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CAPSICUM(4) Device Drivers Manual CAPSICUM(4)


Capsicumlightweight OS capability and sandbox framework




Capsicum is a lightweight OS capability and sandbox framework implementing a hybrid capability system model. Capsicum can be used for application and library compartmentalisation, the decomposition of larger bodies of software into isolated (sandboxed) components in order to implement security policies and limit the impact of software vulnerabilities.

Capsicum provides two core kernel primitives:

capability mode
A process mode, entered by invoking cap_enter(2), in which access to global OS namespaces (such as the file system and PID namespaces) is restricted; only explicitly delegated rights, referenced by memory mappings or file descriptors, may be used. Once set, the flag is inherited by future children processes, and may not be cleared.
Limit operations that can be called on file descriptors. For example, a file descriptor returned by open(2) may be refined using cap_rights_limit(2) so that only read(2) and write(2) can be called, but not fchmod(2). The complete list of the capability rights can be found in the rights(4) manual page.

In some cases, Capsicum requires use of alternatives to traditional POSIX APIs in order to name objects using capabilities rather than global namespaces:

process descriptors
File descriptors representing processes, allowing parent processes to manage child processes without requiring access to the PID namespace; described in greater detail in procdesc(4).
anonymous shared memory
An extension to the POSIX shared memory API to support anonymous swap objects associated with file descriptors; described in greater detail in shm_open(2).

In some cases, Capsicum limits the valid values of some parameters to traditional APIs in order to restrict access to global namespaces:

process IDs
Processes can only act upon their own process ID with syscalls such as cpuset_setaffinity(2).


cap_enter(2), cap_fcntls_limit(2), cap_getmode(2), cap_ioctls_limit(2), cap_rights_limit(2), fchmod(2), open(2), pdfork(2), pdgetpid(2), pdkill(2), pdwait4(2), read(2), shm_open(2), write(2), cap_rights_get(3), libcasper(3), procdesc(4)


Capsicum first appeared in FreeBSD 9.0, and was developed at the University of Cambridge.


Capsicum was developed by Robert Watson <> and Jonathan Anderson <> at the University of Cambridge, and Ben Laurie <> and Kris Kennaway <> at Google, Inc., and Pawel Jakub Dawidek <>.

May 18, 2017 Debian