Intruders Detection Systems• Presently there is much interest in systems, which can detect

intrusions, IDS (Intrusion Detection System).

• IDS are of very different character.

• Some focus on one machine and try to stop the intruder from doing damage, such is LIDS for Linux.

• Some can detect a worm attack from the way it spreads from machine to machine, like GrIDS.

• Several are actually data mining, they determine from logfiles if there is an intrusion based on reasoning by an expert system, NSTAT is an example.

• Many IDS implementations are listening passively to some LAN segment, look at the traffic and detect an intrusion. Snort IDS is a popular freeware program of this Network IDS-type.

• Other IDS solutions protect one machine by access controls.

What is Intrusion DetectionWhat is Intrusion Detection

Intrusion detection systems (IDSs) are Intrusion detection systems (IDSs) are designed for designed for

detecting, blocking and reporting unauthorizeddetecting, blocking and reporting unauthorized

activity inactivity in computer networks.computer networks.

““The life expectancy of a default installation of Linux Red The life expectancy of a default installation of Linux Red Hat 6.2 server is estimated to be less than 72 hours.”Hat 6.2 server is estimated to be less than 72 hours.”

““The fastest compromise happened in 15 minutes The fastest compromise happened in 15 minutes (including scanning, probing and attacking)”(including scanning, probing and attacking)”

““Netbios scans affecting Windows computers were Netbios scans affecting Windows computers were executed with the average of 17 per day”executed with the average of 17 per day”

(source: Honeynet Project)(source: Honeynet Project)

Unauthorized Use of Computer Systems Within Last 12 Months (source CSI/FBI Study)

0

10

20

30

40

50

60

70

80

Yes No Don't Know

Percentage of Respondents

1996

1997

1998

1999

2000

2001

2002

1.1. Motivation for Intrusion DetectionMotivation for Intrusion Detection

In year 2002 most common attacks were:

• Virus (78%)

• Insider Abuse of Net Access (78%)

• Laptop theft (55%)

• Denial of Service and System Penetration (40%)

• Unauthorized Access by Insiders (38%)

Most Common Attacks (source CSI/FBI)

1.1. Motivation for Intrusion DetectionMotivation for Intrusion Detection

((Red color shows the attack types, which IDS can decrease)Red color shows the attack types, which IDS can decrease)

Application IDS– Watch application logs– Watch user actions– Stop attacks targeted against an application

• Advantages– Encrypted data can be read

• Problems– Positioned too high in the attack chain (the attacks reach the

application)

Different Types of IDSsDifferent Types of IDSs

There are Application-, Host- and Network IDS

Host IDS– Watch kernel operations– Watch network interface– Stop illegal system operations– Drop attack packets at network driver

• Advantages– Encrypted data can be read– Each host contributes to the detection process

• Problems– Positioned too high in the attack chain (the attacks reach the

network driver)

Different Types of IDSsDifferent Types of IDSs

Application-, Host- and Network IDS

Network IDS– Watch network traffic– Watch active services and servers– Report and possibly stop network level attacks

• Advantages– Attacks can be stopped early enough (before they reach the hosts or

applications)– Attack information from different subnets can be correlated

• Problems– Encrypted data cannot be read– Annoyances to normal traffic if for some reason normal traffic is

dropped

Different Types of IDSsDifferent Types of IDSs

Application-, Host- and Network IDS

2. Different Types of IDSs2. Different Types of IDSs

Application-, Host- and Network IDS - Comparison

Application-based Host-based Network-basedTechnique Application monitoring Host system monitoring Network segment monitoringData Rate Low Moderate HighPlacement Application, userland process Kernel, system process Network nodeCost ($) Low to Moderate Moderate HighMaintenance Effort Moderate Moderate to High LowEncrypted Data Supported Supported UnsupportedSwitched Networks Not problematic Not problematic Problematic

Diagram

Simple Process Model for IDSimple Process Model for ID

Capture Data Analyse Data Respond

IterateFor example applications log For example applications log

network driver, or network cablenetwork driver, or network cable

Parse data, filter data and executeParse data, filter data and execute

Detection AlgorithmsDetection Algorithms

Drop packets, send alerts, Drop packets, send alerts,

update routing tables,update routing tables,

kill processes etc.kill processes etc.

Misuse Detection

IDS principle of detectionIDS principle of detection

Alert tcp !192.168.1.0/24 any -> 192.168.1.0/24 111

(Content: “|00 01 86 A5|”;msg:”External Mountd access”;)

– SSearch attack signatures, which are patterns, byte code or earch attack signatures, which are patterns, byte code or expressions belonging to a specific attack.expressions belonging to a specific attack.– often called signature-based detectionoften called signature-based detection– A sA signature is created by analysing an attack methodignature is created by analysing an attack method–TThe patterns are stored inside the IDShe patterns are stored inside the IDS

Example Rule:Example Rule:

There are two basic methods used by ID Systems: misuse detection and anomaly detection.

Example of a NIDS, snort• Enable NIDS mode of Snort

# ./snort -dev -l ./log -h 192.168.1.0/24 -

c snort.conf • The above command means that let Snort work as NIDS

for the network 192.168.1.0/24 according to the rules inside snort.conf file.

• Sample rule:• alert udp any any -> 192.168.1.0/24 5060 (content:"|01 6a 42 c8|"; msg: “SIP session signaling";)

• The rules are modular and it is easy to add new rules. Typically the rules make alarms of all old security breaches so that you cannot notice any new breaches.

