ns-h0503-02/11041 intruder. ns-h0503-02/11042 intruders three classes of intruders (hackers or...

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Intruder

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Intruders

• Three classes of intruders (hackers or crackers):– Masquerader– Misfeasor– Clandestine user

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Intrusion Techniques

• System maintain a file that associates a password with each authorized user.

• Password file can be protected with:– One-way encryption– Access Control

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Intrusion Techniques

• Techniques for guessing passwords:• Try default passwords.• Try all short words, 1 to 3 characters long.• Try all the words in an electronic

dictionary(60,000).• Collect information about the user’s hobbies,

family names, birthday, etc.• Try user’s phone number, social security number,

street address, etc.• Try all license plate numbers (MUP103).• Use a Trojan horse• Tap the line between a remote user and the host

system.• Prevention:

Enforce good password selection (Ij4Gf4Se%f#)

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The Stages of a Network Intrusion

1. Scan the network to:• locate which IP addresses are in use, • what operating system is in use, • what TCP or UDP ports are “open” (being listened to by Servers).

2. Run “Exploit” scripts against open ports3. Get access to Shell program which is “suid” (has

“root” privileges).4. Download from Hacker Web site special versions of

systems files that will let Cracker have free access in the future without his cpu time or disk storage space being noticed by auditing programs.

5. Use IRC (Internet Relay Chat) to invite friends to the feast.

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Intusion Detection

• The intruder can be identified and ejected from the system.

• An effective intrusion detection can prevent intrusions.

• Intrusion detection enables the collection of information about intrusion techniques that can be used to strengthen the intrusion prevention facility.

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Profiles of BehaviorIntruders and Authorized Users

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Intrusion Detection

• Statistical anomaly detection– Treshold detection– Profile based

• Rule based detection– Anomaly detection– Penetration identidication

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Measures used for Intrusion Detection

• Login frequency by day and time.• Frequency of login at different locations.• Time since last login.• Password failures at login.• Execution frequency.• Execution denials.• Read, write, create, delete frequency.• Failure count for read, write, create and delete.

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Distributed Intrusion Detection

Developed at University of California at Davis

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Distributed Intrusion Detection

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Firewall

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FirewallsFirewalls

Effective means of protection a local system or network of systems from network-based security

threats while affording access to the outside world via WAN`s or the Internet

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Firewall Design PrinciplesFirewall Design Principles

• Information systems undergo a steady evolution (from small LAN`s to Internet connectivity)

• Strong security features for all workstations and servers not established

• The firewall is inserted between the premises network and the Internet

• Aims:– Establish a controlled link– Protect the premises network from Internet-

based attacks– Provide a single choke point

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Firewall CharacteristicsFirewall Characteristics

• Design goals:– All traffic from inside to outside must pass

through the firewall (physically blocking all access to the local network except via the firewall)

– Only authorized traffic (defined by the local security policy) will be allowed to pass

– The firewall itself is immune to penetration (use of trusted system with a secure operating system)

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• Four general techniques:• Service control– Determines the types of Internet services that

can be accessed, inbound or outbound• Direction control– Determines the direction in which particular

service requests are allowed to flow• User control– Controls access to a service according to

which user is attempting to access it• Behavior control– Controls how particular services are used (e.g.

filter e-mail)

Firewall CharacteristicsFirewall Characteristics

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Types of FirewallsTypes of Firewalls

• Three common types of Firewalls:– Packet-filtering routers– Application-level gateways– Circuit-level gateways

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Packet-filtering Router

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• Packet-filtering Router– Applies a set of rules to each incoming IP

packet and then forwards or discards the packet

– Filter packets going in both directions– The packet filter is typically set up as a list of

rules based on matches to fields in the IP or TCP header

– Two default policies (discard or forward)


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• Advantages:– Simplicity– Transparency to users– High speed

• Disadvantages:– Difficulty of setting up packet filter rules– Lack of Authentication

• Possible attacks and appropriate countermeasures– IP address spoofing– Source routing attacks– Tiny fragment attacks


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Application-level Gateway


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• Application-level Gateway– Also called proxy server– Acts as a relay of application-level traffic

• Advantages:– Higher security than packet filters– Only need to scrutinize a few allowable

applications– Easy to log and audit all incoming traffic

• Disadvantages:– Additional processing overhead on each

connection (gateway as splice point)


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• Circuit-level Gateway


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• Circuit-level Gateway– Stand-alone system or– Specialized function performed by an

Application-level Gateway– Sets up two TCP connections– The gateway typically relays TCP segments

from one connection to the other without examining the contents

• Circuit-level Gateway– The security function consists of determining

which connections will be allowed– Typically use is a situation in which the system

administrator trusts the internal users– An example is the SOCKS package


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Screened host firewall system (single-homed bastion host)

Firewall ConfigurationsFirewall Configurations

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• Screened host firewall, single-homed bastion configuration

• Firewall consists of two systems:– A packet-filtering router– A bastion host


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• Configuration for the packet-filtering router:– Only packets from and to the bastion host are

allowed to pass through the router• The bastion host performs authentication and

proxy functions


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• Greater security than single configurations because of two reasons:– This configuration implements both packet-

level and application-level filtering (allowing for flexibility in defining security policy)

– An intruder must generally penetrate two separate systems


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• This configuration also affords flexibility in providing direct Internet access (public information server, e.g. Web server)


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Screened host firewall system (dual-homed bastion host)


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• Screened host firewall, dual-homed bastion configuration– The packet-filtering router is not completely

compromised– Traffic between the Internet and other hosts on

the private network has to flow through the bastion host


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Screened-subnet firewall system


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• Screened subnet firewall configuration– Most secure configuration of the three– Two packet-filtering routers are used– Creation of an isolated sub-network


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• Advantages:– Three levels of defense to thwart intruders– The outside router advertises only the

existence of the screened subnet to the Internet (internal network is invisible to the Internet)


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• Advantages:– The inside router advertises only the existence

of the screened subnet to the internal network (the systems on the inside network cannot construct direct routes to the Internet)


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