Wall Street has found its next big short. With bad news piling up for mall chains like Macy’s and J.C. Penney, bearish bets against commercial mortgage-backed securities are growing. Like the run-up to the housing debacle, a small but growing group of firms are positioning to profit from a collapse that could spur a wave of defaults. Their target: securities backed by loans taken out by beleaguered mall and shopping center operators.

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