# 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 userā€™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 userā€™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): Example Payloads (Linux & MacOS): ``` ### 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 ``` ## 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 ``` {% 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 applicationā€™s own **business logic**. During the creation of new windows, the application checks whether the navigated link should be opened in a desktop applicationā€™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 ``` 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 \ (similar to \ ``` ## **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 ``` ## **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)
Support HackTricks and get benefits! * Do you work in a **cybersecurity company**? Do you want to see your **company advertised in HackTricks**? or do you want to have access to the **latest version of the PEASS or download HackTricks in PDF**? Check the [**SUBSCRIPTION PLANS**](https://github.com/sponsors/carlospolop)! * Discover [**The PEASS Family**](https://opensea.io/collection/the-peass-family), our collection of exclusive [**NFTs**](https://opensea.io/collection/the-peass-family) * Get the [**official PEASS & HackTricks swag**](https://peass.creator-spring.com) * **Join the** [**šŸ’¬**](https://emojipedia.org/speech-balloon/) [**Discord group**](https://discord.gg/hRep4RUj7f) or the [**telegram group**](https://t.me/peass) or **follow** me on **Twitter** [**šŸ¦**](https://github.com/carlospolop/hacktricks/tree/7af18b62b3bdc423e11444677a6a73d4043511e9/\[https:/emojipedia.org/bird/README.md)[**@carlospolopm**](https://twitter.com/carlospolopm)**.** * **Share your hacking tricks by submitting PRs to the** [**hacktricks github repo**](https://github.com/carlospolop/hacktricks)**.**