More like it cannot be Barriss. That's about as likely as her being the one who removed data on Kamino from the Jedi Archives.
Seriously, check out the EU stuff on Barriss, which is canon acknowledged in the movies (mentioned by Mace Windu in Episode 2 specifically) before you make absurd judgements of her character.
Clone Wars overwrites EU all the time, and the pre-Clone Wars EU won't be affected. It didn't say the attacker was a dark side user, just that they were against the war, which Barriss very well could be (of course, that would overwrite some of my favorite EU novels, the MedStar duology).