electron-updater Code Signing Bypass on Windows

Observations

The file packages/electron-updater/src/windowsExecutableCodeSignatureVerifier.ts implements the signature validation routine for Electron applications on Windows. It executes the following command in a new shell (process.env.ComSpec on Windows, usually C:\Windows\System32\cmd.exe):

https://github.com/electron-userland/electron-builder/blob/140e2f0eb0df79c2a46e35024e96d0563355fc89/packages/electron-updater/src/windowsExecutableCodeSignatureVerifier.ts#L35-L41

Because of the surrounding shell, a first pass by cmd.exe expands any environment variable found in command-line above.

Exploitation

This creates a situation where verifySignature() can be tricked into validating the certificate of a different file than the one that was just downloaded. If the step is successful, the malicious update will be executed even if its signature is invalid.

Impact

This attack assumes a compromised update manifest (server compromise, Man-in-the-Middle attack if fetched over HTTP, Cross-Site Scripting to point the application to a malicious updater server, etc.).

Patch

This vulnerability was patched in #8295, by comparing the path in the output of Get-AuthenticodeSignature with the intended one. The patch is available starting from 6.3.0-alpha.6.