diff --git a/src/extension-support/tw-unsandboxed-extension-runner.js b/src/extension-support/tw-unsandboxed-extension-runner.js
index 08da34a3..fa86bb72 100644
--- a/src/extension-support/tw-unsandboxed-extension-runner.js
+++ b/src/extension-support/tw-unsandboxed-extension-runner.js
@@ -97,6 +97,9 @@ const setupUnsandboxedExtensionAPI = vm => new Promise(resolve => {
         if (!await Scratch.canOpenWindow(url)) {
             throw new Error(`Permission to open tab ${url} rejected.`);
         }
+        // Use noreferrer to prevent new tab from accessing `window.opener`
+        const baseFeatures = 'noreferrer';
+        features = features ? `${baseFeatures},${features}` : baseFeatures;
         return window.open(url, '_blank', features);
     };
 
diff --git a/test/unit/tw_unsandboxed_extensions.js b/test/unit/tw_unsandboxed_extensions.js
index 3cfcd0fe..abc0091d 100644
--- a/test/unit/tw_unsandboxed_extensions.js
+++ b/test/unit/tw_unsandboxed_extensions.js
@@ -218,8 +218,8 @@ test('openWindow', async t => {
     UnsandboxedExtensionRunner.setupUnsandboxedExtensionAPI(vm);
     global.Scratch.canOpenWindow = url => url === 'https://example.com/2';
     await t.rejects(global.Scratch.openWindow('https://example.com/1'), /Permission to open tab https:\/\/example.com\/1 rejected/);
-    t.equal(await global.Scratch.openWindow('https://example.com/2'), '[Window https://example.com/2 target=_blank features=]');
-    t.equal(await global.Scratch.openWindow('https://example.com/2', 'popup=1'), '[Window https://example.com/2 target=_blank features=popup=1]');
+    t.equal(await global.Scratch.openWindow('https://example.com/2'), '[Window https://example.com/2 target=_blank features=noreferrer]');
+    t.equal(await global.Scratch.openWindow('https://example.com/2', 'popup=1'), '[Window https://example.com/2 target=_blank features=noreferrer,popup=1]');
     t.end();
 });