Computer Security: Principles and Practice Fourth Edition By: William Stallings and Lawrie Brown Chapter 4 Access Control Access Control Definitions 1/2 NISTIR 7298 defines access control as: “the process of granting or denying specific requests to: (1) obtain and use information and related information processing services; and (2) enter specific physical facilities” Access Control Definitions 2/2 RFC 4949 defines access control as: “a process by which use of system resources is regulated according to a security policy and is permitted only by authorized entities (users, programs, processes, or other systems) according to that policy” Table 4.1 Access Control Security Requirements ( SP 800-171) (Table is on page 107 in the textbook) Access Control Principles • In a broad sense, all of computer security is concerned with access control • RFC 4949 defines computer security as: “measures that implement and assure security services in a computer system, particularly those that assure access control service” Source: Based on [SAND94]. Source: Based on [SAND94]. Access Control Policies • Discretionary access control (DAC) o Controls access based on the identity of the requestor and on access rules (authorizations) stating what requestors are (or are not) allowed to do • Mandatory access control (MAC) o Controls access based on comparing security labels with security clearances • Role-based access control (RBAC) o Controls access based on the roles that users have within the system and on rules stating what accesses are allowed to users in given roles • Attribute-based access control (ABAC) o Controls access based on attributes of the user, the resource to be accessed, and current environmental conditions Subjects, Objects, and Access Rights Subject An entity capable of accessing objects Three classes • Owner • Group • World Object A resource to which access is controlled Access right Describes the way in which a subject may access an object Could include: Entity used to contain and/or receive information •Read •Write •Execute •Delete •Create •Search Discretionary Access Control (DAC) • Scheme in which an entity may be granted access • rights that permit the entity, by its own violation, to enable another entity to access some resource Often provided using an access matrix o One dimension consists of identified subjects that may attempt data access to the resources o The other dimension lists the objects that may be accessed • Each entry in the matrix indicates the access rights of a particular subject for a particular object Figure 4.2 Example of Access Control Structures Table 4.2 Authorization Table for Files in Figure 4.2 (Table is on page 113 in the textbook) Table 4.3 Access Control System Commands (Table is on page 116 in the textbook) Protection Domains • Set of objects together with access rights to those objects • More flexibility when associating capabilities with protection domains • In terms of the access matrix, a row defines a protection domain • User can spawn processes with a subset of the access rights of the user • Association between a process and a domain can be static or dynamic • In user mode certain areas of memory are protected from use and certain instructions may not be executed • In kernel mode privileged instructions may be executed and protected areas of memory may be accessed UNIX File Access Control UNIX files are administered using inodes (index nodes) • • • • Control structures with key information needed for a particular file Several file names may be associated with a single inode An active inode is associated with exactly one file File attributes, permissions and control information are sorted in the inode • On the disk there is an inode table, or inode list, that contains the inodes of all the files in the file system • When a file is opened its inode is brought into main memory and stored in a memory resident inode table Directories are structured in a hierarchical tree • May contain files and/or other directories • Contains file names plus pointers to associated inodes UNIX File Access Control ● Unique user identification number (user ID) ● Member of a primary group identified by a group ID ● Belongs to a specific group ● 12 protection bits ● ● Specify read, write, and execute permission for the owner of the file, members of the group and all other users The owner ID, group ID, and protection bits are part of the file’s inode Figure 4.5 UNIX File Access Control Traditional UNIX File Access Control ● ● “Set user ID”(SetUID) “Set group ID”(SetGID) ● ● ● Sticky bit ● ● System temporarily uses rights of the file owner/group in addition to the real user’s rights when making access control decisions Enables privileged programs to access files/resources not generally accessible When applied to a directory it specifies that only the owner of any file in the directory can rename, move, or delete that file Superuser ● ● Is exempt from usual access control restrictions Has system-wide access Access Control Lists (ACLs) in UNIX Modern UNIX systems support ACLs • FreeBSD, OpenBSD, Linux, Solaris FreeBSD • Setfacl command assigns a list of UNIX user IDs and groups • Any number of users and groups can be associated with a file • Read, write, execute protection bits • A file does not need to have an ACL • Includes an additional protection bit that indicates whether the file has an extended ACL When a process requests access to a file system object two steps are performed: • Step 1 selects the most appropriate ACL • Step 2 checks if the matching entry contains sufficient permissions Table 4.4 Scope RBAC Models Constraints - RBAC • Provide a means of adapting RBAC to the specifics of administrative and security policies of an organization • A defined relationship among roles or a condition related to roles • Types: Mutually exclusive roles •A user can only be assigned to one role in the set (either during a session or statically) •Any permission (access right) can be granted to only one role in the set Cardinality •Setting a maximum number with respect to roles Prerequisite roles •Dictates that a user can only be assigned to a particular role if it is already assigned to some other specified role Attribute-Based Access Control (ABAC) Can define authorizations that express conditions on properties of both the resource and the subject Strength is its flexibility and expressive power Main obstacle to its adoption in real systems has been concern about the performance impact of evaluating predicates on both resource and user properties for each access Web services have been pioneering technologies through the introduction of the eXtensible Access Control Markup Language (XAMCL) There is considerable interest in applying the model to cloud services ABAC Model: Attributes Subject attributes Object attributes Environmen t attributes • A subject is an active entity that causes information to flow among objects or changes the system state • An object (or resource) is a passive information system-related entity containing or receiving information • Describe the operational, technical, and even situational environment or context in which the information access occurs • These attributes have so far been largely ignored in most access control policies • Attributes define the identity and characteristics of the subject • Objects have attributes