Leaving aside futile preemptive attempts to deny terrorists access to secure communications, I have no more problem with cracking a terrorist's vulnerable computer system than I have with cracking a terrorist's vulnerable skull. Provided of course that by "terrorist" we mean a person who is actively initiating violence against others in order to manipulate the innocent through fear. Such attacks invite defensive force against the attacker through whichever vectors are available to most efficiently minimize harm to the innocent.