Editing Capability-based security (section)

==Introduction==
Capabilities achieve their objective of improving system security by being used in place of forgeable [[reference (computer science)|references]]. A forgeable reference (for example, a [[path (computing)|path name]]) identifies an object, but does not specify which access rights are appropriate for that object and the user program which holds that reference. Consequently, any attempt to access the referenced object must be validated by the operating system, based on the [[ambient authority]] of the requesting program, typically via the use of an [[access-control list]] (ACL). Instead, in a system with capabilities, the mere fact that a user program possesses that capability entitles it to use the referenced object in accordance with the rights that are specified by that capability. In theory, a system with capabilities removes the need for any access control list or similar mechanism by giving all entities all and only the capabilities they will actually need.

A capability is typically implemented as a [[privilege (computing)|privileged]] [[data structure]] that consists of a section that specifies access rights, and a section that uniquely identifies the object to be accessed. The user does not access the data structure or object directly, but instead via a [[Handle (computing)|handle]]. In practice, it is used much like a [[file descriptor]] in a traditional operating system (a traditional handle), but to access every object on the system. Capabilities are typically stored by the operating system in a list, with some mechanism in place to prevent the program from directly modifying the contents of the capability (so as to forge access rights or change the object it points to). Some systems have also been based on [[capability-based addressing]] (hardware support for capabilities), such as [[Plessey System 250]].

Programs possessing capabilities can perform functions on them, such as passing them on to other programs, converting them to a less-privileged version, or deleting them. The operating system must ensure that only specific operations can occur to the capabilities in the system, in order to maintain the integrity of the security policy.

Capabilities as discussed in this article should not be confused with Portable Operating System Interface ([[POSIX]]) 1e/2c "[[#POSIX capabilities|Capabilities]]". The latter are coarse-grained privileges that cannot be transferred between processes.