The npm package dotnet-runtime-base versions 1.0.4 and 1.0.5 contains malicious code in its postinstall script. On Windows systems, the script downloads and executes an attacker-controlled binary from a personal GitHub repository, running it hidden with bypassed execution policies. This behavior results in arbitrary code execution on the installer's machine and likely full system compromise. The malicious payload is designed to evade detection in development and CI environments. Removal of the package does not guarantee elimination of all malicious artifacts. Immediate secret and key rotation is recommended if the package was installed.