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](https://github.com/electron/electron/tree/master/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 several weeks or a few months.

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`][browser-window]. 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.

> :warning: 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>`][webview-tag] tag and make sure to disable the `nodeIntegration`.

## Electron Security Warnings

From Electron 2.0 on, developers will see warnings and recommendations printed
to the developer console. They only show up when the binary's name is Electron,
indicating that a developer is currently looking at the console.

You can force-enable or force-disable these warnings by setting
`ELECTRON_ENABLE_SECURITY_WARNINGS` or `ELECTRON_DISABLE_SECURITY_WARNINGS` on
either `process.env` or the `window` object.

## Checklist: Security Recommendations

This is not bulletproof, but at the least, you should follow these steps to
improve the security of your application.

1. [Only load secure content](#1-only-load-secure-content)
2. [Disable the Node.js integration in all renderers that display remote content](#2-disable-nodejs-integration-for-remote-content)
3. [Enable context isolation in all renderers that display remote content](#3-enable-context-isolation-for-remote-content)
4. [Use `ses.setPermissionRequestHandler()` in all sessions that load remote content](#4-handle-session-permission-requests-from-remote-content)
5. [Do not disable `webSecurity`](#5-do-not-disable-websecurity)
6. [Define a `Content-Security-Policy`](#6-define-a-content-security-policy) and use restrictive rules (i.e. `script-src 'self'`)
7. [Override and disable `eval`](#7-override-and-disable-eval), which allows strings to be executed as code.
8. [Do not set `allowRunningInsecureContent` to `true`](#8-do-not-set-allowrunninginsecurecontent-to-true)
9. [Do not enable experimental features](#9-do-not-enable-experimental-features)
10. [Do not use `enableBlinkFeatures`](#10-do-not-use-enableblinkfeatures)
11. [`<webview>`: Do not use `allowpopups`](#11-do-not-use-allowpopups)
12. [`<webview>`: Verify options and params](#12-verify-webview-options-before-creation)
13. [Disable or limit navigation](#13-disable-or-limit-navigation)
14. [Disable or limit creation of new windows](#14-disable-or-limit-creation-of-new-windows)

## 1) 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 recommend 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?

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

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

```html
<!-- 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">
```


## 2) Disable Node.js Integration for Remote Content

It is paramount that you disable Node.js integration in any renderer
([`BrowserWindow`][browser-window], [`BrowserView`][browser-view], or
[`<webview>`][webview-tag]) 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?

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

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

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

```html
<!-- 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.

```js
const { readFileSync } = require('fs')

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


## 3) 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](https://developer.chrome.com/extensions/content_scripts#execution-environment)
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?

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

```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)
})
```


## 4) 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](https://developer.chrome.com/extensions/permissions)
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?

```js
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)
    }
  })
```


## 5) Do Not Disable WebSecurity

_Recommendation is Electron's default_

You may have already guessed that disabling the `webSecurity` property on a
renderer process ([`BrowserWindow`][browser-window],
[`BrowserView`][browser-view], or [`<webview>`][webview-tag]) 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?
```js
// Bad
const mainWindow = new BrowserWindow({
  webPreferences: {
    webSecurity: false
  }
})
```

```js
// Good
const mainWindow = new BrowserWindow()
```

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

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


## 6) 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 application's security.

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

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

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

### CSP HTTP Header

Electron respects the [`Content-Security-Policy` HTTP header](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy)
which can be set using Electron's
[`webRequest.onHeadersReceived`](../api/web-request.md#webrequestonheadersreceivedfilter-listener)
handler:

```javascript
const { session } = require('electron')

session.defaultSession.webRequest.onHeadersReceived((details, callback) => {
  callback({ responseHeaders: `default-src 'none'` })
})
```

### CSP Meta Tag

CSP's preferred delivery mechanism is an HTTP header. It can be useful, however,
to set a policy on a page directly in the markup using a `<meta>` tag:

```html
<meta http-equiv="Content-Security-Policy" content="default-src 'none'">
```

#### `webRequest.onHeadersReceived([filter, ]listener)`


## 7) 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, 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?

```js
// 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().`)
}
```


## 8) Do Not Set `allowRunningInsecureContent` to `true`

_Recommendation is Electron's default_

By default, Electron will not 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?

