DISCRETIONARY ACCESS CONTROL. Tran Thi Que Nguyet Faculty of Computer Science & Engineering HCMC University of Technology ttqnguyet@cse.hcmut.edu.

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1 DISCRETIONARY ACCESS CONTROL Tran Thi Que Nguyet Faculty of Computer Science & Engineering HCMC University of Technology

2 Outline Introduction to Discretionary Access Control Propose Models for DAC SQL for Data Control DAC & Information Flow Controls Homework: Case study in SQL Server 2008 Reading chapter 4 Access control for Databases: Concepts and Systems. Elisa Bertino, et al. 2 2

3 Introduction to DAC Discretionary Access Control (DAC): User can protect what they own. The owner is given all privileges on their own data. The owner can define the type of access (read/write/execute/ ) and grant access to others. The typical method of enforcing DAC in a database system is based on the granting and revoking privileges 3 3

4 Introduction to DAC Types of Discretionary Privileges: The account/system level: The administrator specifies the particular privileges that each account holds independently of the objects in the database system. The object level: The administrator can control the privilege to access each individual object in the database system. 4 4

5 Introduction to DAC The account/system level privileges (example) CREATE SCHEMA CREATE TABLE CREATE VIEW ALTER DROP MODIFY SELECT 5

6 Introduction to DAC The object level privileges Data objects: relation or view Includes: INSERT UPDATE DELETE DELETE REFERENCE 6

7 Outline Introduction to Discretionary Access Control Propose Models for DAC SQL for Data Control DAC & Information Flow Controls 7 7

8 Proposed Models for DAC General definition: security model Access matrix model Take-Grant model 8 8

9 Security model A security model provides a semantically rich representation in that it allows functional and structural properties of the security system to be described. A security model describes the protection needs of the system. It is a high-level, software-independent, conceptual model. Types of security model Discretionary model: DAC model govern access of users to the information on the basis of the users identity and of rules that specify, for each user and object in the system, the types of access the user is allowed for the object. The request of a user to access an object is checked against the specified authorizations. Non-discretionary model 9

10 Access matrix model An access matrix is a matrix correlating the subjects, objects and the authorizations owned by each subject on each object. Authorization state: Q=(S,O,A) S (Subjects): a set of subjects or active entities that use system resources. Ex: user, group, process O (Objects): a set of passive objects which must be protected such as subjects and system resources Ex: OS level: file, memory, segments, process. DB level: database, relation, attribute, record, field 10

11 Access matrix model Authorization state: Q=(S,O,A) A: Access matrix Row: subjects Column: objects A[s,o]: access mode For DBs, A[s,o] also includes conditions that must be satisfied in order for s to exercise the access modes Possible conditions: data-dependent (sal<1000), time-dependent (8:00am- 5:00pm), context-dependent ( namesalary pair is prohibited), historydependent, S1 Si Sn O1 Oi Om A[s1,o 1] A[si,o1] A[sn,o 1] A[s1,o i] A[si,oi ] A[sn,o i] A[s1,om ] A[si,om] A[sn,om ] 11

12 Access matrix model Asset 1 Asset 2 file device Role 1 read, write, execute, own execute read write Role 2 read read, write, execute, own 12

13 Access matrix model Model implementation: S {(O,A)}: capability list Alice {(file X, {read, delete}), (file Y, {update})} O {(S,A)}: ACL (access control list) File X {(Alice, {read, delete}), (Bob, {read})} Each entry in the list specifies a subject and operation(s): for example, the entry (Alice, delete) on the ACL for file X gives Alice permission to delete file X Advantages & disadvantages of the two above & the model? Capability list: compute a set of subjects granted access on a given object all lists must be gone through ACL: find all objects a subject can access 13

14 (a) <S i, O j, A[S i, O j ]> (b) CL (c) ACL 14

15 Take-Grant model Authorization state: G=(S,O,E) V=S U O is the set of vertexes, S O = Ф E is the set of labelled arcs take(d,s,x,y): the subject s takes the right d on the object/subject y from the object/subject x s t x s t x d take(d,s,x,y) d d y y 15 15

16 Take-Grant model grant(d,s,x,y): the subject s grants the right d on the object/subject y to the object/subject x s g x s g x d grant(d,s,x,y) d d y y 16 16

17 Take-Grant model Access modes: read, write, take, grant Read, write: inert rights Take, grant: transport rights Other rights Create(s, x): subject s creates object x (The arc is labelled with p, possess) remove p (s, x): The possess right p on a subject/an object x is removed from a subject s. This model is classifiable as an access matrix model Disadvantages? 17 17

18 Take-Grant model Disadvantages: Non-selectivity of administrative rights: all authorizations of S owning a GRANT authorization can be transferred, and all authorizations of O/S on which a TAKE right is held can be taken No control on propagation of authorizations Non locality: S owning the grant privilege on O can give any of its privileges to O, thus augmenting the domain of O (the set of authorizations associated to O) Reversibility of the privileges transport flow 18 18

