You asked, "is it legal?".  Well, I (happily) no longer have access to the Lexis Law Library computers in prison, so I can't easily look up legal precedent.  However, I don't think that much precedent will exist on this issue:  It is 'cutting edge' law.  Chances are good that it isn't legal, in the sense that it hasn't been declared legal.  From what I read on recent CP messages, it sounds like the Feds are engaging in what would be, if done by non-government types, illegal 'cracking'.  Flooding a server in a denial of service attack is presumably illegal today, if done in America, and also if done in America directed against foreign hardware/software.  Placing malware is probably also illegal in such situations.  And since one of the servers in question is in France, French law would presumably apply as well.  (It is possible those government-types who engaged in these actions are now criminals, from the standpoint of French law, or even American law, although perhaps nothing will be done about that.)
   The next question is, "Can the fruits of these activities be used in court against [fill in the blanks]?".  There is a doctrine in American law called "Fruit of the poisonous tree" see   http://en.wikipedia.org/wiki/Fruit_of_the_poisonous_tree  , which generally prohibits the use of evidence obtained by the commission of a crime.  But since this doctrine was initiated many decades ago, long before the Internet era, it cannot be certain  how this principle will be applied to computer communications.   http://www.law.cornell.edu/wex/fruit_of_the_poisonous_tree
    Actually, I was on numerous occasions the victim precisely this kind of activity:  The car I drove (in June 1998 and April 2000-October 2000) had a GPS tracking transmitter placed on it, without a warrant.  (the then-current legal thinking was that no such warrant was required; in 1999 there was a Ninth Circuit Court of Appeals case, US v. McIver, that upheld this idea, although that case was quite distinguishable from mine because in the McIver case, there was 1.  A real crime.   2.  That the vehicle and the defendant were connected to.  (Neither of which existed in my case June 1998 or April 2000-October 2000.)  Curiously, despite the presence of the tracking device probably as early as April 2000, the Feds went through the motions of getting a (secret, of course) warrant for a DIFFERENT tracking device in late October, 2000, omitting any mention of the fact that they had such a tracker already placed for many months.  A lawyer would use this as evidence of deception by the Feds, and of admission that they knew that the previously-placed tracker was illegal:  Otherwise, why bother obtaining that October 2000 warrant, if they could have used the information from the previous device at any trial?    In January 2012, in a case U.S. v. Jones, the Supreme Court decided that placing such transmitters (even if the car in question is on publicly-accessible property) required a warrant:  In other words, to do so without a warrant is illegal and a violation of the 4th Amendment to the U.S. Constitution.
     Don't think I was unaware of the (likely) placement of that 2000 tracking device.  I virtually assumed such a thing was there.  But, try as I might, I couldn't get my crooked, colluding attorney (Robert Leen, appointed about November 21, 2000) to demand the fruit of that previous (2000, and 1998) tracking devices for trial.  It isn't that the information thus collected was important in itself:  Rather, the mere fact that a tracking device was placed (without probable cause, or reasonable suspicion that any crime had been, was being, or would be committed, would have exposed a lot of the criminality committed by the Feds.  They would have had to answer the question, "Why did you place those tracking devices on Mr. Bell's car, if you had no reason to believe there was a legitimate law-enforcement reason to do so?".    I attempted to fire attorney Leen about December 8, 2000, when he refused to actually defend me by looking for and asking for such evidence.  (The evidence I already knew existed.)
    Curiously, Andy Greenberg (author of "This Machine Kills Secrets") falsely claimed in his book that I had fired every lawyer I was given.  The truth is more enlightening:  In fact, I didn't fire (or even attempt to fire) my first lawyer, Peter Avenia:  He resigned, against my opposition to that, in early-mid-1999.  Every subsequent lawyer I had I ATTEMPTED to fire, yet in each case I was unsuccessful at accomplishing that.  I attempted to fire them when I verified that they were intending to betray me; but I was in all cases entirely unsuccessful at firing them.  They were allowed to stay by the judge, purporting to 'represent' me for many months or even years, until they were successful at doing the damage to me that they intended to do.  At that point, and ONLY at that point, did they leave, usually by requesting the judge to allow them to resign.  So it is accurate to claim that I never 'fired' any lawyer:  I was never allowed to do so.
    Note:  Andy Greenberg could have avoided a great deal of error in his writing if he had merely sent to me a copy of what he intended to write, to give me the opportunity to correct it.  He didn't.
     Jim Bell