electron/docs/tutorial/security.md

Security, Native Capabilities, and Your Responsibility

As web developers, we usually enjoy the strong security net of the browser - the risks associated with the code we write are relatively small. Our websites are granted limited powers in a sandbox, and we trust that our users enjoy a browser built by a large team of engineers that is able to quickly respond to newly discovered security threats.

When working with Electron, it is important to understand that Electron is not a web browser. It allows you to build feature-rich desktop applications with familiar web technologies, but your code wields much greater power. JavaScript can access the filesystem, user shell, and more. This allows you to build high quality native applications, but the inherent security risks scale with the additional powers granted to your code.

With that in mind, be aware that displaying arbitrary content from untrusted sources poses a severe security risk that Electron is not intended to handle. In fact, the most popular Electron apps (Atom, Slack, Visual Studio Code, etc) display primarily local content (or trusted, secure remote content without Node integration) – if your application executes code from an online source, it is your responsibility to ensure that the code is not malicious.

Reporting Security Issues

For information on how to properly disclose an Electron vulnerability, see SECURITY.md

Chromium Security Issues and Upgrades

While Electron strives to support new versions of Chromium as soon as possible, developers should be aware that upgrading is a serious undertaking - involving hand-editing dozens or even hundreds of files. Given the resources and contributions available today, Electron will often not be on the very latest version of Chromium, lagging behind by either days or weeks.

We feel that our current system of updating the Chromium component strikes an appropriate balance between the resources we have available and the needs of the majority of applications built on top of the framework. We definitely are interested in hearing more about specific use cases from the people that build things on top of Electron. Pull requests and contributions supporting this effort are always very welcome.

Ignoring Above Advice

A security issue exists whenever you receive code from a remote destination and execute it locally. As an example, consider a remote website being displayed inside a BrowserWindow. If an attacker somehow manages to change said content (either by attacking the source directly, or by sitting between your app and the actual destination), they will be able to execute native code on the user's machine.

⚠️ Under no circumstances should you load and execute remote code with Node.js integration enabled. Instead, use only local files (packaged together with your application) to execute Node.js code. To display remote content, use the webview tag and make sure to disable the nodeIntegration.

Checklist: Security Recommendations

This is not bulletproof, but at the least, you should attempt the following:

• Only load secure content
• Disable the Node.js integration in all renderers that display remote content
• Enable context isolation in all renderers that display remote content
• Use ses.setPermissionRequestHandler() in all sessions that load remote content
• Do not disable webSecurity
• Define a Content-Security-Policy and use restrictive rules (i.e. script-src 'self')
• Override and disable eval , which allows strings to be executed as code.
• Do not set allowRunningInsecureContent to true
• Do not enable experimental features
• Do not use blinkFeatures
• WebViews: Do not use allowpopups
• WebViews: Verify the options and params of all <webview> tags

Only Load Secure Content

Any resources not included with your application should be loaded using a secure protocol like HTTPS. In other words, do not use insecure protocols like HTTP. Similarly, we recommed the use of WSS over WS, FTPS over FTP, and so on.

Why?

HTTPS has three main benefits:

1. It authenticates the remote server, ensuring your app connects to the correct host instead of an impersonator.
2. It ensures data integrity, asserting that the data was not modified while in transit between your application and the host.
3. It encrypts the traffic between your user and the destination host, making it more difficult to eavesdrop on the information sent between your app and the host.

How?

// Bad
browserWindow.loadURL('http://my-website.com')

// Good
browserWindow.loadURL('https://my-website.com')

<!-- Bad -->
<script crossorigin src="http://cdn.com/react.js"></script>
<link rel="stylesheet" href="http://cdn.com/style.css">

<!-- Good -->
<script crossorigin src="https://cdn.com/react.js"></script>
<link rel="stylesheet" href="https://cdn.com/style.css">

Disable Node.js Integration for Remote Content

It is paramount that you disable Node.js integration in any renderer (BrowserWindow, BrowserView, or WebView) that loads remote content. The goal is to limit the powers you grant to remote content, thus making it dramatically more difficult for an attacker to harm your users should they gain the ability to execute JavaScript on your website.

After this, you can grant additional permissions for specific hosts. For example, if you are opening a BrowserWindow pointed at `https://my-website.com/", you can give that website exactly the abilities it needs, but no more.

Why?

A cross-site-scripting (XSS) attack is more dangerous if an attacker can jump out of the renderer process and execute code on the user's computer. Cross-site-scripting attacks are fairly common - and while an issue, their power is usually limited to messing with the website that they are executed on. Disabling Node.js integration helps prevent an XSS from being escalated into a so-called "Remote Code Execution" (RCE) attack.

