Access control models and policies Aalto University, autumn 2013

Outline Access control Discretionary AC Mandatory AC Other AC models Models and terminology for thinking about security policies

Access control

Access control (AC) Subjects request actions on objects Alice wants to read a file Bob wants to update account balance Process wants to open a socket AC = authentication + authorization authentication = verifying the identity of the subject authorization = checking that the subject has the right to perform the requested action on the subject

Reference monitor Objects Subjects Reference monitor Audit trail Access rules Access requests Reference monitor controls access by subjects to objects Grants or denies access requests Logs events to audit trail Follows rules set by administrators (i.e. implements a policy) Trusted computing base (TCB) = all system components that need to be trusted to implement access control Security kernel = implementation of reference monitor in an OS But more about the implementation later; now we are talking about policies

Access control matrix Access control matrix is the simplest, most general AC model M : Subjects × Objects → P(Actions) Subject S is allowed to request action A on object O iff A ∈ M(S,O) AC matrix represents the protection state of a system Alice Bob Process 4567 Process 6789 file1.txt read, write read append file2.txt write - Socket s open, read, write, close

Discretionary access control

Discretionary access control (DAC) Data owners, usually users, set access rights Subjects are trusted to make decisions about sharing access rights Users decide who is allowed to access their files User or process that can read a secret file can also share it e.g. by email DAC is also called identity-based AC: rights are assigned to users Typical in commercial and consumer systems There may be a policy against sharing and access may be audited, but the policy is not enforced technically Examples of DAC outside computers: Person with a key can open the door to others; door keys can be shared and copied Tell your friend a secret on the condition that he does not tell it to anyone else

Access control list (ACL) ACL = list of the access rights associated with an object ACLs are another way to represent the AC matrix: one row of the matrix is stored for each object file1.txt ACL: Alice: { read, write }; Bob: { read }; Process 4567: { read, write }; Process 6789: { append }. file2.txt ACL: Alice: { write }; Bob: { read }. Socket s ACL: Process 6789: { open, read, write, close }. ACL examples: Key cards, table reservations, Windows file system

Capabilities Capability = an access right associated with the subject Capabilities are another way to represent the AC matrix: one column is stored for each subject Alice’s capabilities: file1.txt: { read, write }; file2.txt: { write }. Bob’s capabilities: file1.txt: { read }; file2.txt: { read }. Process 4567 capabilities: file1.txt: { read, write }. Process 6789 capabilities: file1.txt: { append }; Socket s: {open, read, write, close }. Examples of capabilities: metal keys, driver’s license, parking permit

Mandatory access control

Mandatory access control (MAC) Access rights are based on rules (i.e. policy) set by administration The AC policy is enforced and cannot be changed by users Subjects cannot leak access rights to others User can read a secret file but cannot copy, print or email; file viewer application prevents cut-and-paste and screen shots One process can access the Internet, another write files to the disk, neither is allowed to do both MAC is also called rule-based AC MAC originates from military policies Intelligence officer may not be allowed to read his own reports Officer can read a secret document but cannot take a copy out of the room Officer who has had contact with foreign agents may lose access to classified information

Mandatory access control (MAC) MAC has some uses in commercial systems DRM: Alice can play the music she has purchased, but cannot share it Malware isolation: Host firewall may block potential spyware from making outbound connections to prevent information leaks Examples of MAC-like policies outside computers: Biometric authentication prevents sharing of capabilities, e.g. photo on driver’s license or signature on credit card Admit-one event tickets: UV stamps, shredding bracelets In UK, jurors must not read newspapers or watch TV about the case so that they are not influenced by them

Clearance and classification Mandatory access control rules are often based on security labels on subjects and objects Subject clearance Object classification l : (Subjects ∪ Objects) → Labels MAC based on clearance and classification levels is also called multi-level security (MLS) Simple security property: S can read O iff l(S) ≥ l(O) Top secret Secret Confidential Unclassified High Low

Bell-LaPadula model Bell-LaPadula (BLP) is a MAC policy for protecting secrets Military security model for computers; military is mostly concerned with protecting secrets Observation: the simple security property is not sufficient to prevent secrets from leaking Bell-LaPadula rules: Simple security property: S can read O iff l(S) ≥ l(O) *-property: S can write O iff l(O) ≥ l(S) Also called: no read up, no write down

Biba model In computer systems, integrity of data and the system is often more important than confidentiality Which is more important in a bank IT system? Biba is a MAC policy for protecting integrity of data Biba rules: S can write O iff l(S) ≥ l(O) S can read O iff l(O) ≥ l(S) Also called: no write up, no read down

