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An Introduction to Intrusion Detection/Prevention, Vulnerability Assessment and related Technologies

Network Security

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Contents

• Lecture aims and learning outcomes• Assumptions• Motivation - Why Intrusion Detection and Vulnerability

Assessment• Attack Development• Vulnerability Development• Hacker Strategy

• Detection - Intrusion Detection Systems• Host based IDS• Network Based IDS

• Prevention - Vulnerability Assessment• Software• Services (Audits)• Web-Based Services

• Counter attacks• Honey Pots• Appliances

• Summary

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Lecture aims and learning outcomes

• The lecture aims are:• To describe the problems related to network based attacks• To describe how some these problems may be addressed

• At the end of this lecture you will be able to:• Demonstrate an understanding of the main issues relating

to threats in the context of network attacks• Understand a number of basic design components for

building a network security architecture• Demonstrate an understanding of the importance of a

security policy with reference to the security of a computer network

• Describe the features and security mechanisms which are generally used to implement security policies for dealing with the security of a computer network

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Assumption

• Perimeter security devices (e.g. firewalls) and computer security mechanisms (e.g. application and OS security) can only offer best effort at preventing attacks.

• They may fail to do so: • a firewall may be misconfigured, • a password may be sniffed off the network, • a new attack type may emerge. (cf. Zero-day attacks)

• They do not detect when an attack is underway or has taken place.

• And they do not react to attacks.

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Traditional Methods

• Example:• Imagine continuous inspection of a Unix system by hand

(similar examples for NT, W2K):• The following simplified checklist is taken from CERT

(http://www.cert.org/tech_tips/intruder_detection_checklist.html):

1. Examine log files for connections from unusual locations or other unusual activity. For example, look at your 'last' log, process accounting, all logs created by syslog, and other security logs.

2. Look for setuid and setgid files (especially setuid root files) everywhere on your system. Intruders often leave setuid copies of /bin/sh or /bin/time around to allow them root access at a later time.

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Ad Hoc Intrusion Detection

• Imagine the complexity and degree of expertise needed to carry out the tasks in this checklist for every host and every sensitive network link on a network every single day.

• The ad hoc approach is not recommended!• Automated systems are needed:

• monitor multiple hosts and network links for suspicious behaviour;

• report this behaviour, possibly react to it.

• Hence: Intrusion Detection Systems (IDS).

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Vulnerability Development

0

100

200

300

400

500

600

700

1999 2000 2001 2002 2003

Linux (aggr.)

SolarisWindows

Total

Source: SecurityFocus

Motivation

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Intruder Knowledge

High

Low

1980 1985 1990 1995 2000

Attack Sophistication

AttackSophistication

Cross site scripting

password guessing

self-replicating code

password cracking

exploiting known vulnerabilities

disabling audits

back doors

hijacking sessions

sweepers

sniffers

packet spoofing

GUIautomated probes/scans

denial of service

www attacks

“stealth” / advanced scanning techniques

burglaries

network mgmt. diagnostics

distributedattack tools

Staged

Auto Coordinated

Source: Carnegie Mellon University

Motivation

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Advisory Release

Widespread Awareness

Vulnerability Scannersadding detection signature

Selective AwarenessFirst

Discovery

Vulnerability & Exploit Lifecycle

Motivation

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Unauthorized Use of Computer Systems within the Last 12 Months

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Origin of the Attack

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Which Type of Attacks ?

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Dollar Amount of Losses by Type

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Reactions to attacks

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Primary Target Identification - Identify Hosts ( ) with external visibility

denotes internal hosts with high value data but no external view

CORP

NETWORK

PING

SWEEPInternet

A Typical Hacker Strategy

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Primary Target Analysis - Identify services running on visible hoststo prioritize further probing activities

PORT

SWEEP

CORP

NETWORK

DNS

WEB

NFS

A Typical Hacker Strategy

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Primary Target Selection - Determine vulnerability state of weakest pointand concentrate further activities against this system

FINGER

NFS CORP

NETWORK

A Typical Hacker Strategy

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Primary Target Exploitation - Gain privileges & control of primary target- attacker now controls a ‘trusted’ corporate system !

