Insider trading law is meant to be a shield, protecting the market and investors from unscrupulous traders, but it can also be a sword. Insofar as we penalize trading on the basis of material, nonpublic information, it becomes possible to share information strategically in order to disable or constrain innocent investors. A hostile takeover can be averted, or a bidding war curtailed, because recipients of such information must then refrain from trading. This Article offers the first general account of “insider tainting,” an increasingly pervasive phenomenon of weaponizing insider trading law.