diff --git a/.jules/sentinel.md b/.jules/sentinel.md
index 5e0d967e..24e108fe 100644
--- a/.jules/sentinel.md
+++ b/.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="${escapeHtml(userInput)}">`).
 **Prevention:** Always use `String(text)` cast combined with a comprehensive replace chain for `&`, `<`, `>`, `"`, and `'` (e.g., `&#039;`) in custom string escaping functions.
+
+## 2025-02-14 - Fix Fail Open XSS Vulnerability in Markdown Parsing
+**Vulnerability:** Raw HTML strings generated by `marked.parse` for node specifications were directly assigned to `innerHTML` without sanitization. If `marked` or an adversary injects malicious `<script>` tags or attributes, they execute as part of the page content.
+**Learning:** `marked.parse` itself does not sanitize HTML output by default. When using libraries for rendering untrusted markdown inside a trusted UI frame, conditionally relying on safe fallbacks or specific libraries isn't sufficient without strict conditional sanitization (e.g. `DOMPurify` if available).
+**Prevention:** Always wrap dynamically generated HTML interpolations (like markdown rendering) using `DOMPurify.sanitize(..., { ADD_ATTR: [...] })` before assigning them to the DOM. If the sanitizer library is missing, degrade securely using regex-based HTML escaping `<pre>${str.replace(/</g, '&lt;').replace(/>/g, '&gt;')}</pre>`.
diff --git a/site/app.js b/site/app.js
index 788bd7e7..4bc5c526 100644
--- a/site/app.js
+++ b/site/app.js
@@ -2723,8 +2723,10 @@ function renderSpecsPanel() {
       );
 
       if (typeof marked !== 'undefined') {
+        // SECURITY: Conditionally sanitize raw markdown HTML to prevent XSS.
+        // Falls back to safe text escaping if DOMPurify isn't loaded.
         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>`;
       }