With the growing popularity of cloud computing, more and more enterprises are migrating their collaboration platforms from inenterprise systems to Software as a Service (SaaS) applications. While SaaS collaboration has numerous advantages, it also raises new security challenges. In particular, since SaaS collaboration is increasingly used across enterprise boundaries, organizations are concerned that sensitive information may be leaked to outsiders due to their employees' inadvertent mistakes on information sharing. In this article, we propose to mitigate the data leakage problem in SaaS collaboration systems by reducing human errors. Built on top of the discretionary access control model in existing collaboration systems, we have designed a series of mechanisms to provide defense in depth against information leakage. First, we allow enterprises to encode their organizational security rules as mandatory access control policies, so as to impose coarse-grained restrictions on their employees' discretionary sharing decisions. Second, we design an attribute-based recommender that suggests and prioritizes potential recipients for users' files, reducing errors in the choices of recipients. Third, our system actively examines abnormal recipients entered by a file owner, providing the last line of defense before a file is shared. We have implemented a prototype of our solution and performed experiments on data collected from real-world collaboration systems. © 2011 ACM.