--- all of the above on Saul, although I think he's actually not the mole, he's the red herring for us. We'll know next week, if he reveals the chip to anyone or not, I guess. (Then again, knowing there's a mole he might want to keep it under wraps and work it himself, thus keeping his ambiguity going. And the writers didn't set up the whole deal with Mike and the other Marines just to throw it away immediately.)
But there is for sure a mole in CIA (I vote for the little half-lebanese guy that was assigned to work with Carrie last year, but I don't really trust Estes either), and probably one around the Veep, from things said and done in Season 1 ("The VP will call you soon and ask you to run for congress..."). I suspect the Veep's wife, she has an odd look - although it could have been the woman who got assassinated. Otherwise she was a loose end had the plan gone down as intended.
Plus, recall the brief intense romance between Brody and Carrie, and she actually defended him to Saul at one point, the idea that 'turned' could reflect how confused and desperate he'd been, and he wasn't a traitor -- yet. Their sad, mutually twisted relationship is far from over. And it'll take awhile to rehabilitate Carrie both in her own head, and (if ever) in the agency, and she could go mavericky again as a result. (When someone (incorrectly) said half this season would take place in Israel, I thought she could maybe even be picked up by Mossad on the rebound as an analyst, but turns out it's only the arab/lebanese street scenes they shot there.)
If I were CIA/FBI, once Brody is known, I'd want to bug and track him up the yin-yang and find the other Zahir contacts and how he and Zahir communicate, etc., not bust him outright. That could buy you a season or two.
Also recall from the prisoner Saul interrogated that when Zahir gets annoyed, he takes it out on your family -- especially your kids. Maybe he tries w. Brody? or feds make it look like he does under a false flag. Now Brody would have a reason to work along with Carrie, but maybe both off books...