I think they would also need control of your email/computer.
IB sends a confirmation email to your email of record that has a number "token" that you must put in, before they approve the transfer.
If someone could monitor your email and/or change it (also requires confirmation token sent to current email address), then they could possibly steal money.
But they would also need to, AFAIK, create a withdrawl "instruction" to ACH or wire out money. This usually takes a while and, also, requires confirmation if I remember correctly.
In short, it would generally be hard IMHO. But if someone had complete control of your system, and you were gone for a week or two (vacation?) then it would be fairly easy.
Best solution is the hardware device. It's just a small handheld device you type in the numbers they give you and it creates a "response code". That way, the crook has to have the device and your passwords to steal.