phpMyFAQ: SVG Sanitizer Bypass via HTML Entity Encoding Leads to Stored XSS and Privilege Escalation

Summary

The regex-based SVG sanitizer in phpMyFAQ (SvgSanitizer.php) can be bypassed using HTML entity encoding in javascript: URLs within SVG <a href> attributes. Any user with edit_faq permission can upload a malicious SVG that executes arbitrary JavaScript when viewed, enabling privilege escalation from editor to full admin takeover.

Details

The file phpmyfaq/src/phpMyFAQ/Helper/SvgSanitizer.php (introduced 2026-01-15) uses regex patterns to detect dangerous content in uploaded SVG files. The regex for javascript: URL detection is:

/href\s*=\s*["\']javascript:[^"\']*["\']/i

This pattern matches the literal string javascript: but fails when the URL is HTML entity encoded. For example, javascript: decodes to javascript: in the browser, but does NOT match the regex. The isSafe() method returns true, so the SVG is accepted without sanitization.

Additionally, the DANGEROUS_ELEMENTS blocklist misses <animate>, <set>, and <use> elements which can also be used to execute JavaScript in SVG context.

Uploaded SVG files are served with Content-Type: image/svg+xml and no Content-Disposition: attachment header, so browsers render them inline and execute any JavaScript they contain.

The image upload endpoint (/admin/api/content/images) only requires the edit_faq permission — not full admin — so any editor-level user can upload malicious SVGs.

PoC

Basic XSS (confirmed working in Chrome 146 and Edge)

1. Login to phpMyFAQ admin panel with any account that has edit_faq permission
2. Navigate to Admin → Content → Add New FAQ
3. In the TinyMCE editor, click the image upload button
4. Upload this SVG file:

<?xml version="1.0" encoding="UTF-8"?>
<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 200 200">
  <a href="&#106;&#97;&#118;&#97;&#115;&#99;&#114;&#105;&#112;&#116;&#58;alert(document.domain)">
    <text x="20" y="50" font-size="16" fill="red">Click for XSS</text>
  </a>
</svg>

5. The SVG is uploaded to /content/user/images/<timestamp>_<filename>.svg
6. Open the SVG URL directly in a browser
7. Click the red text → alert(document.domain) executes