19 Other models Acten (Action-Entity) model Wood et al. model See [S. Castano, M. Fugini, G. Martella, and P. Samarati (1995). Database Security, ACM Press & Addison-Wesley, ISBN ] + Internet 19 19

21 SQL for Data Control Commands: GRANT REVOKE Based on three central objects: Users Database objects Privileges: select, modify (insert, update, delete), reference 21 21

22 SQL for Data Control GRANT: pass privileges on their own database objects to other users GRANT <privilege list> ON TO <database objects> <user list> REVOKE: take back (cancel) privileges on their own database objects from other users REVOKE <privilege list> ON FROM <database objects> <user list> 22 22

23 SQL for Data Control Propagation of Privileges using the GRANT OPTION Whenever the owner A of a relation R grants a privilege on R to another account B, privilege can be given to B with or without the GRANT OPTION. If the GRANT OPTION is given, this means that B can also grant that privilege on R to other accounts

24 Limit horizontal propagation 24

25 Limit Vertical Propagation 25

26 Revocation of authorization (b) B revokes D s privilege (cascade) 26

27 SQL for Data Control DAC with views (virtual relations) If the owner A of a relation R wants another account B to be able to retrieve only some fields of R, then A can create a view V of R that includes only those attributes and then grant SELECT on V to B. The same applies to limiting B to retrieving only certain tuples of R; a view V can be created by defining the view by means of a query that selects only those tuples from R that A wants to allow B to access. 27

28 An Example Suppose that the DBA creates four accounts A1, A2, A3, A4 and wants only A1 to be able to create base relations. Then the DBA must issue the following GRANT command in SQL GRANT CREATETAB TO A1; In SQL2 the same effect can be accomplished by having the DBA issue a CREATE SCHEMA command as follows: CREATE SCHEMA EXAMPLE AUTHORIZATION A1; 28 28

29 An Example(2) User account A1 can create tables under the schema called EXAMPLE. Suppose that A1 creates the two base relations EMPLOYEE and DEPARTMENT A1 is then owner of these two relations and hence all the relation privileges on each of them. Suppose that A1 wants to grant A2 the privilege to insert and delete tuples in both of these relations, but A1 does not want A2 to be able to propagate these privileges to additional accounts: GRANT INSERT, DELETE ON EMPLOYEE, DEPARTMENT TO A2; 29 29

30 An Example(3) 30 30

31 An Example(4) Suppose that A1 wants to allow A3 to retrieve information from either of the two tables and also to be able to propagate the SELECT privilege to other accounts. A1 can issue the command: GRANT SELECT ON EMPLOYEE, DEPARTMENT TO A3 WITH GRANT OPTION; A3 can grant the SELECT privilege on the EMPLOYEE relation to A4 by issuing: GRANT SELECT ON EMPLOYEE TO A4; Notice that A4 can t propagate the SELECT privilege because GRANT OPTION was not given to A

32 An Example(5) Suppose that A1 decides to revoke the SELECT privilege on the EMPLOYEE relation from A3; A1 can issue: REVOKE SELECT ON EMPLOYEE FROM A3; The DBMS must now automatically revoke the SELECT privilege on EMPLOYEE from A4, too, because A3 granted that privilege to A4 and A3 does not have the privilege any more

33 An Example(6) Suppose that A1 wants to give back to A3 a limited capability to SELECT from the EMPLOYEE relation and wants to allow A3 to be able to propagate the privilege. The limitation is to retrieve only the NAME, BDATE, and ADDRESS attributes and only for the tuples with DNO=5. A1 then create the view: CREATE VIEW A3EMPLOYEE AS SELECT NAME, BDATE, ADDRESS FROM EMPLOYEE WHERE DNO = 5; After the view is created, A1 can grant SELECT on the view A3EMPLOYEE to A3 as follows: GRANT SELECT ON A3EMPLOYEE TO A3 WITH GRANT OPTION; 33 33

34 An Example(7) Finally, suppose that A1 wants to allow A4 to update only the SALARY attribute of EMPLOYEE; A1 can issue: GRANT UPDATE ON EMPLOYEE (SALARY) TO A4; The UPDATE or INSERT privilege can specify particular attributes that may be updated or inserted in a relation. Other privileges (SELECT, DELETE) are not attribute specific

36 DAC & INFORMATION FLOW CONTROLS Inherent weakness of DAC: Unrestricted DAC allows information from an object which can be read by a subject to be written to any other object Bob is denied access to file A, so he asks cohort Alice to copy A to B that he can access Suppose our users are trusted not to do this deliberately. It is still possible for Trojan Horses to copy information from one object to another

37 Trojan horse Example 37 37

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