As marketers’ use of big data is increasing, their data management efforts may increase customer data vulnerability or at least perceptions of susceptibility to harm. Yet most firms have little insight into the potential negative ramifications of their big data efforts or how to prevent them. In an event study of data security breaches affecting 167 public companies, the authors identify the effects on firm performance and close rival firm performance, as well as managerial tools to suppress this harm. A second study, involving a series of experiments designed to understand the effects of data vulnerability from the customer perspective, shows that firms’ mere access to customers’ personal data inflates feelings of violation and reduces trust, creating negative word-of-mouth and switching behaviors. Collectively, this work makes four managerial contributions. First, it provides a rare customer-centric perspective of firms’ customer data use. Second, the findings show the severity of a data breach (number of customer records affected) aggravates its negative effect on the focal firm’s stock price. However, this effect of data breach severity is opposite for the rival firm: As the severity of the breach at the focal firm increases, it creates a positive effect on the rival firm’s performance. Third, this research identifies and tests two mitigation strategies that are effective across a range of data vulnerabilities: making data management policies more transparent and providing customers with control over their data. Transparency and control interact to suppress the negative performance effects of data breaches at focal firms, spillover to rivals, and even the negative effects of firm access to customer data.