How should one make "man in the middle" plural?

Usually, when a phrase acts as a single word, the noun in the phrase gets made plural. For instance, "mother in law" becomes "mothers in law" when made plural.

When discussing "man in the middle" attacks, it is common to call the attack a "man in the middle" or "a MITM."

The rule above would suggest that the plural would become "men in the middle." But unlike "mother in law," the noun in the phrase is not actually the thing that gets duplicated — it is instances of the whole phrase. That is, saying "men in the middle" seems to indicate a single attack perpetrated by multiple people; but in this case the intent is to refer to multiple attacks carried out by a single person.

As a result, I'm confused as to whether one should use "man in the middles" or "men in the middle" when referring to more than one man in the middle attack.

(Yes, I know the best solution in this case would probably be to avoid the problem and say "man in the middle attacks" which is unambiguous. But I'm curious anyway)

I wouldn't pluralize it, and go with your suggested alternative instead:

man in the middle attacks

On the other hand, the initialism MITM can be pluralized as MITMs (see http://oxforddictionaries.com/us/words/initialisms-american).

If "man in the middle" is an generally accepted term (to your audience) for an attack, then you could talk about man in the middles. For example, I think a class of ship was a man of war (or man o' war), etc. That would be bad enough, but think about fighting against man o' wars.

I would add hyphens:

man-in-the-middles

I might even italicize:

man-in-the-middles

Though I suspect the best answer is, as you suggest, to add "attacks" on to the end.