Intrusion Detection Systems

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Intrusion Detection Systems
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Definitions

• Intrusion
• A set of actions aimed to compromise the security
  goals, namely
• Integrity, confidentiality, or availability, of a
  computing and networking resource
• Intrusion detection
• The process of identifying and responding to
  intrusion activities

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Elements of Intrusion Detection

• Primary assumptions
• System activities are observable
• Normal and intrusive activities have distinct
  evidence
• Components of intrusion detection systems
• From an algorithmic perspective
• Features - capture intrusion evidences
• Models - piece evidences together
• From a system architecture perspective
• Audit data processor, knowledge base, decision
  engine, alarm generation and responses

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Components of Intrusion Detection System
system activities are observable
normal and intrusive activities have distinct
evidence
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Intrusion Detection Approaches

• Modeling
• Features evidences extracted from audit data
• Analysis approach piecing the evidences together
• Misuse detection (a.k.a. signature-based)
• Anomaly detection (a.k.a. statistical-based)
• Deployment Network-based or Host-based
• Development and maintenance
• Hand-coding of expert knowledge
• Learning based on audit data

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Misuse Detection
Example if (src_ip dst_ip) then land attack
Cant detect new attacks
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Anomaly Detection
probable intrusion
activity measures
Relatively high false positive rate -
anomalies can just be new normal activities.
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Key Performance Metrics

• Algorithm
• Alarm A Intrusion I
• Detection (true alarm) rate P(AI)
• False negative rate P(AI)
• False alarm rate P(AI)
• True negative rate P(AI)
• Architecture
• Scalable
• Resilient to attacks

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Host-Based IDSs

• Using OS auditing mechanisms
• E.G., BSM on Solaris logs all direct or indirect
  events generated by a user
• strace for system calls made by a program
• Monitoring user activities
• E.G., Analyze shell commands
• Monitoring executions of system programs
• E.G., Analyze system calls made by sendmail

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Network IDSs

• Deploying sensors at strategic locations
• E.G., Packet sniffing via tcpdump at routers
• Inspecting network traffic
• Watch for violations of protocols and unusual
  connection patterns
• Monitoring user activities
• Look into the data portions of the packets for
  malicious command sequences
• May be easily defeated by encryption
• Data portions and some header information can be
  encrypted
• Other problems

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Architecture of Network IDS
Alerts/notifications
Policy script
Policy Script Interpreter
Event control
Event stream
Event Engine
tcpdump filters
Filtered packet stream
libpcap
Packet stream
Network
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Firewall Versus Network IDS

• Firewall
• Active filtering
• Fail-close
• Network IDS
• Passive monitoring
• Fail-open

IDS
FW
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Requirements of Network IDS

• High-speed, large volume monitoring
• No packet filter drops
• Real-time notification
• Mechanism separate from policy
• Extensible
• Broad detection coverage
• Economy in resource usage
• Resilience to stress
• Resilience to attacks upon the IDS itself!

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Case Study Snort IDS
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Problems with Current IDSs

• Knowledge and signature-based
• We have the largest knowledge/signature base
• Ineffective against new attacks
• Individual attack-based
• Intrusion A detected Intrusion B detected
• No long-term proactive detection/prediction
• Statistical accuracy-based
• x detection rate and y false alarm rate
• Are the most damaging intrusions detected?
• Statically configured.

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Firewalls
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What is a Firewall?

• A choke point of control and monitoring
• Interconnects networks with differing trust
• Imposes restrictions on network services
• only authorized traffic is allowed
• Auditing and controlling access
• can implement alarms for abnormal behavior
• Itself immune to penetration
• Provides perimeter defence

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Classification of Firewall

• Characterized by protocol level it controls in
• Packet filtering
• Circuit gateways
• Application gateways
• Combination of above is dynamic packet filter

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Firewalls Packet Filters
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Firewalls Packet Filters

• Simplest of components
• Uses transport-layer information only
• IP Source Address, Destination Address
• Protocol/Next Header (TCP, UDP, ICMP, etc)
• TCP or UDP source destination ports
• TCP Flags (SYN, ACK, FIN, RST, PSH, etc)
• ICMP message type
• Examples
• DNS uses port 53
• No incoming port 53 packets except known trusted
  servers

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Usage of Packet Filters

• Filtering with incoming or outgoing interfaces
• E.g., Ingress filtering of spoofed IP addresses
• Egress filtering
• Permits or denies certain services
• Requires intimate knowledge of TCP and UDP port
  utilization on a number of operating systems

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How to Configure a Packet Filter

• Start with a security policy
• Specify allowable packets in terms of logical
  expressions on packet fields
• Rewrite expressions in syntax supported by your
  vendor
• General rules - least privilege
• All that is not expressly permitted is prohibited
• If you do not need it, eliminate it

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Every ruleset is followed by an implicit rule
reading like this.
Example 1 Suppose we want to allow inbound
mail (SMTP, port 25) but only to our gateway
machine. Also suppose that mail from some
particular site SPIGOT is to be blocked.
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Solution 1
Example 2 Now suppose that we want to
implement the policy any inside host can send
mail to the outside.
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Solution 2
This solution allows calls to come from any port
on an inside machine, and will direect them to
port 25 on the outside. Simple enough So why
is it wrong?
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• Our defined restriction is based solely on the
  outside hosts port number, which we have no way
  of controlling.
• Now an enemy can access any internal machines and
  port by originating his call from port 25 on the
  outside machine.
• Now for a better solution

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• The ACK signifies that the packet is part of an
  ongoing conversation
• Packets without the ACK are connection
  establishment messages, which we are only
  permitting from internal hosts

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Backup Slides
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SYN Flooding Attack

• 90 of DoS attacks use TCP SYN floods
• Streaming spoofed TCP SYNs
• Takes advantage of three way handshake
• Server start half-open connections
• These build up until queue is full and all
  additional requests are blocked

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Security Performance of Packet Filters

• IP address spoofing
• Fake source address to be trusted
• Add filters on router to block
• Degradation depends on number of rules applied at
  any point
• Order rules so that most common traffic is dealt
  with first
• Correctness is more important than speed

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Port Numbering

• TCP connection
• Server port is number less than 1024
• Client port is number between 1024 and 16383
• Permanent assignment
• Ports lt1024 assigned permanently
• 20,21 for FTP, 23 for Telnet 25 for server SMTP,
  etc.
• Variable use
• Ports gt1024 must be available for client to make
  any connection
• This presents a limitation for stateless packet
  filtering
• If client wants to use port 2048, firewall must
  allow incoming traffic on this port
• Better stateful filtering knows outgoing requests

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Stateful Filtering