Abstract: Capsicum is a lightweight operating system capabil- ity and sandbox framework planned for inclusion in FreeBSD 9. Capsicum extends, rather than replaces, UNIX APIs, providing new kernel primitives (sandboxed capability mode and capabilities) and a userspace sand- box API. These tools support compartmentalisation of monolithic UNIX applications into logical applications, an increasingly common goal supported poorly by dis- cretionary and mandatory access control. We demon- strate our approach by adapting core FreeBSD utilities and Google’s Chromium web browser to use Capsicum primitives, and compare the complexity and robustness of Capsicum with other sandboxing techniques.