# Electron Fuses

> Package time feature toggles

## What are fuses?

For a subset of Electron functionality it makes sense to disable certain features for an entire application.  For example, 99% of apps don't make use of `ELECTRON_RUN_AS_NODE`, these applications want to be able to ship a binary that is incapable of using that feature.  We also don't want Electron consumers building Electron from source as that is both a massive technical challenge and has a high cost of both time and money.

Fuses are the solution to this problem, at a high level they are "magic bits" in the Electron binary that can be flipped when packaging your Electron app to enable / disable certain features / restrictions.  Because they are flipped at package time before you code sign your app the OS becomes responsible for ensuring those bits aren't flipped back via OS level code signing validation (Gatekeeper / App Locker).

## Current Fuses

### `runAsNode`

**Default:** Enabled
**@electron/fuses:** `FuseV1Options.RunAsNode`

The runAsNode fuse toggles whether the `ELECTRON_RUN_AS_NODE` environment variable is respected or not.  Please note that if this fuse is disabled then `process.fork` in the main process will not function as expected as it depends on this environment variable to function.

### `cookieEncryption`

**Default:** Disabled
**@electron/fuses:** `FuseV1Options.EnableCookieEncryption`

The cookieEncryption fuse toggles whether the cookie store on disk is encrypted using OS level cryptography keys.  By default the sqlite database that Chromium uses to store cookies stores the values in plaintext.  If you wish to ensure your apps cookies are encrypted in the same way Chrome does then you should enable this fuse.  Please note it is a one-way transition, if you enable this fuse existing unencrypted cookies will be encrypted-on-write but if you then disable the fuse again your cookie store will effectively be corrupt and useless.  Most apps can safely enable this fuse.

### `nodeOptions`

**Default:** Enabled
**@electron/fuses:** `FuseV1Options.EnableNodeOptionsEnvironmentVariable`

The nodeOptions fuse toggles whether the [`NODE_OPTIONS`](https://nodejs.org/api/cli.html#node_optionsoptions) environment variable is respected or not.  This environment variable can be used to pass all kinds of custom options to the Node.js runtime and isn't typically used by apps in production.  Most apps can safely disable this fuse.

### `nodeCliInspect`

**Default:** Enabled
**@electron/fuses:** `FuseV1Options.EnableNodeCliInspectArguments`

The nodeCliInspect fuse toggles whether the `--inspect`, `--inspect-brk`, etc. flags are respected or not.  When disabled it also ensures that `SIGUSR1` signal does not initialize the main process inspector.  Most apps can safely disable this fuse.

### `embeddedAsarIntegrityValidation`

**Default:** Disabled
**@electron/fuses:** `FuseV1Options.EnableEmbeddedAsarIntegrityValidation`

The embeddedAsarIntegrityValidation fuse toggles an experimental feature on macOS that validates the content of the `app.asar` file when it is loaded.  This feature is designed to have a minimal performance impact but may marginally slow down file reads from inside the `app.asar` archive.

For more information on how to use asar integrity validation please read the [Asar Integrity](asar-integrity.md) documentation.

### `onlyLoadAppFromAsar`

**Default:** Disabled
**@electron/fuses:** `FuseV1Options.OnlyLoadAppFromAsar`

The onlyLoadAppFromAsar fuse changes the search system that Electron uses to locate your app code.  By default Electron will search in the following order `app.asar` -> `app` -> `default_app.asar`.  When this fuse is enabled the search order becomes a single entry `app.asar` thus ensuring that when combined with the `embeddedAsarIntegrityValidation` fuse it is impossible to load non-validated code.

### `loadBrowserProcessSpecificV8Snapshot`

**Default:** Disabled
**@electron/fuses:** `FuseV1Options.LoadBrowserProcessSpecificV8Snapshot`

The loadBrowserProcessSpecificV8Snapshot fuse changes which V8 snapshot file is used for the browser process.  By default Electron's processes will all use the same V8 snapshot file.  When this fuse is enabled the browser process uses the file called `browser_v8_context_snapshot.bin` for its V8 snapshot. The other processes will use the V8 snapshot file that they normally do.

### `grantFileProtocolExtraPrivileges`

**Default:** Enabled
**@electron/fuses:** `FuseV1Options.GrantFileProtocolExtraPrivileges`

The grantFileProtocolExtraPrivileges fuse changes whether pages loaded from the `file://` protocol are given privileges beyond what they would receive in a traditional web browser.  This behavior was core to Electron apps in original versions of Electron but is no longer required as apps should be [serving local files from custom protocols](./security.md#18-avoid-usage-of-the-file-protocol-and-prefer-usage-of-custom-protocols) now instead.  If you aren't serving pages from `file://` you should disable this fuse.

The extra privileges granted to the `file://` protocol by this fuse are incompletely documented below:

* `file://` protocol pages can use `fetch` to load other assets over `file://`
* `file://` protocol pages can use service workers
* `file://` protocol pages have universal access granted to child frames also running on `file://` protocols regardless of sandbox settings

## How do I flip the fuses?

### The easy way

We've made a handy module, [`@electron/fuses`](https://npmjs.com/package/@electron/fuses), to make flipping these fuses easy.  Check out the README of that module for more details on usage and potential error cases.

```js @ts-nocheck
const { flipFuses, FuseVersion, FuseV1Options } = require('@electron/fuses')

flipFuses(
  // Path to electron
  require('electron'),
  // Fuses to flip
  {
    version: FuseVersion.V1,
    [FuseV1Options.RunAsNode]: false
  }
)
```

You can validate the fuses have been flipped or check the fuse status of an arbitrary Electron app using the fuses CLI.

```bash
npx @electron/fuses read --app /Applications/Foo.app
```

### The hard way

#### Quick Glossary

* **Fuse Wire**: A sequence of bytes in the Electron binary used to control the fuses
* **Sentinel**: A static known sequence of bytes you can use to locate the fuse wire
* **Fuse Schema**: The format / allowed values for the fuse wire

Manually flipping fuses requires editing the Electron binary and modifying the fuse wire to be the sequence of bytes that represent the state of the fuses you want.

Somewhere in the Electron binary there will be a sequence of bytes that look like this:

```text
| ...binary | sentinel_bytes | fuse_version | fuse_wire_length | fuse_wire | ...binary |
```

* `sentinel_bytes` is always this exact string `dL7pKGdnNz796PbbjQWNKmHXBZaB9tsX`
* `fuse_version` is a single byte whose unsigned integer value represents the version of the fuse schema
* `fuse_wire_length` is a single byte whose unsigned integer value represents the number of fuses in the following fuse wire
* `fuse_wire` is a sequence of N bytes, each byte represents a single fuse and its state.
  * "0" (0x30) indicates the fuse is disabled
  * "1" (0x31) indicates the fuse is enabled
  * "r" (0x72) indicates the fuse has been removed and changing the byte to either 1 or 0 will have no effect.

To flip a fuse you find its position in the fuse wire and change it to "0" or "1" depending on the state you'd like.

You can view the current schema [here](https://github.com/electron/electron/blob/main/build/fuses/fuses.json5).