Anomaly Detection

IDS principle of detectionIDS principle of detection

“Distinguish abnormal from normal”

Threshold Detection

• X events in Y seconds triggers the alarm

Statistical Measures

• Current traffic profile matches the ”normal” profile

Rule-Based Methods

• Jack never logs in at 6 to 8 AM

• If Jack just sent email from Espoo office, he should not send email from New York office at the same time

• Example: ( anomaly detection engine---SPADE)

[**] [104:1:1] spp_anomsensor: Anomaly threshold exceeded: 3.8919 [**] 08/22-22:37:00.419813 24.234.114.96:3246 -> VICTIM.HOST:80

TCP TTL:116 TOS:0x0 ID:25395 IpLen:20 DgmLen:48 DF ******S* Seq: 0xEBCF8EB7  Ack: 0x0  Win: 0x4000  TcpLen: 28 TCP Options (4) => MSS: 1460 NOP NOP SackOK

[**] [104:1:1] spp_anomsensor: Anomaly threshold exceeded: 10.5464 [**] 08/22-22:22:46.577210 24.41.81.216:2065 ->

VICTIM.HOST:27374 TCP TTL:108 TOS:0x0 ID:10314 IpLen:20 DgmLen:48 DF ******S* Seq: 0x63B97FE2  Ack: 0x0  Win: 0x4000  TcpLen: 28 TCP Options (4) => MSS: 1460 NOP NOP SackOK

[**] [104:1:1] spp_anomsensor: Anomaly threshold exceeded: 7.8051 [**] 08/23-23:04:53.051245 VICTIM.HOST:31337 ->

64.230.133.196:3486 TCP TTL:255 TOS:0x0 ID:0 IpLen:20 DgmLen:40 DF ***A*R** Seq: 0x0 Ack: 0x22676B9  Win: 0x0  TcpLen: 20

[**] [104:1:1] spp_anomsensor: Anomaly threshold exceeded: 9.0907 [**] 09/02-01:30:31.545406 VICTIM.HOST:515 -> 24.42.220.45:1189

TCP TTL:64 TOS:0x0 ID:0 IpLen:20 DgmLen:60 DF ***A**S* Seq: 0x16FC5A7F  Ack: 0x529F8CE7  Win: 0x16A0  TcpLen: 40 TCP Options (5) => MSS: 1460 SackOK TS: 124399151 14755839 NOP TCP Options => WS: 0

Anomaly/Misuse Detection – Comparison

IDS principle of detectionIDS principle of detection

Method Misuse Detection Anomaly DetectionTechnique Detect Patterns of Interest Deviations from Learned Norms

Generalization Problematic YesSpecifity Yes No

Sensitivity High ModerateFalse Alarms Low ModerateAdaptation No Yes

Responses

IDS response principlesIDS response principles

•Alerts and notifications: email, SMS, pager (important issue: alert path must be bulletproof)

•Increase Surveillance: log more

•Throttling: slow down malicious traffic

•Blocking Access: drop data, update firewall/router

• Make Counterattack: Eye for an eye tactics

•Honey Pots and Padded Cells: route the hacker to a fake system and let him play freely

Detection problemsIDS problems in the detection stageIDS problems in the detection stage

•True positive, TP, is a malicious attack that is correctly detected as malicious.•True negative, TN, is a not an attack and is correctly classified as benign.•False positive, FP, is not an attack but has been classified as an attack.•False negative, FN, is an attack that has been incorrectly classified as a benign.

Detection rate is obtained by testing the IDS against set of intrusive scenarios

“…The false alarm rate is the limiting factor for the performance in an IDS”.

Advanced IDS TechniquesAdvanced IDS Techniques

•Stream Reassembly: follow connections and sessions

•Traffic Normalization: see that protocols are followed

• Bayesian Networks: Data mining and decision networks

•Graphical IDSs (for example GrIDS): use graphs to model attacks

•Feature equality heuristics: port stepping, packet gap recognition

•Genetic Programming, Human immune systems

• Tens of research systems exist

For ProtectionFor Protection

For AttacksFor Attacks• Evasion methods (fragmentation, mutation etc.)

•IDS trashing (DoS tools to like stick/snot to crash IDS capability

Detecting Intruders• Commercially the most used IDS systems are probably misuse

based Network ID Systems, but Host-level IDS is also needed.

• As an example of a Host-level IDS let us look at LIDS for Linux.

• The philosophy of LIDS is to have a three layer protection:

– Firewall

– PortSentry

– LIDS

• The firewall limits access to only allowed ports. In a Web-server only the TCP port 80 is absolutely necessary.

• Disable ports which are not used, for instance by removing the daemons or by modifying /etc/inetd.conf. Leave only the basic activities needed.

Detecting Intruders• PortSentry is put to some port, which is often scanned but

not used in the system.

• One should find suitable ports where to put PortSentry by looking at ports which are scanned often, like 143 or 111.

• Typically nowadays hackers do sweep scanning looking at only one port in several machines.

• PortSentry monitors activity on specific TCP/UDP ports. The PortSentry can take actions, like denying further access to the port.

• This is based on the assumption that the hacker will first probe with a scanner the machine for weaknesses.

• You install PortSentry in TCP-mode by

• portsentry -tcp

• ports are in portsentry.conf -file.

Detecting Intruders• LIDS• LIDS is an intrusion detection system that resides in the

Linux kernel.

• It basically limits the rights of a root user to do modifications. It limits root access to direct port access, direct memory access, raw access, modification of log files, limits access to file system. It also prevents installation of sniffers or changing firewall rules.

• An administrator can remove the protection by giving a password to LIDS, but if a hacker breaks into the root, he cannot without LIDS password do much damage.

• Is this good? it certainly makes the life of a hacker more difficult, but what about a hacker getting into the kernel?

• How nice it is being an administrator using LIDS?