My apologies for saying something similar in a different comment, but: Step 1: Deposit money to ExchB Step 2: Withdraw bitcoins Only vulnerability for you is

My apologies for saying something similar in a different comment, but: Step 1: Deposit money to ExchB Step 2: Withdraw bitcoins Only vulnerability for you is when you've done Step 1 but not Step 2 yet. Don't do humongous deposits, and don't have multiple outstanding transactions with ExchB, and you'll be fine.