diff --git a/src/gateway/ui/chat.html b/src/gateway/ui/chat.html
index 647c7ea..84ea3fa 100644
--- a/src/gateway/ui/chat.html
+++ b/src/gateway/ui/chat.html
@@ -42,6 +42,55 @@
 
     let pendingAttachments = [];
 
+    function sanitizeHtml(html) {
+      const root = document.createElement('div');
+      root.innerHTML = html;
+
+      const blockedTags = new Set([
+        'script', 'style', 'iframe', 'object', 'embed', 'link', 'meta', 'base',
+        'form', 'input', 'button', 'textarea', 'select',
+      ]);
+
+      const nodes = root.querySelectorAll('*');
+      for (const el of nodes) {
+        const tag = el.tagName.toLowerCase();
+        if (blockedTags.has(tag)) {
+          el.remove();
+          continue;
+        }
+
+        for (const attr of Array.from(el.attributes)) {
+          const name = attr.name.toLowerCase();
+          const value = attr.value.trim();
+          if (name.startsWith('on') || name === 'style') {
+            el.removeAttribute(attr.name);
+            continue;
+          }
+          if (name === 'href' || name === 'src' || name === 'xlink:href') {
+            const normalized = value.replace(/[\u0000-\u001F\u007F\s]+/g, '').toLowerCase();
+            if (
+              normalized.startsWith('javascript:')
+              || normalized.startsWith('vbscript:')
+              || normalized.startsWith('data:text/html')
+            ) {
+              el.removeAttribute(attr.name);
+            }
+          }
+        }
+
+        if (tag === 'a' && el.getAttribute('target') === '_blank') {
+          el.setAttribute('rel', 'noopener noreferrer');
+        }
+      }
+
+      return root.innerHTML;
+    }
+
+    function renderSafeMarkdown(text) {
+      const html = marked.parse(String(text ?? ''));
+      return sanitizeHtml(html);
+    }
+
     function isSupportedImageMime(mimeType) {
       return mimeType === 'image/jpeg'
         || mimeType === 'image/png'
@@ -280,7 +329,7 @@
         // Render final response as markdown inside the assistant area
         const responseDiv = document.createElement('div');
         responseDiv.className = 'message assistant';
-        responseDiv.innerHTML = marked.parse(data.content || '');
+        responseDiv.innerHTML = renderSafeMarkdown(data.content || '');
         area.appendChild(responseDiv);
       }
 