How?

// Bad
const mainWindow = new BrowserWindow()
mainWindow.loadURL('https://my-website.com')

// Good
const mainWindow = new BrowserWindow({
  webPreferences: {
    nodeIntegration: false,
    preload: './preload.js'
  }
})

mainWindow.loadURL('https://my-website.com')

<!-- Bad -->
<webview nodeIntegration src="page.html"></webview>

<!-- Good -->
<webview src="page.html"></webview>

When disabling Node.js integration, you can still expose APIs to your website that do consume Node.js modules or features. Preload scripts continue to have access to require and other Node.js features, allowing developers to expose a custom API to remotely loaded content.

In the following example preload script, the later loaded website will have access to a window.readConfig() method, but no Node.js features.

const { readFileSync } = require('fs')

window.readConfig = function () {
  const data = readFileSync('./config.json')
  return data
}

Enable Context Isolation for Remote Content

Context isolation is an Electron feature that allows developers to run code in preload scripts and in Electron APIs in a dedicated JavaScript context. In practice, that means that global objects like Array.prototype.push or JSON.parse cannot be modified by scripts running in the renderer process.

Electron uses the same technology as Chromium's Content Scripts to enable this behavior.

Why?

Context isolation allows each the scripts on running in the renderer to make changes to its JavaScript environment without worrying about conflicting with the scripts in the Electron API or the preload script.

While still an experimental Electron feature, context isolation adds an additional layer of security. It creates a new JavaScript world for Electron APIs and preload scripts.

At the same time, preload scripts still have access to the document and window objects. In other words, you're getting a decent return on a likely very small investment.

How?

// Main process
const mainWindow = new BrowserWindow({
  webPreferences: {
    contextIsolation: true,
    preload: 'preload.js'
  }
})

// Preload script

// Set a variable in the page before it loads
webFrame.executeJavaScript('window.foo = "foo";')

// The loaded page will not be able to access this, it is only available
// in this context
window.bar = 'bar'

document.addEventListener('DOMContentLoaded', () => {
  // Will log out 'undefined' since window.foo is only available in the main
  // context
  console.log(window.foo)

  // Will log out 'bar' since window.bar is available in this context
  console.log(window.bar)
})

Handle Session Permission Requests From Remote Content

You may have seen permission requests while using Chrome: They pop up whenever the website attempts to use a feature that the user has to manually approve ( like notifications).

The API is based on the Chromium permissions API and implements the same types of permissions.

Why?

By default, Electron will automatically approve all permission requests unless the developer has manually configured a custom handler. While a solid default, security-conscious developers might want to assume the very opposite.

How?

const { session } = require('electron')

session
  .fromPartition('some-partition')
  .setPermissionRequestHandler((webContents, permission, callback) => {
    const url = webContents.getURL()

    if (permission === 'notifications') {
      // Approves the permissions request
      callback(true)
    }

    if (!url.startsWith('https://my-website.com')) {
      // Denies the permissions request
      return callback(false)
    }
  })

Define a Content Security Policy

A Content Security Policy (CSP) is an additional layer of protection against cross-site-scripting attacks and data injection attacks. We recommend that they be enabled by any website you load inside Electron.

Why?

CSP allows the server serving content to restrict and control the resources Electron can load for that given web page. https://your-page.com should be allowed to load scripts from the origins you defined while scripts from https://evil.attacker.com should not be allowed to run. Defining a CSP is an easy way to improve your applications security.

How?

Electron respects the Content-Security-Policy HTTP header and the respective <meta> tag.

The following CSP will allow Electron to execute scripts from the current website and from apis.mydomain.com.

// Bad
Content-Security-Policy: '*'

// Good
Content-Security-Policy: script-src 'self' https://apis.mydomain.com

Override and Disable eval

eval() is a core JavaScript method that allows the execution of JavaScript from a string. Disabling it disables your app's ability to evaluate JavaScript that is not known in advance.

Why?

The eval() method has precisely one mission: To evaluate a series of characters as JavaScript and execute it. It is a required method whenever you need to evaluate code that is not known ahead of time. While legitimate use cases exist, just like any other code generators, eval() is difficult to harden.

Generally speaking, it is easier to completely disable eval() than to make it bulletproof. Thus, if you do not need it, it is a good idea to disable it.

How?

// ESLint will warn about any use of eval(), even this one
// eslint-disable-next-line
window.eval = global.eval = function () {
  throw new Error(`Sorry, this app does not support window.eval().`)
}

Do Not Set allowRunningInsecureContent to true

Recommendation is Electron's default

By default, Electron will now allow websites loaded over HTTPS to load and execute scripts, CSS, or plugins from insecure sources (HTTP). Setting the property allowRunningInsecureContent to true disables that protection.