Rlogin Root

NFS CORP

NETWORK

A Typical Hacker Strategy

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Secondary Target Identification - Probing for high value information or systems which are then compromised and data stolen or trojan horses planted, etc.

NFS CORP

NETWORK

HR

R&D

$

A Typical Hacker Strategy

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Animated Demo

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Detection

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Intrusion Detection Systems

• Popular second layer of network security enforcement• Passive supervision of exiting network, analogues to intruder

alarms• Creates more work for personal

• There exist 2 different approaches to the implementation of Intrusion Detection Systems (IDS)• Knowledge-based IDS

• Network based• Host based

• Behaviour-based IDS• Statistical anomaly detection

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Intrusion Detection Systems

• An Intrusion Detection System (IDS) is a network security system designed to identify intrusive or malicious behaviour via monitoring of network activity. The IDS identifies suspicious patterns that may indicate an attempt to attack, break in to, or otherwise compromise a system. An IDS can be network-based or host-based, passive or reactive, and can rely on either misuse detection or anomaly detection.

IDS vs Firewalls.   Firewalls specify policies about what traffic may or may not enter a particular computer network. An IDS monitors patterns of traffic and signals an alert once it deems that an attack has taken place.

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Knowledge-based IDS

• ALL commercial IDS look for attack signatures:• specific patterns of network traffic or activity in log files that

indicate suspicious behaviour.

• Called a knowledge-based or misuse detection IDS• Example signatures might include:

• a number of recent failed login attempts on a sensitive host;• a certain pattern of bits in an IP packet, indicating a buffer

overflow attack;• certain types of TCP SYN packets, indicating a SYN flood

DoS attack.

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Knowledge-based IDS

• Knowledge-based IDS uses information such as:• Security policy;• Known vulnerabilities of particular OS and applications;• Known attacks on systems.

• They are only as good as the information in the database of attack signatures:• new vulnerabilities not in the database are constantly being

discovered and exploited;• vendors need to keep up to date with latest attacks and issue

database updates; customers need to install these;• large number of vulnerabilities and different exploitation methods,

so effective database difficult to build;• large database makes IDS slow to use.

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Behaviour-based IDS

• Statistical Anomaly Detection (or behaviour-based detection) is a methodology where statistical techniques are used to detect penetrations and attacks.

• Begin by establishing base-line statistical behaviour: what is normal for this system?

• Then gather new statistical data and measure the deviation from the base-line.

• If a threshold is exceeded, issue an alarm.

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Behaviour-based IDS

• Example: monitor the number of failed login attempts at a sensitive host over a period; • if a burst of failures occurs, an attack may be under way; • or maybe the admin just forgot his password?

• This raises the issue of false positives (an attack is flagged when one was not taking place – a false alarm) and false negatives (an attack was missed because it fell within the bounds of normal behaviour).

• This issue does also apply to knowledge-based systems.

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Behaviour-based IDS

• IDS does not need to know about security vulnerabilities in a particular system • the base-line defines normality;• don’t need to know the details of the construction of a buffer

overflow packet.

• Normal behaviour may overlap with forbidden behaviour.• Legitimate users may deviate from the baseline, causing false

positives (e.g. user goes on holiday, or works late in the office, or forgets password, or starts to use new application).

• If the base-line is adjusted dynamically and automatically, a patient attacker may be able to gradually shift the base-line over time so that his attack does not generate an alarm.

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Host-based and Network-based IDS

• When an IDS looks for attack signatures in network traffic, it is called a network-based IDS (NIDS).

• When an IDS looks for attack signatures in log files of hosts, it is called a host-based IDS (HIDS).

• Naturally, the most effective Intrusion Detection System will make use of both kinds of information.

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IDS Architecture

• Distributed set of sensors – either located on hosts or on network – to gather data.

• Centralised console to manage sensor network, analyze data, report and react.

• Ideally:• Protected communications between sensors and console;• Protected storage for signature database/logs;• Secure console configuration;• Secured signature updates from vendor;• Otherwise, the IDS itself can be attacked and manipulated.

