So, let's tease out a couple ideas here, that way I can address your core idea at its best.
>They are so easily reverse-engineered and repurposed to attack you
You probably meant just third parties here. No one is checking government machine's patch level and demanding an explanation -- if they have the exploit they probably patched any vulnerable systems before they used it.
> I think state-sponsored malware should be banned, by international treaty
So, espionage is largely _already_ banned by international treaty, including fairly specific rules about how, if you are a soldier down behind enemy lines you can escape using a disguise but not if you are a spy (I hope this conveys how bizarre international law is, the idea that if a spy is caught that their use of a disguise will play a factor in their fate).
Suffice it to say, you want a "soft ban", because you want them to not be used in reality (not just that you don't want them used in threat gestures, etc. -- plus you can't really rely on international law for things like this, as we've seen by recent NSA leaks as well as Merkel's response to learning she was personally wiretapped).
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Most importantly: you have an argument that is too general. You could make _precisely the same argument_ in defense of banning pistols in any wars. (1) enemy is symmetrically offensive; (2) if there exist defenses, your opponent is likely to use them; (3) this leaves you largely unprotected to isomorphic attacks for all useful versions of this weapon (soldiers can still die from gunshot wounds); (4) similar technology can be devised by your opponents; (5) guns can be developed by low-budget or non-state actors; (6) direct attribution for gun crime is difficult (about half of murders in the US go unsolved, and that's not in a war zone).
So governments paying for pistols is too dangerous and should be "soft banned" or banned by international treaty.
I believe you have a more complex belief that would provide for much better discussion (subject matter experts typically do). Would you mind expanding on your ideas and sharing?