Merge pull request #8983 from electron/sandbox-initial-documentation
Add initial documentation for `sandbox` option.
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Kevin Sawicki
GitHub
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2 changed files with 193 additions and 1 deletions
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@ -225,6 +225,13 @@ It creates a new `BrowserWindow` with native properties as set by the `options`.
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When node integration is turned off, the preload script can reintroduce
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Node global symbols back to the global scope. See example
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[here](process.md#event-loaded).
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* `sandbox` Boolean (optional) - If set, this will sandbox the renderer
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associated with the window, making it compatible with the Chromium
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OS-level sandbox and disabling the Node.js engine. This is not the same as
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the `nodeIntegration` option and the APIs available to the preload script
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are more limited. Read more about the option [here](sandbox-option.md).
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**Note:** This option is currently experimental and may change or be
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removed in future Electron releases.
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* `session` [Session](session.md#class-session) (optional) - Sets the session used by the
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page. Instead of passing the Session object directly, you can also choose to
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use the `partition` option instead, which accepts a partition string. When
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@ -282,7 +289,6 @@ It creates a new `BrowserWindow` with native properties as set by the `options`.
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window. Defaults to `false`. See the
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[offscreen rendering tutorial](../tutorial/offscreen-rendering.md) for
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more details.
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* `sandbox` Boolean (optional) - Whether to enable Chromium OS-level sandbox.
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* `contextIsolation` Boolean (optional) - Whether to run Electron APIs and
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the specified `preload` script in a separate JavaScript context. Defaults
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to `false`. The context that the `preload` script runs in will still
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186
docs/api/sandbox-option.md
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186
docs/api/sandbox-option.md
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@ -0,0 +1,186 @@
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# `sandbox` Option
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> Create a browser window with renderer that can run inside chromium OS sandbox.
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One of chromium key security features is that all blink rendering/javascript
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code is confined in a sandbox. This sandbox uses OS-specific features to ensure
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that exploits in the renderer process cannot harm the system.
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In other words, when the sandbox is enabled, the renderers can only make changes
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to the system by delegating tasks to the main process via IPC.
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[Here's](https://www.chromium.org/developers/design-documents/sandbox) more
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information about the sandbox.
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Since a major feature in electron is the ability to run node.js in the
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renderer process(making it easier to develop desktop applications using only web
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technologies), the sandbox has to disabled by electron. One of the reasons is
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that most node.js APIs require system access. `require()` for example, is not
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possible without file system permissions, which are unavailable in a sandboxed
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environment.
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Usually this is not a problem for desktop applications since the code is always
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trusted, but it makes electron less secure than chromium for displaying
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untrusted web content. For applications that require more security, the
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`sandbox` flag will force electron to spawn a classic chromium renderer that is
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compatible with the sandbox.
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A sandboxed renderer doesn't have a node.js environment running and doesn't
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expose javascript APIs to client code. The only exception is the preload script,
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which has access to a subset of electron renderer API.
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Another difference is that sandboxed renderers don't modify any of the default
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javascript APIs. Consequently, some APIs such as `window.open` will work as they
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do in chromium(no `BrowserWindowProxy`).
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## Example
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Create a sandboxed window, simply pass `sandbox: true` to `webPreferences`:
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```js
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let win
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app.on('ready', () => {
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win = new BrowserWindow({
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webPreferences: {
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sandbox: true
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}
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})
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w.loadURL('http://google.com')
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})
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```
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This alone won't enable the OS-enforced sandbox. To use it, the
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`--enable-sandbox` command-line argument must be passed to electron, which will
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force `sandbox: true` to all BrowserWindow instances.
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```js
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let win
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app.on('ready', () => {
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// no need to pass `sandbox: true` since `--enable-sandbox` was enabled.