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Placement of Network-based IDS

InternetInternet

FirewallMail server

Web server

Protected Network

Sensor

Sensor

Sensor

Console

Perimeter Network

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Animated Demo

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Host-based IDS

• Typically monitors system, event, and security logs on Windows and syslog in Unix environments.

• Checks key system files and executables via checksums at regular intervals for unexpected changes.

• Some products can use regular-expressions to refine attack signatures (e.g. passwd program executed AND .rhosts file changed).

• Some products listen to port activity and alert when specific ports are accessed – limited NIDS capability.

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Placement of Host-based IDS

InternetInternet

FirewallMail server

Web server

Sensor

Console

Perimeter Network

Sensor

Sensor

Human Resources Network

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IDS as a Response Tool

• Given the (near) real-time nature of IDS alerts, an IDS can be used as a response tool as well as for detection.

• NIDS and HIDS have different response capabilities – because they detect different attacks, or the same attacks but in different ways.

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HIDS and NIDS

• There are attack types that a HIDS can detect but a NIDS cannot:• SYN flood, Land, Smurf and Teardrop attacks, BackOrifice,…

• And vice-versa:• Trojan login script, walk up to unattended keyboard attack,

encrypted traffic,…

• For more reliable detection, combine both types of IDS.

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IDS Response Options

Network-based Host-based

Notification Alarm to console Alarm to console

E-Mail notification E-Mail notification

SNMP trap SNMP trap

View active session

Storage Log summary Log summary

Log raw network data

Active Kill connection (TCP Reset)

Terminate user login

Re-configure firewall Disable user account

Restore index.html

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IDS Response Options

• Dangers of automated response:• Attacker tricks IDS to respond, but response aimed at

innocent target (say, by spoofing source IP address);• Users locked out of their accounts because of false

positives;• Repeated e-mail notification becomes a denial of service

attack on sysadmin’s e-mail account;• Repeated restoration of index.html from CD reduces

website availability.

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What is Snort?

• Snort is a fast, flexible, small-footprint, open-source NIDS developed by the security community and a “benevolent dictator”

• Lead coder: Marty Roesch, now founder of Sourcefire (http://www.sourcefire.com)

• Initially developed in late 1998 as a sniffer with consistent output, unlike protocol-dependent output of TCPDump

• Licensed under GPL, but version 2.0 may change to a different license

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Snort Rules

• Snort rules are extremely flexible and are easy to modify, unlike many commercial NIDS

• Sample rule to detect SubSeven trojan:

alert tcp $EXTERNAL_NET 27374 -> $HOME_NET any (msg:"BACKDOOR subseven 22"; flags: A+; content: "|0d0a5b52504c5d3030320d0a|"; reference:arachnids,485; reference:url,www.hackfix.org/subseven/; sid:103; classtype:misc-activity; rev:4;)

• Elements before parentheses comprise ‘rule header’• Elements in parentheses are ‘rule options’

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Third-Party Enhancements

• Analysis Console for Intrusion Databases (ACID)• http://acidlab.sourceforge.net/• PHP-based analysis engine to search and process a

database of security events generated by various IDSes, firewalls, and network monitoring tools

• Query-builder and search interface, packet viewer (decoder), alert management, chart and statistics generation

• Description and screenshots taken from ACID web

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Third-Party Enhancements

• Demarc• www.demarc.com • NIDS management console, integrating Snort with the

convenience and power of a centralized interface for all network sensors

• Monitor all servers / hosts to make sure network services such as a mail or web servers remain accessible at all times

• Monitor system logs for anomalous log entries that may indicate intruders or system malfunctions

• Description and screenshots taken from demarc web

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IDS – The Future

• Integrated approach to IDS: • Network and host-based in one system (some products

already do this in a limited way);• The strengths of both NIDS and HIDS (but maybe all of the

weaknesses!)

• Better visualisation, management and reporting tools• Event correlation:

• Correlate a number of sub-events which individually do not indicate an attack but which when viewed in combination do;

• Requires much more sophisticated software and data processing.

• Potentially much better attack detection.

