USTAT A Real-time Intrusion Detection System for UNIX By: Koral Ilgun

1. USTAT A Real-time Intrusion Detection System for UNIX By: Koral Ilgun

2. Overview: • Introduction to USTAT -- • State Transition Analysis Tool for Unix • Key issues • System components • Implementation issues • Evaluation of USTAT

3. Introduction to USTAT • Misuse detector • A penetration is (viewed as) a sequence of signature actions and a corresponding sequence of state changes that lead the computer from some initial state to a target compromised state • Basic Properties of USTAT • Real-time expert system intrusion detection tool • Rule-based analysis • Only for known penetrations • Targets abusive insiders

4. Comparison between USTAT and other intrusion detection systems • Statistical anomaly detection • threshold detection • profile-based • Rule-based misuse detection • it is an expert system • Most current intrusion detection tools employ both the anomaly detection and (rule-based) misuse detection components

5. State Transition Analysis • View a penetration as • a (known) sequence of actions S1, S2, …,Sn • that lead from an initial (limited authorized) access state, S1 • to a final compromised state, Sn

6. State Transition Analysis (cont) • State really represents some attribute of the system – not the whole system state • State is generic, e.g. “user is now root” • Penetration sequence represented by finite state machine • node is a state • arc is an action (or transition)

7. Examples: • % ln target -x • % -x User creates link User executes file euid(user) = root euid(user) = not root File ‘target’ is root’s setuid shell script that contains the #!/bin/sh mechanism

8. Examples (cont): • 1. Attacker creates hard link starting with dash to root’s setuid shell script that contains the #!/bin/sh mechanism • 2. Attacker executes “-x” • Insight: Creating hard link ==> new directory entry is created with target’s original privileges and ownership information • Target can be accessed via any link to it • Executing shell script containing #!/bin/sh invokes a sub-shell • Sub-shell becomes interactive (because of the “-”) • Attacker is thus executing a setuid file owned by root, so shell has effective ID of root

9. Examples (cont): • Two actions/transitions: • make hardlink • execute “-x” • Three states: • Initial state: euid = user (not root) • Intermediate state: hardlink established • Final compromise state: not euid = user I.e. a user (non-root) running an interactive shell with an effective user id of root

10. Features of USTAT • Preempts attacks: • USTAT monitors state transitions • Note that they can span multiple sessions • It foresees impending compromise -- at least one transition away • Recognizes cooperative attacks • Note that USTAT is not tied to users or processes • It reflects state of the system • So, it can detect state resulting from actions by multiple users

11. USTAT input • Audit records of the form, <subject, action, object> • Subject is <real userID, effective userID, groupID> • Action is <action, time, processID> • Object is <object name, permissions, owner, group owner, inode #, device #, file systemID, target> • All information can be obtained directly from Unix audit records

12. USTAT input (cont) • Unix audits 239 event types • Only 28 are useful to USTAT • They are mapped down to 10 USTAT action types • Filters out all failed command events early, i.e. all the events with a return value of -1

13. USTAT simplifications (cont) • All (audited) actions mapped to a small set: • read write create execute • exit delete modify-owner rename • modify-permission hardlink • Files are categorized: E.g. all files that should not be accessed via regular utilities (because they hold sensitive data) are mapped to “Fileset1” • 5 “filesets”

14. USTAT monitoring • Monitors for all known penetrations simultaneously • Finite state machine for each known penetration • Maintains state tables • Row represents instance of not yet completed penetration • Column represents states in penetration scenario • Cell holds detailed info, e.g. userID, actual file names, etc

15. USTAT INITIAL state table • One row for each known penetration, I.e. each possible penetration is in its initial state • Initial action/transition for each is anticipated • Inference Engine accepts audit input • For each row, it asks: does this audit event match the “next transition” anticipated for this row such that the next state is satisfied • If so, duplicate the row and mark “details” for the now satisfied state

16. Decision Engine Informs sys admin about results of the inference engine Is compromise about to occur? Has compromise occurred? Play an active role in preempting the attack! However, note that USTAT input comes from the OS (Unix) audit log

17. Strength and weakness of USTAT • Strength: • Flexibility • Real-time -- preempts attack before system is damaged • Detects cooperative attacks • Weakness - Cannot cope with the following attacks: • Manipulation of components outside the system’s execution domain, e.g., wiretapping • Denial of service attacks • Failures

18. Evaluation of USTAT • Massive amount of data that was collected by the audit daemon limited extensive testing • Limiting factor is the transfer rate of the disk that is extensively used by USTAT and the audit daemon • USTAT functionality costs 13% of machine • Papers do not indicate number of penetrations that can be described as state transition diagram

19. NSTAT – USTAT for Distributed Systems • Similar “state-based” approach • Input: audit data from multiple hosts • Objective: detect coordinate attack • Central server processes input • Time – how to deal with skewed clocks? • Vulnerability of NSTAT server

20. Compare! • Tripwire -- Integrity of file data • GrIDs -- graph nodes (or accumulated groups of nodes) with arcs depicting message traffic • USTAT -- monitor for known penetrations, tracking state changes that progress toward compromise