I agree with janey B--the characters' flaws are neither weakness nor passitity, but they are flawed. If there is a weakness, it is in each character's willingness (?) to act in predictable ways.
Roderigo could not have been exploited had he not (predictably) been out of his mind for Desdemona; Cassio could not have been exploited had he not been drinking and then acted as he and everyone else knew he would when he was drunk (he means it when he says he has "poor and unhappy brains" for drinking); Emilia acts in obedience to her husband which, even for Emilia, is predictable; however, she is definitely not passive;even Desdemona is predictable in her goodness( Iago says it is her virtue that will become the net that he will use to ensnare them all.
So if there is a weakness in the characters, I would say it is in their inability to see how their qualities, good and bad, can be used against them. For some, (Othello, Cassio and Roderigo, for instance) it is also in their decisions to act in ways that are contrary to their thoughts. (All three characters voice thoughts that would allow them to back out of their situations, and each of them ignores their better judgement and continues with the actions they thought better of.