• Commercial Statistical Anomaly Detection

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Prevention

Vulnerability AssessmentIntrusion Prevention Systems

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Vulnerability Assessment

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Vulnerability Assessment

• An examination of the ability of a system or application, including current security procedures and controls, to withstand assault.

• A vulnerability assessment may be used to: • identify weaknesses that could be exploited; • predict the effectiveness of additional security measures in

protecting information resources from attack.

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Vulnerability Assessment

• Vulnerability Assessment Methods• Software solutions (ISS Scanner, Stat, Nessus etc.)• Audit Services (manual Penetration tests etc)• Web based commercial (Qualys, Security Point etc.)

• Use a database of vulnerability signatures• Usually perform a port scan to detect which services

available• Try to identify if service is vulnerable by:

• Looking for banner information• Sending a harmless request and analysing the response• Actually performing the attack!

• Offer various reporting and management facilities

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Animated Demo

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Lesson learnt from VA

• Keep up-to-date with security (and other) patches• Form Microsoft OS www.windowsupdate.com

• Enterprise version available – Windows Update Services (WUS)

• Microsoft Baseline Security Advisor• Includes hfnetcheck.exe (from Shavlik)

• Similar for SUN, HP, IBM, CISCO etc. OSs

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Intrusion Prevention Systems

Rate basedContent based

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Intrusion Prevention System - IPS

• Relatively new (marketing) term • Essentially a combination of access control

(firewall/router) and intrusion detection systems• Often shared technologies between stateful inspection and

signature recognition (“looking deep into the packet”)• Inline network IDS allows for instant access control policy

modification

• 2004 Gartner study claims by 2005 only integrated firewalls with IDS (i.e. IPS) will survive

• Most success to-date with “flood” (DoS) attacks

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Definition of an IPS

• Can be defined as an in-line product that focuses on identifying and blocking malicious network activity in real time.

• Two general categories: • rate-based products• content-based (also referred to as signature- and anomaly-

based)

• Often look like firewalls and often have some basic firewall functionality.

• But firewalls block all traffic except that which they have a reason to pass;

• IPSs pass all traffic except that which they have a reason to block.

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Rate-based IPS

• Block traffic based on load: • too many packets, • too many connects, • too many errors.

• In the presence of too much of anything, the rate-based IPS kicks in and blocks, throttles or otherwise mediates the traffic.

• Most useful rate-based IPS include a combination of powerful configuration options with range of response technologies • For example, limit queries to your DNS server to 1,000 per

second• Other simple rules covering bandwidth and connection

limiting

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Disadvantages of Rate-based IPS

• Biggest problem with deploying rate-based IPS products is deciding what constitutes an overload.

• For any rate-based IPS to work properly, need to know not only what "normal" traffic levels are (on a host-by-host and port-by-port basis) but also other network details such as how many connections your Web servers can handle.

• Most products do not provide any help but require a “trained” system engineer

• Because rate-based IPSs require frequent tuning and adjustment, they will be most useful in very high-volume Web, application and mail server environments.

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Content-based products

• Block traffic based on attack signatures and protocol anomalies • Worms, e.g. Blaster and MyDoom, that match a signature can

be blocked. • Packets that do not comply to TCP/IP RFCs can be dropped. • Suspicious behaviour such as port scanning triggers the IPS to

block future traffic from a single host • The best content-based IPSs offer a range of techniques for

identifying malicious content and many options for how to handle the attacks, • simply dropping bad packets to • dropping future packets from the same attacker, and • reporting and alerting strategies.

• IDS-like technology for identifying threats and blocking them, content-based IPSs can be used deep inside the network to complement firewalls and provide security policy enforcement.

