()` - reused or mutated sanitization policy objects - custom policies that preserve foreign namespaces such as SVG or MathML ## Affected versions - `justhtml` `<= 1.15.0` ## Fixed version - `justhtml` `1.16.0` released on April 12, 2026 ## Impact ### Policy reuse and mutation Nested mu

O3 Security · Impact-Aware SCA

Is GHSA-4p64-v8f5-r2gx in your dependencies?

O3 detects GHSA-4p64-v8f5-r2gx across PyPI dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.

Scan my dependencies How O3 SCA works