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7.5
Justhtml 1.16.0 fixes security risks in HTML sanitization
GHSA-4p64-v8f5-r2gx
Summary
Justhtml versions 1.15.0 and earlier have security weaknesses in how they handle certain HTML inputs. This can lead to malicious code being executed when sanitizing HTML. To fix this, update to Justhtml version 1.16.0, released on April 12, 2026. This version includes security patches to prevent potential attacks.
What to do
- Update justhtml to version 1.16.0.
Affected software
| Vendor | Product | Affected versions | Fix available |
|---|---|---|---|
| – | justhtml | <= 1.16.0 | 1.16.0 |
Original title
Multiple security fixes in justhtml
Original 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`.
`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`.
osv CVSS4.0
7.5
Vulnerability type
CWE-178
CWE-436
CWE-471
CWE-79
Cross-site Scripting (XSS)
Published: 14 Apr 2026 · Updated: 14 Apr 2026 · First seen: 14 Apr 2026