MAL-2025-4029 Malicious code in ts-runtime-compat-check (npm)

The npm package ts-runtime-compat-check is a malicious package that functions as a key component in a remote code execution attack chain. This package:

1. Contains a postinstall script that executes lib/install.js

2. The install script makes HTTP requests to a server specified by an environment variable APP_PROXY (defaults to 'http://localhost' but typically set to https://proxy.eslint-proxy.site by parent packages)

3. Sends the victim's application path as a query parameter to the malicious server

4. When the server returns specially crafted error messages, the package decodes them from base64 and executes them using new Function.constructor()

5. This creates a direct remote code execution capability

This package is utilized by other malicious packages like eslint-config-airbnb-compat to establish a covert channel for executing arbitrary code. The separation of concerns across multiple packages appears designed to evade detection by security scanners.

Any computer that has this package installed or running should be considered fully compromised. All secrets and keys stored on that computer should be rotated immediately from a different computer. The package should be removed, but as full control of the computer may have been given to an outside entity, there is no guarantee that removing the package will remove all malicious software resulting from installing it.