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.
Well I don't think you could argue that they are all weak and passive (look at Iago, he's the character that causes the plot to progress). The other characters (some more than others) are not really passive, but more than that, they were manipulated by Iago. The other characters try to make their own actions, but are led astray by Iago. Take Cassio for example, after losing his position with Othello as a favorite lieutenant, he goes to Desdemona and asks her to help him. He is active in trying to regain his position. Unfortunately, he is active as Othello's puppet so it does him no good. Another example would be Emilia, as Iago's wife, she is also not weak or passive. She tries hard to please Iago (by stealing Desdemona's handkerchief) but she also is a strong-willed woman who apparently will not suffer her husband to abuse her.She is a woman who believes that men and women experience the same passions and desires. Near the end of the play, Emilia will not be silenced in her efforts to bring Desdemona's killers to justice. She even defies Othello in his efforts to physically intimidate her. The characters in Othello may be easily manipulative, but I don't think they are all weak and passive.