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Quantitative Evaluation for Operational Security - an Experiment

Quantitative Evaluation for Operational Security - an Experiment. [Ortalo et al., IEEE Transactions on Software Engineering, Sept/Oct 1999] Group Meeting, Mar 7, 2000. Outline. Introduction The Approach: Privilege graphs Attack state graphs Mathematical model The experiment

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Quantitative Evaluation for Operational Security - an Experiment

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  1. Quantitative Evaluation for Operational Security -an Experiment [Ortalo et al., IEEE Transactions on Software Engineering, Sept/Oct 1999] Group Meeting, Mar 7, 2000 H.W. Chan, CSE Dept., CUHK

  2. Outline • Introduction • The Approach: • Privilege graphs • Attack state graphs • Mathematical model • The experiment • setup and results • Discussion H.W. Chan, CSE Dept., CUHK

  3. Introduction • System security has been usually discussed in terms of security requirements and policy • requires cooperation of all users • difficult for ordinary users to comprehend • A quantitative measure for system security is easier to comprehend • a figure representing the ‘degree of security’ of the system can be useful H.W. Chan, CSE Dept., CUHK

  4. Quantifying security • Borrowing software reliability theory: • In reliability, a piece of software fails upon time of usage; the Mean Time To Failure quantify the reliability of the software • Similar, in security, a system can be breached upon effort of attacks; the Mean Effort to Breach can quantify the security of the system H.W. Chan, CSE Dept., CUHK

  5. The Approach • Privilege graph: • node: a set of privileges owned by a user or set of users (e.g., a group in Unix) • arc: a vulnerability that cause a user owning one privilege to obtain another, e.g., Y X There is a method allowing a user owning privilege X to obtain privilege Y. H.W. Chan, CSE Dept., CUHK

  6. Examples of vulnerabilities • Privilege subsets directly issued from the protection scheme • Direct security flaws, e.g., Trojan horse • System features exploited for attack • .rhosts, .xinitrc, setuid programs hwchan1 gds H.W. Chan, CSE Dept., CUHK

  7. Privilege graph - example A 6 3 P B Xadmin Key 1: Y’s .rhosts is writable by X 2: X can guess Y’s password 3: X can modify Y’s .tcshrc 4: X is a member of Y 5: Y uses a program managed by X 6: X can modify a setuid program owned by Y 7: X is in Y’s .rhosts 7 5 1 4 insider F 2 H.W. Chan, CSE Dept., CUHK

  8. Quantifying vulnerabilities • Each arc in the privilege graph should be assigned a weight to quantify the effort required for exploiting the vulnerability • Different factors should be considered, e.g., expertise, time and equipment • No good methods to do this yet! H.W. Chan, CSE Dept., CUHK

  9. Attacker behavior • In an attack, an attacker begins with some minimal privileges, and wants to obtain some protected privileges. • In a privilege graph, the path from the attacker node to the target node describes the progress of attack: target attacker H.W. Chan, CSE Dept., CUHK

  10. There can be more than one paths from the attacker node to the target node • assumption: attacker does not know the shortest path • Two assumptions for attacker behavior • Total memory (TM): all possibilities of attack are considered at any stage of attack • Memoryless (ML): at each newly visited node, only attacks possible from that node are considered H.W. Chan, CSE Dept., CUHK

  11. Attack state graphs (ML) I FI ABFIPX IP FIX BFIPX AIP BFIX AFIX H.W. Chan, CSE Dept., CUHK

  12. Attack state graph (TM) I FI ABFIPX IP FIX FIP BFIPX AIP BFIX AFIX AFIP H.W. Chan, CSE Dept., CUHK

  13. Mathematical Model • Assume the Markov model: • Probability of success in an attack before an amount of effort ‘e’ is spent is: P(e) = 1 - exp(-Le) • L is the rate of attack, and can be assigned as the weight of the vulnerability • thus, mean effort to succeed is 1/L H.W. Chan, CSE Dept., CUHK

  14. mean effort spent in state j is Ej = 1/summation(Lji), for all i belongs to out(j) • Mean Effort To security Failure (METF) from initial state k to state i is METFk = Ek + summation(Lki*Ek*METFi), for all i belongs to out(k) H.W. Chan, CSE Dept., CUHK

  15. The experiment • Setup: • Several hundred different workstations • 700 users sharing one global file system • privilege graphs, attacker state graph and METF computed every day from June 95 to Mar 97 (674 days) • vulnerabilities are classified into four levels and given rates 10^-1, 10^-2, 10^-3, 10^-4 H.W. Chan, CSE Dept., CUHK

  16. Results H.W. Chan, CSE Dept., CUHK

  17. Conclusion and discussion • A preliminary investigation about the security evaluation of operational systems • The assignment of rates of the vulnerabilities is pretty arbitrary, but is key to the validity of the measurement H.W. Chan, CSE Dept., CUHK

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