Monitoring Systems

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IDS - Intrusion Detection Systems

Motivation

  • Protective measures like firewalle reduce the attack risk, but cannot totally eliminate it
    • e.g. bogus packer headers can lead to an attack not being detected
  • are principally "burglar alarms"
  • are employed for automated attack detection -> by monitoring accordance with provided security guidelines / rules
  • possible statements produced by an analysis are e.g.:
    • accordance with security guidelines
    • violation of security guidelines
    • suspective behaviour
  • about attackers:
    • from the outside
    • from the inside: according to a study by the FBI, the larger part of network attacks originates from its inside (Insiders know more about a systems architecture, protective measures etc.)
    • types: vandalism / espionage / just for fun / find security flaws in cooperation with the host
  • maybe a better name would be "Signature Compare System" or "Signature Compare Automaton"


timing behaviour

  • Realtime IDS - reports suspected intrusions which are proceeding (e.g. by network packet analysis)
  • Standard IDS - reports sucessful attacks (e.g. by logfile analysis)


architectures

HIDS - host-based IDS

  • receive their audit files from a host
  • use e.g. log files
  • system manipulations can be detected using checksums
  • by these mechanisms, attacks can only be detected when they already have occured -> forensic use
  • examples:
    • sXid - monitors SUID/GUID files
    • Tripwire - monitors file integrity (employing a database and hash functions to detect changes)

logcheck - logfile analysis by provided strings which are classified as security relevant: denied / failed; security violations: login root refused


NIDS - network-based IDS

  • analyzes network data
  • NIDS come in different flavours:
    • sensor-based system: has a network interface to "`sniff"' on the traffic
    • agent-based system: agents are installed on the monitored systems to catch the packets (from the different of the TCP/IP stack) and send them to a centralised unit for analyis.
  • Position:
    • between "outside world" and firewall, but before the internal network -> interesting to get an overview, but not a good choice on the long run because too much data is collected (..every portscan is recorded)
    • behind firewall, inside the local network -> can be used as a controlling device, unusal activities from local hosts are also reported
    • between 2 firewalls -> does not have to react on every single portscan, can be used to monitor inbound as well as outbound traffic
    • multiple sensors -> are useful in larger networks, collected data can be sent to a central analysis unit


hybrid forms

  • combinations of HIDS and NIDS
  • by partial data analyis on each host the central analysis unit can be relieved of some load


analysis techniques

misuse detection

  • works similar to virus scanners
  • detection by known signatures
  • pattern matching -> incoming pattern is compared to known attack patterns from a database
  • signs of an attack:
    • malformed network packets (big / small / characteristic sub-strings in packet data)
    • unusual protocols
    • access to certain ports
  • Disadvantage: only known attack methods can be detected. An up-to-date attack database is essential!


State Transition

  • another misuse detection technique
  • every state represents a state of the system / each transition is an action
  • state-transition diagram for every attack pattern
  • when system activity occurs, the analysis unit can change the state of the state automaton
  • (+) probability and proceeding of an attack can be modelled / some prediction can be made on the attackers next actions
  • (+) an attack is modelled on a very abstract basis / attack variants can be modelled with small additional automatons
  • (+) coordinated and slowed attacks can be detected!
  • next stage of development: colored Petri nets


Anomaly Detection

  • is not based on a static dataset
  • assumption: every behaviour that is not normal is an attack / suspective behaviour
  • the system has to "learn" for severeal weeks what normal behaviour means
  • this must be done while the systems integrity is certainz
  • an IDS may register e.g.
    • protocols
    • ports
    • attributes of system critical files (rights / users / size / modification date etc.)
  • when the learing phase is completed, learned behaviour is considered normal, everything else as anomalous.


Quantitative Analysis

  • Treshold Detection:
    • system and user activities are represented by counters:
    • exceeding a treshold is reported as possible attack
    • simple example: system log in, threshold of 3 -> 3 failed logins - user account is blocked
  • extension: heuristical treshold analyis
    • treshold is not fixed, but adjusted dynamically
    • in our example: the treshold is computed from the users past login activities
  • integrity checking
    • changes to system objects that may not be changed (data, programs, hardware) are monitored


Statistical Analysis

  • IDS keeps statistical profiles on each users normal behaviour
  • security alert is defined as strong deviation from this normal behaviour
  • (+) attackers using a "hijacked" account can be detected
  • (+) previously unknown patterns and methods of attack can be recognized
  • (-) bad real-time behaviour: all actions must have already happend to see if they are normal
  • (-) may pose too many restrictions on users
  • (-) may simplify "social monitoring" ("`..user X sends emails after lunch break"')


rule-based systems

  • operation similar to statistical systems (rules are defined instead of statistics)
  • rules may be user-defined or derived from data analyis
  • special subclass: Time-Based-Indictive IDS
    • monitors sequence of system operations
    • an anomaly is detected if order of events is not correct


neuronal networks

  • network is trained on "clean" data
  • learning ability makes neuronal networks wells suited for anomaly detection
  • (-) does not report type of attack
  • solution: one network for each type of attack (e.g. SYN-flooding)


Strict Anomaly Detection (Burglar Alarm / Passive Traps)

  • assumption: everything that is not explicitely permitted is an attack
  • employs pattern matching
  • normal system behaviour provided in a database
  • every deviant system behaviour is reported as an attack
  • (+) fewer patterns must be saved in the database
  • (+) updates to the database are only needed when the system is changed
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