It's rather an anti evil maid tool, or an evil maid defense. :)
sorry for being pedantic, but with the arms race within cybersecurity, "anti something defense" sounds like double negation to me.
While thinking about it, this phrase occurred to me: “silver bullets are a defense against zombies.” It is not the same phrase structure as the original, but it also has the double-negative vibe, yet it feels more reasonable to me than “…are a defense for zombies”, which to me suggests that zombies would employ them against their enemies.
I think the resolution here is that defense is inherently against something, so these phrases are not unequivocally double negatives - though I also agree with nine_k’s point about a better way to say it.
EDIT: Duh! The fact that defense is inherently against something is precisely what makes these phrases look like double negatives! The resolution must be something else - maybe agreement in mood or sentiment…