Final answer:
Cassio's weakness, which Iago exploits in Shakespeare's 'Othello', is his poor tolerance for alcohol, leading to his downfall.
Step-by-step explanation:
The question pertains to a character flaw in one of Shakespeare's well-known plays, Othello.
The weakness of Cassio that Iago exploits is Cassio's inability to handle his liquor; Cassio is aware that drinking too much can lead to problems, but Iago manipulates the situation to ensure that Cassio does drink and thus behaves inappropriately, which ultimately compromises his position and reputation.
Iago's cunning use of Cassio's vulnerability forms a pivotal part of the play's plot development, leading to tragic consequences.