Intrusion Detection System

Resides on computer or appliance connected to segment of an organization’s network; looks for signs of attacks

When examining packets, a NIDS looks for attack patterns

Installed at specific place in the network where it can watch traffic going into and out of particular network segment

NIDS Signature Matching

To detect an attack, NIDSs look for attack patterns

Done by using special implementation of TCP/IP stack:

In process of protocol stack verification, NIDSs look for invalid data packets

In application protocol verification, higher-order protocols are examined for unexpected packet behavior or improper use

Advantages and Disadvantages of NIDSs

Good network design and placement of NIDS can enable organization to use a few devices to monitor large network

NIDSs are usually passive and can be deployed into existing networks with little disruption to normal network operations

NIDSs not usually susceptible to direct attack and may not be detectable by attackers

Can become overwhelmed by network volume and fail to recognize attacks

Require access to all traffic to be monitored

Cannot analyze encrypted packets

Cannot reliably ascertain if attack was successful or not

Some forms of attack are not easily discerned by NIDSs, specifically those involving fragmented packets

4 Host-Based IDS

 Host-based IDS (HIDS) resides on a particular computer or server and monitors activity only on that system

Benchmark and monitor the status of key system files and detect when intruder creates, modifies, or deletes files

Most HIDSs work on the principle of configuration or change management

Advantage over NIDS: can usually be installed so that it can access information encrypted when traveling over network

          Advantages and Disadvantages of HIDSs

          Can detect local events on host systems and detect attacks that may elude a network-based IDS

Functions on host system, where encrypted traffic will have been decrypted and is available for processing

Not affected by use of switched network protocols

Can detect inconsistencies in how applications and systems programs were used by examining records stored in audit logs

Pose more management issues

Vulnerable both to direct attacks and attacks against host operating system

Does not detect multi-host scanning, nor scanning of non-host network devices

Susceptible to some denial-of-service attacks

Can use large amounts of disk space

Can inflict a performance overhead on its host systems

5 Application-Based IDS

Application-based IDS (AppIDS) examines application for abnormal events

AppIDS may be configured to intercept requests:

File System

Network

Configuration

Execution Space

Advantages and Disadvantages of AppIDSs

Advantages

Aware of specific users; can observe interaction between application and user

Able to operate even when incoming data is encrypted

Disadvantages

More susceptible to attack

Less capable of detecting software tampering

May be taken in by forms of spoofing

Selecting IDS Approaches and Products

ü Technical and policy considerations

What is your systems environment?

What are your security goals and objectives?

What is your existing security policy?

          Organizational requirements and constraints

What are requirements that are levied from outside the organization?

What are your organization’s resource constraints?

IDS Control Strategies

          An IDS can be implemented via one of three basic control strategies

Centralized: all IDS control functions are implemented and managed in a central location

Fully distributed: all control functions are applied at the physical location of each IDS component

Partially distributed: combines the two; while individual agents can still analyze and respond to local threats, they report to a hierarchical central facility to enable organization to detect widespread attacks

IDS Deployment Overview

Like decision regarding control strategies, decisions about where to locate elements of intrusion detection systems can be art in itself

Planners must select deployment strategy based on careful analysis of organization’s information security requirements but, at the same time, causes minimal impact

NIDS and HIDS can be used in tandem to cover both individual systems that connect to an organization’s networks and networks themselves

Deploying Network-Based IDSs

NIST recommends four locations for NIDS sensors

Location 1: behind each external firewall, in the network DMZ

Location 2: outside an external firewall

Location 3: On major network backbones

Location 4: On critical subnets

Deploying Host-Based IDSs

Proper implementation of HIDSs can be painstaking and time-consuming task

Deployment begins with implementing most critical systems first

Installation continues until either all systems are installed, or the organization reaches planned degree of coverage it is willing to live with

Measuring the Effectiveness of IDSs

IDSs are evaluated using two dominant metrics:

Administrators evaluate the number of attacks detected in a known collection of probes

Administrators examine the level of use at which IDSs fail

          Evaluation of IDS might read: at 100 Mb/s, IDS was able to detect 97% of directed attacks

          Since developing this collection can be tedious, most IDS vendors provide testing mechanisms that verify systems are performing as expected

          Some of these testing processes will enable the administrator to:

Record and retransmit packets from real virus or worm scan

Record and retransmit packets from a real virus or worm scan with incomplete TCP/IP session connections (missing SYN packets)

Conduct a real virus or worm scan against an invulnerable system

Honey Pots, Honey Nets, and Padded Cell Systems

          Honey pots: decoy systems designed to lure potential attackers away from critical systems and encourage attacks against the themselves

          Honey nets: collection of honey pots connecting several honey pot systems on a subnet

          Honey pots designed to:

Divert attacker from accessing critical systems

Collect information about attacker’s activity

Encourage attacker to stay on system long enough for administrators to document event and, perhaps, respond

Padded cell: honey pot that has been protected so it cannot be easily compromised

In addition to attracting attackers with tempting data, a padded cell operates in tandem with a traditional IDS

When the IDS detects attackers, it seamlessly transfers them to a special simulated environment where they can cause no harm—the nature of this host environment is what gives approach the name padded cell

          Advantages

Attackers can be diverted to targets they cannot damage

Administrators have time to decide how to respond to attacker

Attackers’ actions can be easily and more extensively monitored, and records can be used to refine threat models and improve system protections

Honey pots may be effective at catching insiders who are snooping around a network

Disadvantages

Legal implications of using such devices are not well defined

Honey pots and padded cells have not yet been shown to be generally useful security technologies

Expert attacker, once diverted into a decoy system, may become angry and launch a more hostile attack against an organization’s systems

Administrators and security managers will need a high level of expertise to use these systems

Trap and Trace Systems

Use combination of techniques to detect an intrusion and trace it back to its source

Trap usually consists of honey pot or padded cell and alarm

Legal drawbacks to trap and trace

Enticement: process of attracting attention to system by placing tantalizing bits of information in key locations

Entrapment: action of luring an individual into committing a crime to get a conviction.

Enticement is legal and ethical, whereas entrapment is not