CAPSICUM(4)            FreeBSD Kernel Interfaces Manual            CAPSICUM(4)

NAME

     Capsicum - lightweight OS capability and sandbox framework

SYNOPSIS

     options CAPABILITY_MODE
     options CAPABILITIES

DESCRIPTION

     Capsicum is a lightweight OS capability and sandbox framework
     implementing a hybrid capability system model.  Capsicum is designed to
     blend capabilities with UNIX.  This approach achieves many of the
     benefits of least-privilege operation, while preserving existing UNIX
     APIs and performance, and presents application authors with an adoption
     path for capability-oriented design.

     Capabilities are unforgeable tokens of authority that can be delegated
     and must be presented to perform an action.  Capsicum makes file
     descriptors into capabilities.

     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.

             Access to system calls in capability mode is restricted: some
             system calls requiring global namespace access are unavailable,
             while others are constrained.  For instance, sysctl(2) can be
             used to query process-local information such as address space
             layout, but also to monitor a system's network connections.
             sysctl(2) is constrained by explicitly marking ~~60 of over 15000
             parameters as permitted in capability mode; all others are
             denied.

             The system calls which require constraints are sysctl(2),
             shm_open(2) (which is permitted to create anonymous memory
             objects but not named ones) and the openat(2) family of system
             calls.  The openat(2) calls already accept a file descriptor
             argument as the directory to perform the open(2), rename(2), etc.
             relative to; in capability mode the openat(2) family of system
             calls are constrained so that they can only operate on objects
             "under" the provided file descriptor.

     capabilities
             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).

     FreeBSD provides some additional functionality to support application
     sandboxing that is not part of Capsicum itself:

     capsicum_helpers(3)
             A set of a inline functions which simplify modifying programs to
             use Capsicum.

     libcasper(3)
             A library that provides services for sandboxed applications, such
             as operating on files specified on a command line or establishing
             network connections.

SEE ALSO

     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),
     capsicum_helpers(3), libcasper(3), procdesc(4)

HISTORY

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

AUTHORS

     Capsicum was developed by Robert Watson <rwatson@FreeBSD.org> and
     Jonathan Anderson <jonathan@FreeBSD.org> at the University of Cambridge,
     and Ben Laurie <benl@FreeBSD.org> and Kris Kennaway <kris@FreeBSD.org> at
     Google, Inc., and Pawel Jakub Dawidek <pawel@dawidek.net>.  Portions of
     this manual page are drawn from Robert N. M. Watson, Jonathan Anderson,
     Ben Laurie, and Kris Kennaway, "Capsicum: practical capabilities for
     UNIX", USENIX Security Symposium, August 2010, DOI:
     10.5555/1929820.1929824.

FreeBSD 15.1-STABLE-HBSD       January 23, 2026                    CAPSICUM(4)