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Diving Into Electron Web API Permissions

26 September 2022 at 22:00

Introduction

When a Chrome user opens a site on the Internet that requests a permission, Chrome displays a large prompt in the top left corner. The prompt remains visible on the page until the user interacts with it, reloads the page, or navigates away. The permission prompt has Block and Allow buttons, and an option to close it. On top of this, Chrome 98 displays the full prompt only if the permission was triggered β€œthrough a user gesture when interacting with the site itself”. These precautionary measures are the only things preventing a malicious site from using APIs that could affect user privacy or security.

Chrome Permission Prompt

Since Chrome implements this pop-up box, how does Electron handle permissions? From Electron’s documentation:

β€œIn Electron the permission API is based on Chromium and implements the same types of permissions. By default, Electron will automatically approve all permission requests unless the developer has manually configured a custom handler. While a solid default, security-conscious developers might want to assume the very opposite.”

This approval can lead to serious security and privacy consequences if the renderer process of the Electron application were to be compromised via unsafe navigation (e.g., open redirect, clicking links) or cross-site scripting. We decided to investigate how Electron implements various permission checks to compare Electron’s behavior to that of Chrome and determine how a compromised renderer process may be able to abuse web APIs.

Webcam, Microphone, and Screen Recording Permissions

The webcam, microphone, and screen recording functionalities present a serious risk to users when approval is granted by default. Without implementing a permission handler, an Electron app’s renderer process will have access to a user’s webcam and microphone. However, screen recording requires the Electron app to have configured a source via a desktopCapturer in the main process. This leaves little room for exploitability from the renderer process, unless the application already needs to record a user’s screen.

Electron groups these three into one permission, β€œmedia”. In Chrome, these permissions are separate. Electron’s lack of separation between these three is problematic because there may be cases where an application only requires the microphone, for example, but must also be granted access to record video. By default, the application would not have the capability to deny access to video without also denying access to audio. For those wondering, modern Electron apps seemingly handling microphone & video permissions separately, are only tracking and respecting the user choices in their UI. An attacker with a compromised renderer could still access any media.

It is also possible for media devices to be enumerated even when permission has not been granted. In Chrome however, an origin can only see devices that it has permission to use. The API navigator.mediaDevices.enumerateDevices() will return all of the user’s media devices, which can be used to fingerprint the user’s devices. For example, we can see a label of β€œDefault - MacBook Pro Microphone (Built-in)”, despite having a deny-all permission handler.

navigator.mediaDevices.enumerateDevices()

To deny access to all media devices (but not prevent enumerating the devices), a permission handler must be implemented in the main process that rejects requests for the β€œmedia” permission.

File System Access API

The File System Access API normally allows access to read and write to local files. In Electron, reading files has been implemented but writing to files has not been implemented and permission to write to files is always denied. However, access to read files is always granted when a user selects a file or directory. In Chrome, when a user selects a file or directory, Chrome notifies you that you are granting access to a specific file or directory until all tabs of that site are closed. In addition, Chrome prevents access to directories or files that are deemed too sensitive to disclose to a site. These are both considerations mentioned in the API’s standard (discussed by the WICG).

  • β€œUser agents should try to make sure that users are aware of what exactly they’re giving websites access to” – implemented in Chrome with the notification after choosing a file or directory
Chrome's prompt: Let site view files?
  • β€œUser agents are encouraged to restrict which directories a user is allowed to select in a directory picker, and potentially even restrict which files the user is allowed to select” – implemented in Chrome by preventing users from sharing certain directories containing system files. In Electron, there is no such notification or prevention. A user is allowed to select their root directory or any other sensitive directory, potentially granting more access than intended to a website. There will be no notification alerting the user of the level of access they will be granting.

Clipboard, Notification, and Idle Detection APIs

For these three APIs, the renderer process is granted access by default. This means a compromised renderer process can read the clipboard, send desktop notifications, and detect when a user is idle.

Clipboard

Access to the user’s clipboard is extremely security-relevant because some users will copy passwords or other sensitive information to the clipboard. Normally, Chromium denies access to reading the clipboard unless it was triggered by a user’s action. However, we found that adding an event handler for the window’s load event would allow us to read the clipboard without user interaction.

Cliboard Reading Callback
Cliboard Reading Callback

To deny access to this API, deny access to the β€œclipboard-read” permission.

Notifications

Sending desktop notifications is another security-relevant feature because desktop notifications can be used to increase the success rate of phishing or other social engineering attempts.

PoC for Notification API Attack

To deny access to this API, deny access to the β€œnotifications” permission.

Idle Detection

The Idle Detection API is much less security-relevant, but its abuse still represents a violation of user privacy.

Idle Detection API abuse

To deny access to this API, deny access to the β€œidle-detection” permission.

Local Font Access API

For this API, the renderer process is granted access by default. Furthermore, the main process never receives a permission request. This means that a compromised renderer process can always read a user’s fonts. This behavior has significant privacy implications because the user’s local fonts can be used as a fingerprint for tracking purposes and they may even reveal that a user works for a specific company or organization. Yes, we do use custom fonts for our reports!

Local Font Access API abuse

Security Hardening for Electron App Permissions

What can you do to reduce your Electron application’s risk? You can quickly assess if you are mitigating these issues and the effectiveness of your current mitigations using ElectroNG, the first SAST tool capable of rapid vulnerability detection and identifying missing hardening configurations. Among its many features, ElectroNG features a dedicated check designed to identify if your application is secure from permission-related abuses:

ElectroNG Permission Check

A secure application will usually deny all the permissions for dangerous web APIs by default. This can be achieved by adding a permission handler to a Session as follows:

  ses.setPermissionRequestHandler((webContents, permission, callback) => {
    return callback(false);
  })

If your application needs to allow the renderer process permission to access some web APIs, you can add exceptions by modifying the permission handler. We recommend checking if the origin requesting permission matches an expected origin. It’s a good practice to also set the permission request handler to null first to force any permission to be requested again. Without this, revoked permissions might still be available if they’ve already been used successfully.

session.defaultSession.setPermissionRequestHandler(null);

Conclusions

As we discussed, these permissions present significant risk to users even in Electron applications setting the most restrictive webPreferences settings. Because of this, it’s important for security teams & developers to strictly manage the permissions that Electron will automatically approve unless the developer has manually configured a custom handler.

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