GHSA-4p64-v8f5-r2gx — justhtml
GHSA-4p64-v8f5-r2gx is a Improper Input Validation vulnerability in justhtml. A fix is available for justhtml — see the affected versions and patch details below.
Multiple security fixes in justhtml
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-4p64-v8f5-r2gx.
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 1.16.0 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()orsanitize_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
justhtml1.16.0released 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, andcursorcontaining externalurl(...)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
styleorscriptin custom policies for untrusted input - avoid serializing untrusted programmatic doctypes or DOM trees
Credit
Discovered during an internal security review of justhtml.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | justhtml | all versions | 1.16.0pip install --upgrade 'justhtml==1.16.0' |
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 1.16.0 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 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like GHSA-4p64-v8f5-r2gx can be triaged on real exposure rather than presence alone.
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
Is GHSA-4p64-v8f5-r2gx in your dependencies?
O3 Security finds GHSA-4p64-v8f5-r2gx across PyPI dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.