1. Access Control

2. What is Access Control? The ability to allow only authorized users, programs or processes system or resource access The ability to disallow unauthorized users, programs or process system or resource access An entire set of procedures performed by hardware, software and administrators – To monitor access – Identify users requesting access – Record access attempts – Grant or deny access based on pre-established rules

3. Examples of Access Control Social Networks: login system, e.g., only account owner can log on to his/her account Web Browsers: same-origin poliey Operating Systems: process isolation; memory protection Networking: firewall

4. Design Principles Economy of mechanism: keep the design as simple and small as possible Fail-safe defaults: base access decisions on permission not exclusion Complete mediation: access to an object must be checked for authority Open design: the design should not be secret Separation of privilege: a protection mechanism that requires two keys to unlock it is more robust Least privilege: every program and every user of the system should operate using the least set of privileges necessary to complete the job Least common mechanism: minimize the amount of mechanism common to more than one user and depended on by all users Psychological acceptability: it is essential that the human interface be designed for ease of use, so that users routinely and automatically apply the protection mechanisms correctly.

5. Design and Implementation How access control policies are configured: – Discretionary Access Control (DAC) – Mandatory Access Control (MAC) How access control is implemented: – Access Control Matrices – Access Control List – Capability List – Role-Based Access Control

6. DAC: Discretionary Access Control Individual user can set an access control mechanism to allow/deny access to an object Rely on the object owner to control access Widely implemented in most operating systems Strength of DAC: flexibility, a key reason why it is widely known and implemented in mainstream operating systems

7. MAC: Mandatory Access Control A policy mandates who is allowed to have access; individual user cannot alter that access Rely on the system to control access. MAC mechanisms have been tightly coupled to a few security models – Bell-LaPadula (BLP) – Biba Systems supporting MAC, e.g., SELinux, TrustedBSD

8. Access Control Methods I Access Control Matrices – Disadvantage: large matrix size; mostly sparse matrix Access Control List: column of access control matrix – Advantage: Easy to determine who can access a given object Easy to revoke all access to an object – Disadvantage: Difficult to know the access right of a given subject Difficult to revoke a user’s right on all objects

9. Access Control Methods II Capability List: row of access control matrix. – Advantage: Easy to know the access right of a given subject Easy to revoke a users access right on all objects – Disadvantage: Difficult to know who can access a given object Difficult to revoke all access right to an object

10. Access Control List Examples UNIX ACL – Abbreviations of Access Control Lists: three classes: owner, group, and other users – Full Access Control Lists Windows NT – Generic rights: No access, Read, Change, Full control. – Built-in Groups; each with different privileges Everyone: all users Interactive: users logged on locally Network: users logged on over the network System: the operating system Creator / Owner: creator or owner of a file or a resource Social networks – Most social networks use ACL as its main access control model. Users can specify who can access their profiles, friend lists, etc. How is the ACL implemented in operating systems? – ACL is saved in the i-node data structure

12. Exercise