ACP_Commander can be used to remotely control an ARM9-based LS Pro/LS Live/Tera Pro v2/Tera Live. It connects to a stock process so it even works with the stock firmware and in EM Mode.
Is ACP Commander a Security Risk?
Clear answer: No.
The security risk is the Linkstation/Terrastation implementing a server for doing this kind of stuff. ACP_Commander is just the tool which utilizes this weakness.