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 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 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` 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 display secure (https) content](#only-display-secure-content)
* [Disable the Node integration in all renderers that display remote content](#disable-node-integration-for-remote-content)
  (setting `nodeIntegration` to `false` in `webPreferences`)
* Enable context isolation in all renderers that display remote content
  (setting `contextIsolation` to `true` in `webPreferences`)
* Use `ses.setPermissionRequestHandler()` in all sessions that load remote content
* Do not disable `webSecurity`. Disabling it will disable the same-origin policy.
* Define a [`Content-Security-Policy`](http://www.html5rocks.com/en/tutorials/security/content-security-policy/)
, and use restrictive rules (i.e. `script-src 'self'`)
* [Override and disable `eval`](https://github.com/nylas/N1/blob/0abc5d5defcdb057120d726b271933425b75b415/static/index.js#L6-L8)
, which allows strings to be executed as code.
* Do not set `allowRunningInsecureContent` to true.
* Do not enable `experimentalFeatures` or `experimentalCanvasFeatures` unless
  you know what you're doing.
* Do not use `blinkFeatures` unless you know what you're doing.
* WebViews: Do not add the `nodeintegration` attribute.
* WebViews: Do not use `disablewebsecurity`
* WebViews: Do not use `allowpopups`
* WebViews: Do not use `insertCSS` or `executeJavaScript` with remote CSS/JS.
* [WebViews: Verify the options and params of all `<webview>` tags](#verify-webview-options-before-creation)

## Only Display Secure Content
Any resources not included with your application should be loaded using a secure
protocol like `HTTPS`. Furthermore, avoid "mixed content", which occurs when the
initial HTML is loaded over an `HTTPS` connection, but additional resources
(scripts, stylesheets, etc) are loaded over an insecure connection.

### Why?
`HTTPS` has three main benefits:

1) It authenticates the remote server, ensuring that the host is actually who it
   claims to be. When loading a resource from an `HTTPS` host, it prevents an
   attacker from impersonating that host, thus ensuring that the computer your
   app's users are connecting to is actually the host you wanted them to connect
   to.
2) It ensures data integrity, asserting that the data was not modified while in
   transit between your application and the host.
3) It encryps the traffic between your user and the destination host, making it
   more difficult to eavesdropping 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">
```

## Disable Node Integration for Remote Content
It is paramount that you disable Node integration in any renderer (`BrowserWindow`,
`BrowserView`, or `WebView`) that loads remote content. The goal of disabling Node
integration 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.

Disabling Node integration does not mean that you cannot grant additional powers
to the website you are loading. If you are opening a `BrowserWindow` pointed at
`https://my-website.com`, the goal is to give that website exactly the abilities
it needs, but no more.

### Why?
A cross-site-scripting (XSS) attack becomes dramatically 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. However, in a renderer process with Node.js integration enabled, an XSS attack
becomes a whole different class of attack: A so-called "Remote Code Execution"
(RCE) attack. Disabling Node.js integration limits the power of successful XSS
attacks.

### How?
```js
// 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')
```

When disabling Node 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;
}
```

## Enable Context Isolation for Remote Content

## 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)
    }
  })
```


## Define a Content Security Policy

A Content Security Policy (CSP) is an additional layer of protection against
cross-site-scripting attacks (XSS) and data injection attacks. They can be
enabled by websites and we recommend that any website you load inside Electron
does so.

### Why?

CSP allows the server serving content to restrict and control the resources
Electron will load for that given web page. `https://your-page.com` should have
be allowed to 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](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy)
as well as the respective `<meta>` tag.

The following CSP will allow Electron to execute scripts from the current
website as well as 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 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?

```js
window.eval = global.eval = function() {
  throw new Error(`Sorry, this app does not support window.eval().`);
}
```


## 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 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.

```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 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.