diff --git a/public/js/app.js b/public/js/app.js
index 7434ba5b..1fb3f32c 100644
--- a/public/js/app.js
+++ b/public/js/app.js
@@ -684,6 +684,30 @@ function applyUserFeatures(features) {
 document.addEventListener('tabChanged', loadUserFeatures);
 
 // ── Announcement banner ────────────────────────────────────
+// The banner is admin-authored and shown on every page, so it renders
+// Markdown but only ever emits inline formatting and links — no images, no
+// headings, no block layout that could push the app around. Anything else,
+// and any failure to sanitise, falls back to the literal text.
+function renderAnnouncement(el, raw) {
+  var markdown = String(raw == null ? '' : raw);
+  if (!window.marked || typeof window.marked.parseInline !== 'function' ||
+      !window.DOMPurify || typeof window.DOMPurify.sanitize !== 'function') {
+    el.textContent = markdown;
+    return;
+  }
+  try {
+    el.innerHTML = window.DOMPurify.sanitize(window.marked.parseInline(markdown, { gfm: true }), {
+      ALLOWED_TAGS: ['strong', 'em', 'b', 'i', 'u', 's', 'code', 'a', 'br'],
+      ALLOWED_ATTR: ['href', 'target', 'rel'],
+      ADD_ATTR: ['target'],
+      FORBID_ATTR: ['style', 'onerror', 'onload', 'onclick', 'onmouseover'],
+      ALLOW_DATA_ATTR: false
+    });
+  } catch (e) {
+    el.textContent = markdown;
+  }
+}
+
 function loadAnnouncement() {
   loadUserFeatures();
   fetch('/api/admin/config/announcement', { headers: getAuthHeaders() })
@@ -695,7 +719,7 @@ function loadAnnouncement() {
       var icon   = document.getElementById('announcement-icon');
       if (!banner || !text) return;
       if (data.enabled && data.text && data.text.trim()) {
-        text.textContent = data.text;
+        renderAnnouncement(text, data.text);
         // type → icon + class
         var icons = { info: 'fa-info-circle', warning: 'fa-triangle-exclamation', error: 'fa-circle-xmark', success: 'fa-circle-check' };
         var type = data.type || 'info';
diff --git a/test/frontend-rendering-safety.test.js b/test/frontend-rendering-safety.test.js
index 53cd6daa..ab5ff2c3 100644
--- a/test/frontend-rendering-safety.test.js
+++ b/test/frontend-rendering-safety.test.js
@@ -18,3 +18,27 @@ test('billing E/M suggestion fields are escaped before innerHTML insertion', ()
   assert.match(source, /escHtml\(data\.emLevel\.rosCount\)/);
   assert.match(source, /escHtml\(data\.emLevel\.peCount\)/);
 });
+
+test('the announcement banner formats without becoming an injection point', () => {
+  const fs = require('node:fs');
+  const path = require('node:path');
+  const app = fs.readFileSync(path.join(__dirname, '..', 'public/js/app.js'), 'utf8');
+  const fn = app.slice(app.indexOf('function renderAnnouncement(el, raw)'), app.indexOf('function loadAnnouncement()'));
+
+  // Inline only: this text sits on every page, so it must not be able to
+  // introduce headings, images or block layout that shifts the app around.
+  assert.match(fn, /window\.marked\.parseInline\(markdown, \{ gfm: true \}\)/);
+  assert.match(fn, /ALLOWED_TAGS: \['strong', 'em', 'b', 'i', 'u', 's', 'code', 'a', 'br'\]/);
+  assert.doesNotMatch(fn, /'img'|'script'|'iframe'|'h1'|'h2'/);
+  assert.match(fn, /FORBID_ATTR: \['style', 'onerror', 'onload', 'onclick', 'onmouseover'\]/);
+  assert.match(fn, /ALLOW_DATA_ATTR: false/);
+
+  // Sanitising is not optional: without either library, or on any failure, the
+  // text is shown literally rather than as markup.
+  assert.match(fn, /if \(!window\.marked \|\| typeof window\.marked\.parseInline !== 'function' \|\|\s*\n\s*!window\.DOMPurify \|\| typeof window\.DOMPurify\.sanitize !== 'function'\) \{\s*\n\s*el\.textContent = markdown;/);
+  assert.match(fn, /catch \(e\) \{\s*\n\s*el\.textContent = markdown;/);
+  // innerHTML is only ever reached through sanitize().
+  const assignments = fn.match(/\.innerHTML\s*=/g) || [];
+  assert.equal(assignments.length, 1, 'one innerHTML assignment');
+  assert.match(fn, /el\.innerHTML = window\.DOMPurify\.sanitize\(/);
+});