I think these would be hard targets for a botnet. Anyone who owns an ASIC miner would likely know a lot about it and know about the risks.
Ya. Currently.
How bout a 0 day attack?
0 day of what? Some flaw in the ASIC miners code? Or a flaw in the host machine? These are not typically the leads to botnets anyway.
Here's the thing, what are botnets used for? They are used for DDoS, and spam-mail traditionally. Why? Because every computer that is on the internet, has access to the internet. Traditionally low-hanging fruit is picked from this bunch, such that easy vulnerabilities that have remained unpatched for long periods of time allow for continuous usage of the targets.
So let's look at this: Every computer on the net has an internet connection, only a fraction are part of botnets.
Every computer has a GPU, only a fraction are "botnettable", only some portion of these GPUs could be used for effective mining, and it's a high-risk venture that would likely lose the machine, and thus is less profitable than other botnet ventures. This is likely why we've not seen any evidence of a GPU mining botnet on bitcoin.
An extremely tiny fraction of computers will have ASICs connected to them, of these some fraction will be botnettable at some point, and most use of this botnet will result in the botnet being broken up in very short time-scale. The difficulty will both be in even locating computers with ASICs, let alone exploiting them, let alone exploiting them effectively.
Is it
impossible? I will go ahead and say it is not, just so I don't have egg on my face at some point. But will it happen? I will again say no, it is too difficult, with too little reward.
The only scenario I could really foresee would be some disruption hacktivist, who manages some malicious code that can collect the IPs of the vastly reduced mining world, and utilizing some "zero-day attack" disrupts the block chain, either taking control, forking it, or generally fouling it up. This would likely be as easily or more easily accomplished by finding the IPs and DDoS'ing the heaviest ASIC miners connections though.
Always ask yourself for the motivation behind actions, they are very important.