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win = new BrowserWindow()
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w.loadURL('http://google.com')
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})
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```
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Note that it is not enough to call
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`app.commandLine.appendSwitch('--enable-sandbox')`, as electron/node startup
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code runs after it is possible to make changes to chromium sandbox settings. The
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switch must be passed to electron command-line:
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```
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electron --enable-sandbox app.js
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```
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It is not possible to have the OS sandbox active only for some renderers, if
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`--enable-sandbox` is enabled, normal electron windows cannot be created.
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If you need to mix sandboxed and non-sandboxed renderers in one application,
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simply omit the `--enable-sandbox` argument. Without this argument, windows
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created with `sandbox: true` will still have node.js disabled and communicate
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only via IPC, which by itself is already a gain from security POV.
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## Preload
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An app can make customizations to sandboxed renderers using a preload script.
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Here's an example:
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```js
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let win
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app.on('ready', () => {
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win = new BrowserWindow({
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webPreferences: {
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sandbox: true,
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preload: 'preload.js'
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}
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})
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w.loadURL('http://google.com')
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})
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```
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and preload.js:
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```js
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// This file is loaded whenever a javascript context is created. It runs in a
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// private scope that can access a subset of electron renderer APIs. We must be
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// careful to not leak any objects into the global scope!
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const fs = require('fs')
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const {ipcRenderer} = require('electron')
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// read a configuration file using the `fs` module
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const buf = fs.readFileSync('allowed-popup-urls.json')
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const allowedUrls = JSON.parse(buf.toString('utf8'))
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const defaultWindowOpen = window.open
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function customWindowOpen (url, ...args) {
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if (allowedUrls.indexOf(url) === -1) {
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ipcRenderer.sendSync('blocked-popup-notification', location.origin, url)
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return null
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}
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return defaultWindowOpen(url, ...args)
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}
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window.open = customWindowOpen
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```
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Important things to notice in the preload script:
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- Even though the sandboxed renderer doesn't have node.js running, it still has
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access to a limited node-like environment:`Buffer`, `process`, `setImmediate`
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and `require` are available.
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- The preload can indirectly access all APIs from the main process through the
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`remote` and `ipcRenderer` modules. This is how `fs`(used above) and other
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modules are implemented: They are proxies to remote counterparts in the main
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process.
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- The preload must be contained in a single script, but it is possible to have
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complex preload code composed with multiple modules by using a tool like
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browserify, as explained below. In fact, browserify is already used by
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electron to provide a node-like environment to the preload script.
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To create a browserify bundle and use it as a preload script, something like
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the following should be used:
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browserify preload/index.js \
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-x electron \
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-x fs \
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--insert-global-vars=__filename,__dirname -o preload.js
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The `-x` flag should be used with any required module that is already exposed in
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the preload scope, and tells browserify to use the enclosing `require` function
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for it. `--insert-global-vars` will ensure that `process`,`Buffer` and
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`setImmediate` are also taken from the enclosing scope(normally browserify
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injects code for those).
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Currently the `require` function provided in the preload scope exposes the
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following modules:
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- `child_process`
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- `electron`(crashReporter, remote and ipcRenderer)
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- `fs`
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- `os`
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- `timers`
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- `url`
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More may be added as needed to expose more electron APIs in the sandbox, but any
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module in the main process can already be used through
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`electron.remote.require`.
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## Status
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Please use the `sandbox` option with care, as it still is an experimental
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feature. We are still not aware of the security implications of exposing some
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electron renderer APIs to the preload script, but here are some things to
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consider before rendering untrusted content:
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- A preload script can accidentaly leak privileged APIs to untrusted code.
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- Some bug in V8 engine may allow malicious code to access the renderer preload
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APIs, effectively granting full access to the system through the `remote`
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module.
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Since renderering untrusted content in electron is still uncharted territory,
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the APIs exposed to the sandbox preload script should be considered more
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unstable than the rest of electron APIs, and may have breaking changes to fix
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security issues.
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One planned enhancement that should greatly increase security is to block IPC
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messages from sandboxed renderers by default, allowing the main process
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explicitly define a set of messages the renderer is allowed to send.
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