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](#1-only-load-secure-content) for more details.

### How?

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

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


## 9) Do Not Enable Experimental Features

_Recommendation is Electron's default_

Advanced users of Electron can enable experimental Chromium features using the
`experimentalFeatures` property.

### Why?

Experimental features are, as the name suggests, experimental and have not been
enabled for all Chromium users. Furthermore, 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?

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

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


## 10) Do Not Use `enableBlinkFeatures`

_Recommendation is Electron's default_

Blink is the name of the rendering engine behind Chromium. As with
`experimentalFeatures`, the `enableBlinkFeatures` 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?
```js
// Bad
const mainWindow = new BrowserWindow({
  webPreferences: {
    enableBlinkFeatures: ['ExecCommandInJavaScript']
  }
})
```

```js
// Good
const mainWindow = new BrowserWindow()
```


## 11) Do Not Use `allowpopups`

_Recommendation is Electron's default_

If you are using [`<webview>`][webview-tag], 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`][browser-window] using the
`window.open()` method. `<webview>` tags 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`][browser-window] 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?

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

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


## 12) 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 [`<webview>`][webview-tag] tags
from the main process and to verify that their webPreferences do not disable
security features.

### Why?

Since `<webview>` 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>`][webview-tag] tags.

### How?

Before a [`<webview>`][webview-tag] 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.

```js
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.

## 13) Disable or limit navigation

If your app has no need to navigate or only needs to navigate to known pages,
it is a good idea to limit navigation outright to that known scope, disallowing
any other kinds of navigation.

### Why?

Navigation is a common attack vector. If an attacker can convince your app to
navigate away from its current page, they can possibly force your app to open
web sites on the Internet. Even if your `webContents` are configured to be more
secure (like having `nodeIntegration` disabled or `contextIsolation` enabled),
getting your app to open a random web site will make the work of exploiting your
app a lot easier.

A common attack pattern is that the attacker convinces your app's users to
interact with the app in such a way that it navigates to one of the attacker's
pages. This is usually done via links, plugins, or other user-generated content.

### How?

If your app has no need for navigation, you can call `event.preventDefault()`
in a [`will-navigate`][will-navigate] handler. If you know which pages your app
might navigate to, check the URL in the event handler and only let navigation
occur if it matches the URLs you're expecting.

We recommend that you use Node's parser for URLs. Simple string comparisons can
sometimes be fooled - a `startsWith('https://google.com')` test would let
`https://google.com.attacker.com` through.

```js
const URL = require('url').URL

app.on('web-contents-created', (event, contents) => {
  contents.on('will-navigate', (event, navigationUrl) => {
    const parsedUrl = new URL(navigationUrl)

    if (parsedUrl.origin !== 'https://my-own-server.com') {
      event.preventDefault()
    }
  })
})
```

## 14) Disable or limit creation of new windows

If you have a known set of windows, it's a good idea to limit the creation of
additional windows in your app.

### Why?

Much like navigation, the creation of new `webContents` is a common attack
vector. Attackers attempt to convince your app to create new windows, frames,
or other renderer processes with more privileges than they had before; or
with pages opened that they couldn't open before.

If you have no need to create windows in addition to the ones you know you'll
need to create, disabling the creation buys you a little bit of extra
security at no cost. This is commonly the case for apps that open one
`BrowserWindow` and do not need to open an arbitrary number of additional
windows at runtime.

### How?

[`webContents`][web-contents] will emit the [`new-window`][new-window] event
before creating new windows. That event will be passed, amongst other
parameters, the `url` the window was requested to open and the options used to
create it. We recommend that you use the event to scrutinize the creation of
windows, limiting it to only what you need.

```js
const { shell } = require('electron')

app.on('web-contents-created', (event, contents) => {
  contents.on('new-window', (event, navigationUrl) => {
    // In this example, we'll ask the operating system
    // to open this event's url in the default browser.
    event.preventDefault()

    shell.openExternal(navigationUrl)
  })
})
```

[browser-window]: ../api/browser-window.md
[browser-view]: ../api/browser-view.md
[webview-tag]: ../api/webview-tag.md
[web-contents]: ../api/web-contents.md
[new-window]: ../api/web-contents.md#event-new-window
[will-navigate]: ../api/web-contents.md#event-will-navigate