🛡️ Sentinel: [HIGH] Fix XSS in markdown rendering

.jules/sentinel.md

@@ -14,3 +14,8 @@
 **Vulnerability:** Several `escapeHtml` implementations used DOM manipulation (`document.createElement('div').innerHTML`) or incomplete string replacement, failing to escape single and double quotes.
 **Learning:** Incomplete escaping allows XSS payloads to break out of HTML attributes (e.g., `<input value="${escapeHtml(userInput)}">`).
 **Prevention:** Always use `String(text)` cast combined with a comprehensive replace chain for `&`, `<`, `>`, `"`, and `'` (e.g., `&#039;`) in custom string escaping functions.
+
+## 2026-07-06 - XSS vulnerability in Markdown rendering fallback
+**Vulnerability:** The application was vulnerable to Cross-Site Scripting (XSS) when rendering user-provided Markdown notes due to missing HTML sanitization on the `marked.parse` output. A 'fail open' state could exist if fallback mechanisms returned unsanitized raw HTML.
+**Learning:** When using external parsers like `marked.parse`, their raw output is inherently unsafe and requires sanitization. Furthermore, conditional sanitization (e.g. `window.DOMPurify ? sanitize(...) : fallback(...)`) MUST use a secure fallback mechanism such as regex-based HTML escaping rather than falling back to the raw unsanitized HTML.
+**Prevention:** Always wrap dynamically rendered content with `DOMPurify.sanitize()` using required attribute allowlists. Ensure any fallback logic provides escaped text rather than unsanitized raw data to prevent 'fail open' scenarios.

site/app.js

@@ -2724,7 +2724,7 @@ function renderSpecsPanel() {
 
       if (typeof marked !== 'undefined') {
         const rendered = marked.parse(processedNote);
-        html += rendered;
+        html += window.DOMPurify ? window.DOMPurify.sanitize(rendered, { ADD_ATTR: ['data-note-node', 'data-node'] }) : `<pre>${processedNote.replace(/</g, '&lt;').replace(/>/g, '&gt;')}</pre>`;
       } else {
         html += `<pre>${processedNote.replace(/</g, '&lt;').replace(/>/g, '&gt;')}</pre>`;
       }