World Library  
Flag as Inappropriate
Email this Article

Capability-based security

Article Id: WHEBN0000539717
Reproduction Date:

Title: Capability-based security  
Author: World Heritage Encyclopedia
Language: English
Subject: CapROS, GNOSIS, Capability-based addressing, Discretionary access control, Access control
Collection: Access Control, Computer Security Models
Publisher: World Heritage Encyclopedia

Capability-based security

Capability-based security is a concept in the design of token of authority. It refers to a value that references an object along with an associated set of access rights. A user program on a capability-based operating system must use a capability to access an object. Capability-based security refers to the principle of designing user programs such that they directly share capabilities with each other according to the principle of least privilege, and to the operating system infrastructure necessary to make such transactions efficient and secure. Capability-based security is to be contrasted with an approach that uses hierarchical protection domains.

Although most operating systems implement a facility which resembles capabilities, they typically do not provide enough support to allow for the exchange of capabilities among possibly mutually untrusting entities to be the primary means of granting and distributing access rights throughout the system. A capability-based system, in contrast, is designed with that goal in mind.

Capabilities as discussed in this article should not be confused with POSIX 1e/2c "Capabilities". The latter are coarse-grained privileges that cannot be transferred between processes.


  • Capabilities and capability-based security 1
  • Introduction to capability-based security 2
  • Sharing of capabilities between processes 3
  • POSIX vs. Capsicum capabilities 4
  • Research and commercial systems 5
  • See also 6
  • References 7
  • External links 8

Capabilities and capability-based security

Capabilities achieve their objective of improving system security by being used in place of forgeable 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, 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 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.

Introduction to capability-based security

A capability is defined to be a protected object reference which, by virtue of its possession by a user process, grants that process the capability (hence the name) to interact with an object in certain ways. Those ways might include reading data associated with an object, modifying the object, executing the data in the object as a process, and other conceivable access rights. The capability logically consists of a reference that uniquely identifies a particular object and a set of one or more of these rights.

Suppose that, in a user process's memory space, there exists the following string:


Although this identifies a unique object on the system, it does not specify access rights and hence is not a capability. Suppose there is instead the following two values:


This identifies an object along with a set of access rights. It, however, is still not a capability because the user process's possession of these values says nothing about whether that access would actually be legitimate.

Now suppose that the user program successfully executes the following statement:

int fd = open("/etc/passwd", O_RDWR);

The variable fd now contains the index of a file descriptor in the process's file descriptor table. This file descriptor is a capability. Its existence in the process's file descriptor table is sufficient to know that the process does indeed have legitimate access to the object. A key feature of this arrangement is that the file descriptor table is in kernel memory and cannot be directly manipulated by the user program.

Sharing of capabilities between processes

In traditional operating systems, programs often communicate with each other and with storage using references like those in the first two examples. Path names are often passed as command-line parameters, sent via sockets, and stored on disk. These references are not capabilities, and must be validated before they can be used. In these systems, a central question is "on whose authority is a given reference to be evaluated?" This becomes a critical issue especially for processes which must act on behalf of two different authority-bearing entities. They become susceptible to a programming error known as the confused deputy problem, very frequently resulting in a security hole.

In a capability-based system, the capabilities themselves are passed between processes and storage using a mechanism that is known by the operating system to maintain the integrity of those capabilities.

Although many operating systems implement facilities very similar to capabilities through the use of file descriptors or file handles — for example, in UNIX, file descriptors can be discarded (closed), inherited by child processes, and even sent to other processes via sockets — there are several obstacles that prevent all of the benefits of a capability-based addressing system from being realized in a traditional operating system environment. Chief among these obstacles is the fact that entities which might hold capabilities (such as processes and files) cannot be made persistent in such a way that maintains the integrity of the secure information that a capability represents. The operating system cannot trust a user program to read back a capability and not tamper with the object reference or the access rights, and has no built-in facilities to control such tampering. Consequently, when a program wishes to regain access to an object that is referenced on disk, the operating system must have some way of validating that access request, and an access control list or similar mechanism is mandated.

One novel approach to solving this problem involves the use of an orthogonally persistent operating system. (This was realised in the Flex machine. See Ten15). In such a system, there is no need for entities to be discarded and their capabilities be invalidated, and hence require an ACL-like mechanism to restore those capabilities at a later time. The operating system maintains the integrity and security of the capabilities contained within all storage, both volatile and nonvolatile, at all times; in part by performing all serialization tasks by itself, rather than requiring user programs to do so, as is the case in most operating systems. Because user programs are relieved of this responsibility, there is no need to trust them to reproduce only legal capabilities, nor to validate requests for access using an access control mechanism.

POSIX vs. Capsicum capabilities

POSIX draft 1003.1e specifies a concept of permissions called "capabilities". However, POSIX capabilities differ from capabilities in this article—POSIX capability is not associated with any object; a process having CAP_NET_BIND_SERVICE capability can listen on any TCP port under 1024. In contrast, Capsicum capabilities on FreeBSD and Linux hybridize a true capability-system model with the UNIX design and POSIX API. Capsicum capabilities are a refined form of file descriptor, a delegable right between processes and additional object types beyond classic POSIX, such as processes, can be referenced via capabilities. In Capsicum capability mode, processes are unable to utilize global name spaces (such as the filesystem name space) to look up objects, and must instead inherit or be delegated them.

Research and commercial systems

See also


  • Levy, Henry M., *Capability-Based Computer Systems, Digital Equipment Corporation 1984. ISBN 0-932376-22-3. An electronic version is available here.
  • The EROS Project
  • E, a programming language based around capability security (
  • Mark S. Miller, Ka-Ping Yee, Jonathan Shapiro. Capability Myths Demolished, Technical Report SRL2003-02, Systems Research Laboratory, Johns Hopkins University. Available online.
  • The Cambridge CAP Computer, Levy, 1988

not currently used:

  • Capability-based addressing:
  • Li Gong, A Secure Identity-Based Capability System, sp, p. 56, 1989 IEEE Symposium on Security and Privacy, 1989
  • Capability-based addressing
  • A hardware implementation of capability-based addressing
  • An implementation of capabilities on the PDP-11/45
  • IBM System/38 support for capability-based addressing
  • EROS: a fast capability system

POSIX "capabilities" in Linux:

  • POSIX Capabilities & Files
  • POSIX file capabilities: Parceling the power of root
  • Making Root Unprivileged
  • Security issues and new risks linked to POSIX file capabilities

External links

  • "What is a Capability?"
  • Reviews of 'Capability Myths Demolished'
  • Capability Theory by Sound Bytes
This article was sourced from Creative Commons Attribution-ShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for and content contributors is made possible from the U.S. Congress, E-Government Act of 2002.
Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.
By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a non-profit organization.

Copyright © World Library Foundation. All rights reserved. eBooks from Project Gutenberg are sponsored by the World Library Foundation,
a 501c(4) Member's Support Non-Profit Organization, and is NOT affiliated with any governmental agency or department.