diff --git a/web/public/assets/js/app/__tests__/instance-selector.test.js b/web/public/assets/js/app/__tests__/instance-selector.test.js
index f4e0650..bc0c869 100644
--- a/web/public/assets/js/app/__tests__/instance-selector.test.js
+++ b/web/public/assets/js/app/__tests__/instance-selector.test.js
@@ -90,10 +90,29 @@ test('resolveInstanceLabel falls back to the domain when the name is missing', (
 test('buildInstanceUrl normalises domains into navigable HTTPS URLs', () => {
   assert.equal(buildInstanceUrl('mesh.example'), 'https://mesh.example');
   assert.equal(buildInstanceUrl(' https://mesh.example '), 'https://mesh.example');
+  assert.equal(buildInstanceUrl('https://mesh.example/path?query#fragment'), 'https://mesh.example');
+  assert.equal(buildInstanceUrl('javascript:alert(1)'), null);
+  assert.equal(buildInstanceUrl('ftp://mesh.example'), null);
+  assert.equal(buildInstanceUrl('mesh.example:8080'), 'https://mesh.example:8080');
+  assert.equal(buildInstanceUrl('mesh.example<script>'), null);
   assert.equal(buildInstanceUrl(''), null);
   assert.equal(buildInstanceUrl(null), null);
 });
 
+test('buildInstanceUrl rejects malformed HTTP URLs safely', () => {
+  const originalWarn = console.warn;
+  const warnings = [];
+  console.warn = message => warnings.push(message);
+
+  try {
+    assert.equal(buildInstanceUrl('http://[::1'), null);
+    assert.equal(buildInstanceUrl('https://bad host.example'), null);
+    assert.ok(warnings.length >= 1);
+  } finally {
+    console.warn = originalWarn;
+  }
+});
+
 test('initializeInstanceSelector populates options alphabetically and selects the configured domain', async () => {
   const env = createDomEnvironment();
   const select = setupSelectElement(env.document);
diff --git a/web/public/assets/js/app/instance-selector.js b/web/public/assets/js/app/instance-selector.js
index 12ad4f2..0b17c0c 100644
--- a/web/public/assets/js/app/instance-selector.js
+++ b/web/public/assets/js/app/instance-selector.js
@@ -34,12 +34,15 @@ function resolveInstanceLabel(entry) {
   return domain;
 }
 
-/**
- * Construct a navigable URL for the provided instance domain.
- *
- * @param {string} domain Instance domain as returned by the federation catalog.
- * @returns {string|null} Navigable absolute URL or ``null`` when the domain is empty.
- */
+ /**
+  * Construct a navigable URL for the provided instance domain.
+  *
+  * The returned URL is guaranteed to use HTTP(S) and a host-only component to avoid
+  * interpreting arbitrary DOM-controlled text as executable content.
+  *
+  * @param {string} domain Instance domain as returned by the federation catalog.
+  * @returns {string|null} Navigable absolute URL or ``null`` when the domain is empty or unsafe.
+  */
 export function buildInstanceUrl(domain) {
   if (typeof domain !== 'string') {
     return null;
@@ -50,8 +53,29 @@ export function buildInstanceUrl(domain) {
     return null;
   }
 
-  if (/^[a-zA-Z][a-zA-Z\d+.-]*:\/\//.test(trimmed)) {
-    return trimmed;
+  const allowedHostPattern = /^[a-zA-Z0-9.-]+(?::\d{1,5})?$/;
+
+  if (/^https?:\/\//i.test(trimmed)) {
+    try {
+      const parsed = new URL(trimmed);
+      if (!['http:', 'https:'].includes(parsed.protocol)) {
+        return null;
+      }
+
+      const sanitizedHost = parsed.host.trim();
+      if (!allowedHostPattern.test(sanitizedHost)) {
+        return null;
+      }
+
+      return `${parsed.protocol}//${sanitizedHost}`;
+    } catch (error) {
+      console.warn('Rejected invalid instance URL', error);
+      return null;
+    }
+  }
+
+  if (!allowedHostPattern.test(trimmed)) {
+    return null;
   }
 
   return `https://${trimmed}`;