Loading the initial HTML of a website over HTTPS and attempting to load subsequent resources via HTTP is also known as "mixed content".

Why?

Simply put, loading content over HTTPS assures the authenticity and integrity of the loaded resources while encrypting the traffic itself. See the section on only displaying secure content for more details.

How?

// Bad
const mainWindow = new BrowserWindow({
  webPreferences: {
    allowRunningInsecureContent: true
  }
})

// Good
const mainWindow = new BrowserWindow({})

Do Not Enable Experimental Features

Recommendation is Electron's default

Advanced users of Electron can enable experimental Chromium features using the experimentalFeatures and experimentalCanvasFeatures properties.

Why?

Experimental features are, as the name suggests, experimental and have not been enabled for all Chromium users. Futhermore, their impact on Electron as a whole has likely not been tested.

Legitimate use cases exist, but unless you know what you are doing, you should not enable this property.

How?

// Bad
const mainWindow = new BrowserWindow({
  webPreferences: {
    experimentalFeatures: true
  }
})

// Good
const mainWindow = new BrowserWindow({})

Do Not Use blinkFeatures

Recommendation is Electron's default

Blink is the name of the rendering engine behind Chromium. As with experimentalFeatures, the blinkFeatures property allows developers to enable features that have been disabled by default.

Why?

Generally speaking, there are likely good reasons if a feature was not enabled by default. Legitimate use cases for enabling specific features exist. As a developer, you should know exactly why you need to enable a feature, what the ramifications are, and how it impacts the security of your application. Under no circumstances should you enable features speculatively.

How?

// Bad
const mainWindow = new BrowserWindow({
  webPreferences: {
    blinkFeatures: ['ExecCommandInJavaScript']
  }
})

// Good
const mainWindow = new BrowserWindow()

Do Not Disable WebSecurity

Recommendation is Electron's default

You may have already guessed that disabling the webSecurity property on a renderer process (BrowserWindow, BrowserView, or WebView) disables crucial security features.

Do not disable webSecurity in production applications.

Why?

Disabling webSecurity will disable the same-origin policy and set allowRunningInsecureContent property to true. In other words, it allows the execution of insecure code from different domains.

How?

// Bad
const mainWindow = new BrowserWindow({
  webPreferences: {
    webSecurity: false
  }
})

// Good
const mainWindow = new BrowserWindow()

<!-- Bad -->
<webview disablewebsecurity src="page.html"></webview>

<!-- Good -->
<webview src="page.html"></webview>

Do Not Use allowpopups

Recommendation is Electron's default

If you are using WebViews, you might need the pages and scripts loaded in your <webview> tag to open new windows. The allowpopups attribute enables them to create new BrowserWindows using the window.open() method. WebViews are otherwise not allowed to create new windows.

Why?

If you do not need popups, you are better off not allowing the creation of new BrowserWindows by default. This follows the principle of minimally required access: Don't let a website create new popups unless you know it needs that feature.

How?

<!-- Bad -->
<webview allowpopups src="page.html"></webview>

<!-- Good -->
<webview src="page.html"></webview>

Verify WebView Options Before Creation

A WebView created in a renderer process that does not have Node.js integration enabled will not be able to enable integration itself. However, a WebView will always create an independent renderer process with its own webPreferences.

It is a good idea to control the creation of new WebViews from the main process and to verify that their webPreferences do not disable security features.

Why?

Since WebViews live in the DOM, they can be created by a script running on your website even if Node.js integration is otherwise disabled.

Electron enables developers to disable various security features that control a renderer process. In most cases, developers do not need to disable any of those features - and you should therefore not allow different configurations for newly created <WebView> tags.

How?

Before a <WebView> tag is attached, Electron will fire the will-attach-webview event on the hosting webContents. Use the event to prevent the creation of WebViews with possibly insecure options.

app.on('web-contents-created', (event, contents) => {
  contents.on('will-attach-webview', (event, webPreferences, params) => {
    // Strip away preload scripts if unused or verify their location is legitimate
    delete webPreferences.preload
    delete webPreferences.preloadURL

    // Disable Node.js integration
    webPreferences.nodeIntegration = false

    // Verify URL being loaded
    if (!params.src.startsWith('https://yourapp.com/')) {
      event.preventDefault()
    }
  })
})

Again, this list merely minimizes the risk, it does not remove it. If your goal is to display a website, a browser will be a more secure option.