that can be leverages to make access control decisions ABAC Distinguishable because it controls access to objects by evaluating rules against the attributes of entities, operations, and the environment relevant to a request Relies upon the evaluation of attributes of the subject, attributes of the object, and a formal relationship or access control rule defining the allowable operations for subject-object attribute combinations in a given environment Systems are capable of enforcing DAC, RBAC, and MAC concepts Allows an unlimited number of attributes to be combined to satisfy any access control rule ABAC Policies A policy is a set of rules and relationships that govern allowable behavior within an organization, based on the privileges of subjects and how resources or objects are to be protected under which environment conditions Typically written from the perspectiv e of the object that needs protecting and the privileges available to subjects Privileges represent the authorized behavior of a subject and are defined by an authority and embodied in a policy Other terms commonly used instead of privileges are: rights, authorizations, and entitlements Identity, Credential, and Access Management (ICAM) • A comprehensive approach to managing and implementing digital identities, credentials, and access control • Developed by the U.S. government • Designed to: o Create trusted digital identity representations of individuals and nonperson entities (NPEs) o Bind those identities to credentials that may serve as a proxy for the individual of NPE in access transactions • A credential is an object or data structure that authoritatively binds an identity to a token possessed and controlled by a subscriber o Use the credentials to provide authorized access to an agency’s resources Identity Management Concerned with assigning attributes to a digital identity and connecting that digital identity to an individual or NPE Goal is to establish a trustworthy digital identity that is independent of a specific application or context Most common approach to access control for applications and programs is to create a digital representation of an identity for the specific use of the application or program Maintenance and protection of the identity itself is treated as secondary to the mission associated with the application Final element is lifecycle management which includes: • Mechanisms, policies, and procedures for protecting personal identity information • Controlling access to identity data • Techniques for sharing authoritative identity data with applications that need it • Revocation of an enterprise identity Credential Management The management of the life cycle of the credential Encompasses five logical components: Examples of credentials are smart cards, private/public cryptographic keys, and digital certificates An authorized individual sponsors an individual or entity for a credential to establish the need for the credential The sponsored individual enrolls for the credential • Process typically consists of identity proofing and the capture of biographic and biometric data • This step may also involve incorporating authoritative attribute data, maintained by the identity management component A credential is produced • Depending on the credential type, production may involve encryption, the use of a digital signature, the production of a smart card or other functions The credential is issued to the individual or NPE A credential must be maintained over its life cycle • Might include revocation, reissuance/replacement, reenrollment, expiration, personal identification number (PIN) reset, suspension, or reinstatement Access Management Deals with the management and control of the ways entities are granted access to resources Covers both logical and physical access May be internal to a system or an external element Purpose is to ensure that the proper identity verification is made when an individual attempts to access a security sensitive building, computer systems, or data Three support elements are needed for an enterprisewide access control facility: • Resource management • Privilege management • Policy management Three support elements are needed for an enterprise-wide access control facility: Resource management • Concerned with defining rules for a resource that requires access control • Rules would include credential requirements and what user attributes, resource attributes, and environmental conditions are required for access of a given resource for a given function Privilege management • Concerned with establishing and maintaining the entitlement or privilege attributes that comprise an individual’s access profile • These attributes represent features of an individual that can be used as the basis for determining access decisions to both physical and logical resources • Privileges are considered attributes that can be linked to a digital identity Policy management • Governs what is allowable and unallowable in an access transaction Identity Federation • Term used to describe the technology, standards, policies, and processes that allow an organization to trust digital identities, identity attributes, and credentials created and issued by another organization • Addresses two questions: o How do you trust identities of individuals from external organizations who need access to your systems o How do you vouch for identities of individuals in your organization when they need to collaborate with external organizations Open Identity Trust Framework OpenID OIDF ICF • An open standard that allows users to be authenticated by certain cooperating sites using a third party service • OpenID Foundation is an international nonprofit organization of individuals and companies committed to enabling, promoting, and protecting OpenID technologies • Information Card Foundation is a nonprofit community of companies and individuals working together to evolve the Information Card ecosystem OITF OIX AXN • Open Identity Trust Framework is a standardized, open specification of a trust framework for identity and attribute exchange, developed jointly by OIDF and ICF • Open Identity Exchange Corporation is an independent, neutral, international provider of certification trust frameworks conforming to the OITF model • Attribute Exchange Network is an online Internet-scale gateway for identity service providers and relying parties to efficiently access user asserted, permissioned, and verified online identity attributes in high volumes at affordable costs Table 4.5 Functions and Roles for Banking Example Table 4.5 Functions and Roles for Banking Example Summary • Access control principles o Access control context o Access control policies • Subjects, objects, and access rights • Discretionary access control o Access control model o Protection domains • UNIX file access control o Traditional UNIX file access control o Access control lists in UNIX • Role-based access control o RBAC reference models • Attribute-based access control o o o Attributes ABAC logical architecture ABAC policies • Identity, credential, and access management o o o o Identity management Credential management Access management Identity federation • Trust frameworks o o Traditional identity exchange approach Open identity trust framework • Bank RBAC system 
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