OS Access Control Mauricio Sifontes

Introduction Access Control is a job performed by the operating system in collaboration with the hardware that deals with granting different users different permissions to own, read and write files or other resources. This is an integral in the security of the system

Concepts Roles: Actions: Owner Identification Custodian Authentication End User Actions: Identification Authentication Authorization Access

Lampson’s Access Matrix Is a 2 dimensional matrix that consists of the objects in one dimension and the subjects in the other It defines the different access rights that the subjects have on the objects Subjects can be for example processes, users, groups, etc. Objects can be for example processes, files, resources, etc. Access rights can be for example own, read, write, execute, etc.

Access Control Lists Set of permissions attached to an object (Column of Lampson’s Matrix) The system has to check the list when a subject wants to use the resource In windows, an ACL Table contains: SID: ID used to identify user, group or session in all interactions Access Mask: Value that determines what rights are allowed or denied Type Flag: Indicates the type of the object which determines what operations can be performed on it Inheritance Flags: determines the inheritance property of the object

Capability Tickets Authorized objects and operations for every user (Row of Lampson’s Matrix) Capability tickets present a greater security vulnerability because they might be dispersed through a system To overcome this, capability tickets should be managed by the OS and located in a part of memory inaccessible to users In networks, encryption can de used for data distribution since only certain users should have access to the data

Access Control Models Access control models are frameworks that the custodian can use to give different types of access to different subjects The access control model is both embedded in the software and the hardware Discretionary Access Control (DAC) Mandatory Access Control (MAC) Role Based Access Control (RBAC) Rule Based Access Control (RBAC)

Discretionary Access Control (DAC) Developed by Lampson, Graham and Denning Least Restrictive Every object has an owner with full control over it Owner can give different access types to other users Used in most Windows and Unix Systems Weaknesses: End User is responsible for setting proper level security Subject permissions will be inherited by programs that the user executes Step 1: Subject a request object alpha in cretain operation Step 2: A message is created with these 3 variables Step 3: Matrix interrogated and access granted or denied. If denied, a warning is triggered

Discretionary Access Control (DAC) Protection State: The set of information at any given point that specifies the access right for each subject in respect to the objects. Rules for modifying the Access Matrix: Entries in the matrix are treated as object themselves The owners and control properties must be specified in the access Control Matrix A table of rules must be determined for transferring access capabilities

Access Control Rules (DAC)

Mandatory Access Control (MAC) Most restrictive model Only custodian is capable of assigning access levels Model based on “labels” i.e. confidential, secret, top secret, etc. Also based on “levels” Commonly used in military applications Lattice Model (multiple lattices) Bell-LaPadula Model (restricts creation of objects)

Role Based Access Control (RBAC) More relevant to real world applications Permissions are assigned depending on the users role in the system A type of user is created with specific permissions and then users are assigned a type

Rule Based Access Control (RBAC) Also referred to as Rule Based Role Based Access Control (RB-RBAC) Can dynamically assign roles to users based on a set of rules determined by the custodian Each object has access properties The system has to check on a case by case basis Used to manage user access to one or more systems

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