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Current Web service platforms (WSPs) often perform all Web services-related processing, including security-sensitive information handling, in the same protection domain. Consequently, the entire WSP may have access to security-sensitive information such as credit card numbers, forcing us to trust a large and complex piece of software. To address this problem, we propose ISO-WSP, a new information flow architecture that decomposes current WSPs into two parts executing in separate protection domains: (1) a small trusted T-WSP to handle security-sensitive data, and (2) a large, legacy untrusted U-WSP that provides the normal WSP functionality, but uses the T-WSP for security-sensitive data handling. By restricting security-sensitive data access to T-WSP, ISO-WSP reduces the software complexity of trusted code, thereby improving the testability of ISO-WSP. Using a prototype implementation based on the Apache Axis2 WSP, we show that ISO-WSP reduces software complexity of trusted components by a factor of five, while incurring a modest performance overhead of few milliseconds per request.