Learning outcomes

Fundamentals of The Internet

Learning outcomes

After this session, you should be able to:

•Identify the threat of intruders in systems and networks and explain how to protect them through password management

•Describe the operation of viruses, Trojans and worms and identify relevant software to counteract them

•Explain the effect of unwanted network connections in an organisation and how to design a firewall that will balance user freedom in relation to network security


Taxanomy of Malicious Programs

Need Host

Program

Independent

Trapdoors Logic Bombs

TrojanHorses

Viruses Zombie Worms

Malicious Programs


Definitions

•Trojan Horse - instructions in an otherwise good program that cause bad things to happen (sending your data or password to an attacker over the net).

•Logic Bomb - malicious code that activates on an event (e.g., date, specific key sequence, absence of a file etc ).

•Trap Door (or Back Door) - undocumented entry point written into code for debugging that can allow unwanted users.


Definitions

• Virus - code that infects other executable files by copying itself.

• A “Bacteria” replicates until it fills all disk space, or CPU cycles.

• Payload - harmful things the malicious program does, after it has had time to spread.

• Worm - a program that replicates itself across the network (usually riding on email messages or attached documents (e.g., macro viruses).

• Zombie – a program that takes over other Internet-attached computers to launch attacks that are difficult to trace back to the original creator of the program. Typically used in Denial-of-Service Attacks


Virus Phases

• Dormant phase - the virus is idle

• Propagation phase - the virus places an identical copy of itself into other programs

• Triggering phase – the virus is activated to perform the function for which it was intended

• Execution phase – the function is performed

Four different phases exist for a virus. However, not all viruses have all of these four phases.


Virus Protection

Have a well-known virus protection program, configured to scan disks and downloads automatically for known viruses.

Do not execute programs (or "macro's") from unknown sources (e.g., PS files, Hypercard files, MS Office documents). Do not download .dll, .lib., .hlp, .obj files from unknown sources.

Avoid the most common operating systems and email programs, if possible.


Virus Structure

Increases the length of the host program !!

And, this segment will be the same in all infected files !! It is called the signature of the virus.


A Compression Virus


Types of Viruses

• Parasitic Virus - attaches itself to executable files as part of their code. Runs whenever the host program runs.

• Memory-resident Virus - Lodges in main memory as part of the residual operating system.

• Boot Sector Virus - infects the boot sector of a disk, and spreads when the operating system boots up (original DOS viruses).

• Stealth Virus - explicitly designed to hide from Virus Scanning programs. Compression and controlling disk I/O are the most common techniques.

• Polymorphic Virus - mutates with every new host to prevent signature detection. It is achieved either by randomly shuffling independent instructions in the virus, or by adding superfluous instructions, or by encryption.


Macro Viruses

• Microsoft Office applications allow “macros” to be part of the document. The macro could run whenever the document is opened, or when a certain command is selected (Save File).

• Platform independent.

• Infect documents, delete files, generate email and edit letters.


Antivirus Approaches

1st Generation, Scanners: searched files for any of a library of known virus “signatures.” Checked executable files for length changes.

2nd Generation, Heuristic Scanners: looks for more general signs than specific signatures (code segments common to many viruses). Checked files for checksum or hash changes.

3rd Generation, Activity Traps: stay resident in memory and look for certain patterns of software behavior (e.g., scanning files).

4th Generation, Full Featured: combine the best of the techniques above.


Advanced Antivirus Techniques

• Generic Decryption (GD)– CPU Emulator– Virus Signature Scanner– Emulation Control Module

• If the virus is encrypted it will decrypt and reveal itself.

• Key question: how long should a GD scanner run each interpretation?


Firewall Design Principles

• Too many computers with different Operating Systems exist in corporate networks today.

• They all need Internet access.• 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


Firewall 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).


Firewall Characteristics

Four general techniques:

•User control–Controls access to a service according to which user is attempting to access it (may need authentication procedures).

•Behavior control–Controls how particular services are used (e.g. filter e-mail).

•Service control–Determines the types of Internet services that can be accessed (such as FTP, HTTP) , inbound or outbound.

•Direction control–Determines the direction in which particular service requests are allowed to flow.


Types of Firewalls

• Three common types of Firewalls:

– Packet-filtering routers

– Stateful Inspection Firewalls

– Application-level gateways

– Circuit-level gateway


Types of Firewalls

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)


Types of Firewalls

action src port dest port comment

blockSPIGO

T*

{our hosts}

*we don’t trust these

people

allow * *{our

hosts}25

connection to our SMTP port

•Source and Destination IP address

•Source and Destination Port Numbers

•IP Protocol field, which defines the higher-level protocol in the IP packet

•MAC address in case there are more than one.

All these fields can be used in a packet filter:

Packet filtering example:


Types of Firewalls

Stateful Inspection Firewalls:

A traditional packet filter allows or denies packet on an individual basis after analysing IP and TCP packet headers in the arriving packet. A stateful inspection firewall, on the other hand, considers the status of ongoing TCP connections in addition to the header information in the arriving packets.

Source Address

Source PortDestination

AddressDestination

PortConnection

State

192.168.1.100 1030 210.9.88.29 80 Active

192.168.1.101 80 216.32.42.123 2552 Released

192.168.1.105 1990 192.168.1.6 79 Active

223.43.21.231 2112 192.168.1.6 80 Active

210.99.212.18 3321 192.168.1.6 80 Active

Example Stateful Furewall Connection State Table:Block packets that scan this port !!


Types of Firewalls

Application-level Gateway (proxy):

– Also called proxy server– Acts as a relay of application-level traffic– Requires user authentication– More secure than packet filtering and stateful inspection– More processing overhead as well.


Types of Firewalls

Circuit-level Gateway:

– Sets up two TCP connections– Requires user authentication– The gateway typically relays TCP segments from one connection to

the other without examining the contents– 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


Firewall Configurations

• In addition to the use of simple configuration of a single system (single packet filtering router or single gateway), more complex configurations are possible.

• A “Bastion Host” (means a well-fortified area) serves as a platform for an application-level or circuit-level gateway.

– they use a secure operating system,

– only the essential services are installed (including proxies for Telnet, DNS, FTP, SMTP, and user authentication),

– each proxy module is a very small software package (fewer than 1000 lines) to minimise any security flaw

– each proxy is independent

– proxies have no disk access and they run as nonprivileged users



Screened host firewall system (single-homed bastion host):

– Only packets from and to the bastion host are allowed to pass through the packet filter

– The bastion host performs authentication and proxy functions

– Both packet-level and application-level filtering



Screened host firewall system (dual-homed bastion host):

– Traffic between the Internet and other hosts on the private network has to flow through the bastion host

– Even if the packet filter is compromised, the private network is physically isolated from the Internet by the Bastion host



Screened-subnet firewall system:

– Most secure configuration of the three

– An isolated sub-network is created between the Internet and the private network

– The private network is not visible to the Internet

– The Internet is not visible to the private network

Documents

Learning outcomes