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Counter attacks

The Problem of originHoneypots/nets

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Problem of origin

• Denial of Service attacks (DoS)In contrast to unauthorised access attacks a DoS attack does not need to contain method for communicating back to the attacker

• Distributed Denial of Service (DDoS) attacks• Trin00/Stacheldraht (Feb 2000)

• Attacks on ebay, amazon.com and etrade.com

• MS.Blaster (August 2003)

• Problem of lack of metrics to measure the impact of Denial of Service attacks – more research required

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What is a DDoS Attack ?• In a Denial of Service (DoS) attack,

• The attacker overwhelms a targeted system with a flood of packets to deny availability of services to legitimate users

• In a Distributed Denial of Service (DDoS) attack,• The attacker uses dozens or even hundreds of ‘zombie’

machines to multiply the force of the attack

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Motives Behind DDoS Attacks

• Until recently attacks appear to be motivated by:• Desire for attention • Notoriety • Fun

• Long term, DDoS type attacks could become motivated by:• Economic warfare between competition• Disgruntled employees/customers • Monetary gains (i.e. stock market manipulation/online betting)• Political sabotage and vandalism (party websites during election

campaigns)

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DDoS Components

• All DDoS attacks consist of three parts:• Client Program• Master Server• Agent (Zombie) Program

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DDoS Attack Illustrated

ScanningProgram

Unsecured Computers

Hacker Hacker scans Internet for unsecured systems that can be compromised

1

Internet

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Hacker

Zombies

Hacker secretly installs zombie agent programs, turning unsecured computers into zombies

2

Internet

DDoS Attack Illustrated

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Hacker

Hacker selects a Master Server to send commands to the zombies

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ZombiesMasterServer

Internet

DDoS Attack Illustrated

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Hacker

Using Client program, Hacker sends commands to Master Server to launch zombie attack against a targeted system

4

Zombies

TargetedSystem

MasterServer

Internet

DDoS Attack Illustrated

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Targeted SystemSystem

Hacker

Master Server sends signal to zombies to launch attack on targeted system

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MasterServer

Internet

Zombies

DDoS Attack Illustrated

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TargetedSystem

Hacker

Targeted system is overwhelmed by bogus requests that shut it down for legitimate users

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MasterServer

User

Request Denied

Internet

Zombies

DDoS Attack Illustrated

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Minimizing Risk

• Prevent yourself from being victimized• Ensure your computers are not zombies• Perform periodic assessments via automated scanning

services

• Implement an early warning system• Automated Intrusion Detection & Response tools• Collect forensic data to prosecute hackers later

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Honeypots

• Technology used to track, learn and gather evidence of hacker activities

• Definition• “… a resource whose value is being attacked or compromised”

Laurence Spitzner, “The value of honeypots”, SecurityFocus, October 2001

• Strategically placed systems designed to mimic production systems, but not reveal “real” data

• Modes of operation• Baiting• Waiting• Collating• Disseminating

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Honeypot types of implementation

• Level of Involvement• Low Involvement: Port Listeners• Mid Involvement: Fake Daemons• High Involvement: Real Services

• Risk increases with level of involvement

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Honeynet

• Network of honeypots• Supplemented by firewalls and intrusion detection

systems - Honeywall

• Advantages:• “More realistic” environment• Improved possibilities to collect data

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Honeynet

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Sebek

• Sebek is a data capture tool designed to capture all of the attackers activities on a honeypot, without the attacker knowing it.

• 2 components. • Client that runs on the honeypots, its purpose is to capture

all of the attackers activities (keystrokes, file uploads, passwords) then covertly send the data to the server.

• Server which collects the data from the honeypots. The server normally runs on the Honeywall gateway.

• Since the Sebek client runs as a kernel module on the honeypots, it can capture all activity, including encrypted, such as SSH, IPSec

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Honeynet using a Honeywall

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Summary

• Threats are both internal and external.• Prevention, detection and reaction are needed in

combination. • Intrusion detection systems are a very useful second

line of defence (in addition to firewalls and other safeguards).

• IDS deployment, customisation and management is generally not straightforward.

• Vulnerability Assessment and Patch Management are King.

• Newer technologies such as IPS and Honeynets can remove the burden from over worked system and network administrators.

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IDS Further Reading

• Stallings Chapter 9, pp.292-303 (possibly too much emphasis on statistical approach; research-focussed rather than commercially focussed).

• An article: “The future of IDS” by Matthew Tanase at SecurityFocus.com:• http://online.securityfocus.com/infocus/1518

• An evaluation of IDS products by Kathleen A. Jackson:• http://www.sekure.net/ids/00416750.pdf

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Questions

Thank You !

Merry Christmas&Happy New Year