@@ -326,7 +375,7 @@
     function appendAssistantMessage(content) {
       const div = document.createElement('div');
       div.className = 'message assistant';
-      div.innerHTML = marked.parse(content);
+      div.innerHTML = renderSafeMarkdown(content);
       messagesEl.appendChild(div);
       scrollToBottom();
     }
diff --git a/src/gateway/ui/lib/markdown.d.ts b/src/gateway/ui/lib/markdown.d.ts
new file mode 100644
index 0000000..ca34253
--- /dev/null
+++ b/src/gateway/ui/lib/markdown.d.ts
@@ -0,0 +1 @@
+export function renderSafeMarkdown(text: unknown): string;
diff --git a/src/gateway/ui/lib/markdown.js b/src/gateway/ui/lib/markdown.js
new file mode 100644
index 0000000..4d06811
--- /dev/null
+++ b/src/gateway/ui/lib/markdown.js
@@ -0,0 +1,92 @@
+/**
+ * Render markdown and sanitize the resulting HTML before DOM insertion.
+ * This protects the web UI from untrusted model/tool output.
+ */
+export function renderSafeMarkdown(text) {
+  const raw = String(text ?? '');
+  let html = '';
+  const md = globalThis.marked;
+
+  try {
+    if (md) {
+      html = md.parse(raw);
+    }
+  } catch {
+    // Fall through to plain-text fallback
+  }
+
+  if (!html) {
+    html = `<p>${escapeHtml(raw)}</p>`;
+  }
+
+  return sanitizeHtml(html);
+}
+
+function escapeHtml(text) {
+  const div = document.createElement('div');
+  div.textContent = text;
+  return div.innerHTML;
+}
+
+function sanitizeHtml(html) {
+  const root = document.createElement('div');
+  root.innerHTML = html;
+
+  // Drop elements that can execute script or embed active content.
+  const blockedTags = new Set([
+    'script',
+    'style',
+    'iframe',
+    'object',
+    'embed',
+    'link',
+    'meta',
+    'base',
+    'form',
+    'input',
+    'button',
+    'textarea',
+    'select',
+  ]);
+
+  const nodes = root.querySelectorAll('*');
+  for (const el of nodes) {
+    const tag = el.tagName.toLowerCase();
+    if (blockedTags.has(tag)) {
+      el.remove();
+      continue;
+    }
+
+    for (const attr of Array.from(el.attributes)) {
+      const name = attr.name.toLowerCase();
+      const value = attr.value.trim();
+
+      // Remove event handlers and inline styles.
+      if (name.startsWith('on') || name === 'style') {
+        el.removeAttribute(attr.name);
+        continue;
+      }
+
+      // Strip scriptable URL schemes.
+      if (name === 'href' || name === 'src' || name === 'xlink:href') {
+        if (isUnsafeUrl(value)) {
+          el.removeAttribute(attr.name);
+        }
+      }
+    }
+
+    if (tag === 'a' && el.getAttribute('target') === '_blank') {
+      const rel = (el.getAttribute('rel') ?? '').toLowerCase();
+      if (!rel.includes('noopener') || !rel.includes('noreferrer')) {
+        el.setAttribute('rel', 'noopener noreferrer');
+      }
+    }
+  }
+
+  return root.innerHTML;
+}
+
+function isUnsafeUrl(value) {
+  const normalized = value.replace(/[\u0000-\u001F\u007F\s]+/g, '').toLowerCase();
+  return normalized.startsWith('javascript:') || normalized.startsWith('vbscript:') || normalized.startsWith('data:text/html');
+}
diff --git a/src/gateway/ui/lib/markdown.test.ts b/src/gateway/ui/lib/markdown.test.ts
new file mode 100644
index 0000000..76778e4
--- /dev/null
+++ b/src/gateway/ui/lib/markdown.test.ts
@@ -0,0 +1,35 @@
+import { describe, it, expect } from 'vitest';
+import { parseHTML } from 'linkedom';
+import { renderSafeMarkdown } from './markdown.js';
+
+describe('renderSafeMarkdown', () => {
+  const { document, window } = parseHTML('<!doctype html><html><body></body></html>');
+  (globalThis as { document?: unknown }).document = document;
+  (globalThis as { window?: unknown }).window = window;
+  (globalThis as { marked?: { parse: (text: string) => string } }).marked = {
+    parse: (text: string) => text,
+  };
+
+  it('removes script tags', () => {
+    const html = renderSafeMarkdown('hello<script>alert(1)</script>');
+    expect(html).not.toContain('<script');
+    expect(html).toContain('hello');
+  });
+
+  it('removes inline event handlers', () => {
+    const html = renderSafeMarkdown('<img src="https://example.com/x.png" onerror="alert(1)">');
+    expect(html).toContain('<img');
+    expect(html).not.toContain('onerror=');
+  });
+
+  it('removes javascript: href values', () => {
+    const html = renderSafeMarkdown('<a href="javascript:alert(1)">click</a>');
+    expect(html).toContain('<a');
+    expect(html).not.toContain('javascript:');
+  });
+
+  it('adds rel noopener noreferrer for target=_blank links', () => {
+    const html = renderSafeMarkdown('<a href="https://example.com" target="_blank">x</a>');
+    expect(html).toContain('rel="noopener noreferrer"');
+  });
+});
diff --git a/src/gateway/ui/pages/chat.js b/src/gateway/ui/pages/chat.js
index 651c983..a400701 100644
--- a/src/gateway/ui/pages/chat.js
+++ b/src/gateway/ui/pages/chat.js
@@ -5,7 +5,8 @@
  * markdown-rendered responses, slash commands, and web search.
  */
 
-/* global marked, hljs */
+/* global hljs */
+import { renderSafeMarkdown } from '../lib/markdown.js';
 
 let _currentSession = null;
 let _sending = false;
@@ -33,17 +34,6 @@ function escapeHtml(text) {
   return div.innerHTML;
 }
 
-function renderMarkdown(text) {
-  try {
-    if (typeof marked !== 'undefined') {
-      return marked.parse(text);
-    }
-  } catch {
-    // Fall through to plain text
-  }
-  return `<p>${escapeHtml(text)}</p>`;
-}
-
 function highlightCode() {
   if (typeof hljs !== 'undefined') {
     document.querySelectorAll('.chat-messages pre code').forEach(block => {
@@ -60,7 +50,7 @@ function createMessageEl(role, content) {
   div.className = `message ${role}`;
 
   if (role === 'assistant' || role === 'system') {
-    div.innerHTML = renderMarkdown(content);
+    div.innerHTML = renderSafeMarkdown(content);
     setTimeout(highlightCode, 0);
   } else {
     div.textContent = content;
@@ -607,7 +597,7 @@ async function sendMessage(client, overrideText) {
     // Replace placeholder with actual response
     placeholder.classList.remove('streaming-cursor');
     const content = done?.content ?? done?.text ?? '(no response)';
-    placeholder.innerHTML = renderMarkdown(content);
+    placeholder.innerHTML = renderSafeMarkdown(content);
     placeholder.appendChild(createMessageActions('assistant'));
     setTimeout(highlightCode, 0);
   } catch (err) {