* Users not in the ACTIVE witness set should not have to worry about their
* key getting compromised and being used to produced multiple blocks so
* the attacker can report it and steel their vesting steem.
*
* The result of the operation is to transfer the full VESTING STEEM balance
* of the block producer to the reporter.
Does it mean it is theoretically possible for the users who ARE in the active witness set,
should worry about their key getting compromised and being used to double-produce blocks, so all their VESTING STEEM balance could be transferred to the malicious reporter? I mean is such kind of attack still possible?
You should guard your block producing keys like your balance keys. It is no different than someone having their bitcoin wallet key compromised and allowing the attacker to drain your account.
This is how we prevent the nothing-at-stake attack and the "produce on multiple forks" attack.