Today, Jonathan Anderson, Ben Laurie, Kris Kennaway, and I presented Capsicum: practical capabilities for UNIX at the 19th USENIX Security Symposium in Washington, DC; the slides can be found on the Capsicum web site. We argue that capability design principles fill a gap left by discretionary access control (DAC) and mandatory access control (MAC) in operating systems when supporting security-critical and security-aware applications.

Capsicum responds to the trend of application compartmentalisation (sometimes called privilege separation) by providing strong and well-defined isolation primitives, and by facilitating rights delegation driven by the application (and eventually, user). These facilities prove invaluable, not just for traditional security-critical programs such as tcpdump and OpenSSH, but also complex security-aware applications that map distributed security policies into local primitives, such as Google’s Chromium web browser, which implement the same-origin policy when sandboxing JavaScript execution.

Capsicum extends POSIX with a new capability mode for processes, and capability file descriptor type, as well as supporting primitives such as process descriptors. Capability mode denies access to global operating system namespaces, such as the file system and IPC namespaces: only delegated rights (typically via file descriptors or more refined capabilities) are available to sandboxes. We prototyped Capsicum on FreeBSD 9.x, and have extended a variety of applications, including Google’s Chromium web browser, to use Capsicum for sandboxing. Our paper discusses design trade-offs, both in Capsicum and in applications, as well as a performance analysis. Capsicum is available under a BSD license.

Capsicum is collaborative research between the University of Cambridge and Google, and has been sponsored by Google, and will be a foundation for future work on application security, sandboxing, and security usability at Cambridge and Google. Capsicum has also been backported to FreeBSD 8.x, and Heradon Douglas at Google has an in-progress port to Linux.

We’re also pleased to report the Capsicum paper won Best Student Paper award at the conference!