Information flow security BLP and Biba are information flow policies BLP prevents flow of information from high to low Biba prevents flow if information from low to high Information flow policies are the basis for many security proofs. Typical proofs show non-interference: view of one subject is not affected by the data of the other low output does not depend on high input, or high output does not depend on low input How to use BLP and Biba in the same system? System high input high output low input low output

High water mark, low water mark How to classify an object that is created combining low and high information?  High water mark policy for secrecy: always set the classification to the highest input  Low water mark policy for integrity: always set the classification if to the lowest input Problem: Over time, all documents will become top secret with the lowest integrity level

Upgrading and downgrading Upgrading, downgrading: In practice, security levels need to be changed by humans E.g. downgrading documents for publication E.g. upgrading intelligence reports that aggregate a lot of low-level data Documents may need to be sanitized i.e. redacted before downgrading E.g. removing personal names from military documents before publication Sanitization may be difficult E.g. US military painting black box over text in PDF; AOL publishing anonymized web search data High subjects can use covert channels to leak data intentionally, e.g. hide data in photos

Other access control models

Clark-Wilson model Data integrity cannot always be expressed in terms of MLS, i.e. who has access to what data E.g. transfers between bank accounts must not change the total balance Integrity in many commercial systems depends on following the correct procedures Clark-Wilson model defines rules for commercial systems for how to maintain data integrity: Transactions must transform data items from a consistent state to another consistent state Auditing and procedural controls to enforce this (The specific rules could be different in each system) Clark-Wilson model has not really been implemented; it is important because of the idea of using accounting rules as a model for security policy

Chinese Wall model Conflicts of interest are common in business: Consulting company, investment bank, or law office may be advising competing clients and must keep their information separate The clients are assigned to different employees who do not speak to each other To avoid conflicts of interest, the access control policy must take into account the information previously accessed by the subject Chinese Wall model: If subject S has previously accessed an object O1 and the objects O1 and O2 are in a conflict of interest, then S may not access O2 Idea: subject can fall to either side of the wall but cannot change sides later

Separation of duty Chinese Wall is an example of separation of duty Other separation of duty policies: Expense claim requires two signatures: the claimant and an authorized approver, e.g. department manager; one person cannot act in both roles for the same expense claim Auditors are often required to be from outside the company Some safes have two locks, and the keys are given to two different persons Lecturers issue grades to students but only study office staff can enter them into the study register Unlike BLP and Biba, separation of duty policies are stateful

Role-based accesss control

Groups and roles Adding structure to policies Group = set of subjects E.g. Administrators, T-110.4206-students Object ACL can list groups in addition to individual users Both group membership and ACLs change over time Role = set of permissions (i.e. permitted actions on objects) E.g. Administrator, T-110.4206-teacher, SCI-professor Roles are usually relatively static; their assignment to users changes Both are forms of indirection Objects × Actions Subjects Roles or Groups *

Role-based access control (RBAC) NIST standard Modeling high-level roles in an organization E.g. Doctor, Nurse, Student, Lecturer, Course-assistant Roles defined once; changed infrequently Roles may be parameterized E.g. Treating-doctor of Mr. Smith, Lecturer of T-110.4206, Student of T-110.4206 Roles may form a hierarchy with inheritance E.g. Lecturer and Teaching-assistant are Teaching-staff Roles are assigned to users for longer term but activated on demand for each session Constraints on role assignment and activation can implement separation of duty

Example: University of Turku has implemented identity management based on RBAC Source: http://www.come.uw.edu.pl/eunis/pandp/paper/kmiika_RBAC-In-Prodution.doc (link broken)

Still other access control models Originator-controlled AC (ORCON) Creator of data retains control over access to it Attribute-based AC Access control is based in subject attributes instead of subject identity AC = attribute verification + authorization E.g. need to be 18 to buy tobacco; need to be an Aalto student to access course material Enables anonymous access Double-blinded review for scientific journals Many other AC models have been proposed

Reading material Dieter Gollmann: Computer Security, 2nd ed., chapters 4, 8, 9; 3rd ed. chapters 5–6 Edward Amoroso: Fundamentals of Computer Security Technology, chapters 6-13 Ross Anderson: Security Engineering, 2nd ed., chapter 8

Exercises What are the subjects, object and actions in Noppa? Can you think of security mechanisms outside computers which would need MAC but actually implement DAC? What security labels and MAC policy would be suitable for Noppa? Give examples of systems that require confidentiality or integrity but not both. Which AC model and what kind of security labels could be used to describe virtual machine isolation? What label would be hypervisor or VM monitor get? Could you define different confidentiality labels and integrity labels and then use both Bell-LaPadula and Biba policies in the same system? Give an example. Define RBAC roles that could be used in the implementations of Noppa. To what extent can your RBAC policy (above) be implemented with groups?