In the near future, clouds will provide situational monitoring services such as health monitoring, stock market monitoring, shopping cart monitoring, and emergency control and threat management. Offering such services requires securely processing data streams generated by multiple, possibly competing and/or complementing, organizations, such that there is no overt or covert leakage of sensitive information. We demonstrate how an information flow control model adapted from the Chinese Wall policy can be used to protect against sensitive data disclosure in data stream management system. We also develop a language based on Continuous Query Language that can be used to express information flow constraints in stream processing and provide its formal semantics.