diff --git a/shell/browser/electron_browser_client.cc b/shell/browser/electron_browser_client.cc
index 5426960fb4..85b537dc84 100644
--- a/shell/browser/electron_browser_client.cc
+++ b/shell/browser/electron_browser_client.cc
@@ -72,6 +72,7 @@
 #include "services/network/public/cpp/resource_request_body.h"
 #include "services/network/public/cpp/self_deleting_url_loader_factory.h"
 #include "services/network/public/cpp/url_loader_factory_builder.h"
+#include "services/network/public/cpp/web_sandbox_flags.h"
 #include "shell/app/electron_crash_reporter_client.h"
 #include "shell/browser/api/electron_api_app.h"
 #include "shell/browser/api/electron_api_crash_reporter.h"
@@ -128,6 +129,7 @@
 #include "third_party/blink/public/common/tokens/tokens.h"
 #include "third_party/blink/public/common/web_preferences/web_preferences.h"
 #include "third_party/blink/public/mojom/badging/badging.mojom.h"
+#include "third_party/blink/public/mojom/devtools/console_message.mojom.h"
 #include "ui/base/resource/resource_bundle.h"
 #include "ui/native_theme/native_theme.h"
 #include "v8/include/v8.h"
@@ -926,7 +928,9 @@ void HandleExternalProtocolInUI(
     const GURL& url,
     content::WeakDocumentPtr document_ptr,
     content::WebContents::OnceGetter web_contents_getter,
-    bool has_user_gesture) {
+    bool has_user_gesture,
+    bool is_primary_main_frame,
+    network::mojom::WebSandboxFlags sandbox_flags) {
   content::WebContents* web_contents = std::move(web_contents_getter).Run();
   if (!web_contents)
     return;
@@ -945,6 +949,30 @@ void HandleExternalProtocolInUI(
     rfh = web_contents->GetPrimaryMainFrame();
   }
 
+  // Sandboxed iframes without one of the appropriate sandbox-escape tokens
+  // must not be able to launch external protocol handlers. This mirrors
+  // chrome/browser/chrome_content_browser_client.cc; see crbug.com/1148777.
+  if (!is_primary_main_frame) {
+    using SandboxFlags = network::mojom::WebSandboxFlags;
+    auto allow = [sandbox_flags](SandboxFlags flag) {
+      return (sandbox_flags & flag) == SandboxFlags::kNone;
+    };
+    const bool allowed = allow(SandboxFlags::kTopNavigationToCustomProtocols) ||
+                         (allow(SandboxFlags::kTopNavigationByUserActivation) &&
+                          has_user_gesture);
+    if (!allowed) {
+      rfh->AddMessageToConsole(
+          blink::mojom::ConsoleMessageLevel::kError,
+          "Navigation to external protocol blocked by sandbox, because it "
+          "doesn't contain any of: "
+          "'allow-top-navigation-to-custom-protocols', "
+          "'allow-top-navigation-by-user-activation', "
+          "'allow-top-navigation', or 'allow-popups'. See "
+          "https://chromestatus.com/feature/5680742077038592");
+      return;
+    }
+  }
+
   GURL escaped_url(base::EscapeExternalHandlerValue(url.spec()));
   auto callback = base::BindOnce(&OnOpenExternal, escaped_url);
   permission_helper->RequestOpenExternalPermission(rfh, std::move(callback),
@@ -973,7 +1001,8 @@ bool ElectronBrowserClient::HandleExternalProtocol(
                      initiator_document
                          ? initiator_document->GetWeakDocumentPtr()
                          : content::WeakDocumentPtr(),
-                     std::move(web_contents_getter), has_user_gesture));
+                     std::move(web_contents_getter), has_user_gesture,
+                     is_primary_main_frame, sandbox_flags));
   return true;
 }
 
diff --git a/spec/chromium-spec.ts b/spec/chromium-spec.ts
index 40cb2a726c..79b5c50d26 100644
--- a/spec/chromium-spec.ts
+++ b/spec/chromium-spec.ts
@@ -4558,3 +4558,81 @@ describe('navigator.usb', () => {
     }
   });
 });
+
+describe('iframe sandbox external protocols', () => {
+  let server: http.Server;
+  let serverUrl: string;
+  let w: BrowserWindow;
+  let openExternalRequests: string[];
+
+  before(async () => {
+    server = http.createServer((req, res) => {
+      res.setHeader('Content-Type', 'text/html');
+      if (req.url === '/child') {
+        res.end('<script>location.href = "magnet:sandbox-test"</script>');
+      } else {
+        const sandbox = new URL(req.url!, serverUrl).searchParams.get('sandbox') ?? '';
+        res.end(`<iframe sandbox="${sandbox}" src="/child"></iframe>`);
+      }
+    });
+    serverUrl = (await listen(server)).url;
+  });
+
+  after(() => {
+    server.close();
+  });
+
+  beforeEach(() => {
+    openExternalRequests = [];
+    w = new BrowserWindow({ show: false });
+    w.webContents.session.setPermissionRequestHandler((_wc, permission, callback, details) => {
+      if (permission === 'openExternal') {
+        openExternalRequests.push((details as any).externalURL);
+      }
+      callback(false);
+    });
+  });
+
+  afterEach(() => {
+    w.webContents.session.setPermissionRequestHandler(null);
+    return closeAllWindows();
+  });
+
+  it('blocks navigation to external protocol from a sandboxed iframe', async () => {
+    const consoleMessage = once(w.webContents, 'console-message');
+    await w.loadURL(`${serverUrl}/?sandbox=${encodeURIComponent('allow-scripts')}`);
+    const [{ message }] = await consoleMessage;
+    expect(message).to.match(/external protocol blocked by sandbox/);
+    expect(openExternalRequests).to.be.empty();
+  });
+
+  it('allows navigation to external protocol with allow-top-navigation-to-custom-protocols', async () => {
+    const requested = new Promise<void>(resolve => {
+      w.webContents.session.setPermissionRequestHandler((_wc, permission, callback, details) => {
+        if (permission === 'openExternal') {
+          openExternalRequests.push((details as any).externalURL);
+          resolve();
+        }
+        callback(false);
+      });
+    });
+    await w.loadURL(`${serverUrl}/?sandbox=${encodeURIComponent('allow-scripts allow-top-navigation-to-custom-protocols')}`);
+    await requested;
+    expect(openExternalRequests).to.deep.equal(['magnet:sandbox-test']);
+  });
+
+  it('allows navigation to external protocol with allow-popups', async () => {
+    const requested = new Promise<void>(resolve => {
+      w.webContents.session.setPermissionRequestHandler((_wc, permission, callback, details) => {
+        if (permission === 'openExternal') {
+          openExternalRequests.push((details as any).externalURL);
+          resolve();
+        }
+        callback(false);
+      });
+    });
+    await w.loadURL(`${serverUrl}/?sandbox=${encodeURIComponent('allow-scripts allow-popups')}`);
+    await requested;
+    expect(openExternalRequests).to.deep.equal(['magnet:sandbox-test']);
+  });
+});