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Malicious Software By Kavita Khanna ( kavita\[email protected] ) & Himani Singh ( [email protected] ) (CS-265, Fall-2003). Malicious Software – “Presentation Outline” What is malicious software? Categories of malicious software.

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Malicious Software


Kavita Khanna

([email protected])


Himani Singh

([email protected])

(CS-265, Fall-2003)

Malicious Software –“Presentation Outline”
  • What is malicious software?
  • Categories of malicious software.
  • Different malicious software – viruses, worms, Trojan Horse etc.
  • More description about viruses :
  • Desirable properties of viruses.
  • Identifying infected files and programs.
  • Where do viruses reside.
  • Identifying and detecting viruses – virus signature.
  • Effect of Virus attack on computer system.
  • Protection against attacks by malicious software – preventing infection.
  • References.
what is malicious software
What is Malicious Software:
  • Software deliberately designed to harm

computer systems.

  • Malicious software program causes undesired actions in information systems.
  • Spreads from one system to another through:
  • E-mail (through attachments)
  • Infected floppy disks
  • Downloading / Exchanging of corrupted files
  • Embedded into computer games
types of malicious software
Types of Malicious Software
  • Virus :These are the programs that spread to other software in the system .i.e., program that incorporates copies of itself into other programs.

Two major categories of viruses:

  • Boot sector virus : infect boot sector of systems.

become resident.

activate while booting machine

  • File virus : infects program files.

activates when program is run.

categories of viruses
Categories of Viruses



  • Hides

modifications it

has made to

files or to the


  • Reports

false values to

programs as

they read files

or data from

storage media. 



  • Produces

modified & fully

operational code.

  • Produces new

& different code

every time when

virus is copied &

transmitted to a

new host.

  • Difficult to

detect & remove.



  • Programming

tricks make the

tracing and


the code difficult.

  • Complex


methods used to

design code, so

difficult to repair

infected file.



  • Creates new

program instead

of modifying

existing program.

  • Contains all

virus code.

  • Executed by

shell, instead of

original program.

Rabbit :This malicious software replicates itself without limits. Depletes some or all the system’s resources.
  • Re-attacks the infected systems – difficult recovery.
  • Exhausts all the system’s resources such as CPU time, memory, disk space.
  • Depletion of resources thus denying user access to those resources.
Hoaxes :False alerts of spreading viruses.
  • e.g., sending chain letters.
  • message seems to be important to recipient, forwards it to other users – becomes a chain.
  • Exchanging large number of messages (in chain) floods the network resources – bandwidth wastage.
  • Blocks the systems on network – access denied due to heavy network traffic.
Trojan Horse :This is a malicious program with unexpected additional functionality. It includes harmful features of which the user is not aware.
  • Perform a different function than what these are advertised to do (some malicious action e.g., steal the passwords).
  • Neither self-replicating nor self-propagating.
  • User assistance required for infection.
  • Infects when user installs and executes infected programs.
  • Some types of trojan horses include Remote Access Trojans (RAT), KeyLoggers, Password-Stealers (PSW), and logic bombs.
Transmitting medium :
  • spam or e-mail
  • a downloaded file
  • a disk from a trusted source
  • a legitimate program with the Trojan inside.
  • Trojan looks for your personal information and sends it to the Trojan writer (hacker). It can also allow the hacker to take full control of your system.
  • Different types of Trojan Horses :

1. Remote access Trojantakes full control of your

system and passes it to the hacker.

2.The data-sending Trojan sends data back to the hacker by means of e-mail.

e.g., Key-loggers – log and transmit each keystroke.

The destructive Trojan has only one purpose: to destroy and delete files. Unlikely to be detected by anti-virus software.
  • The denial-of-service (DOS) attack Trojans combines computing power of all computers/systems it infects to launch an attack on another computer system. Floods the system with traffic, hence it crashes.
  • The proxy Trojans allows a hacker to turn user’s computer into HIS (Host Integration Server) server – to make purchases with stolen credit cards and run other organized criminal enterprises in particular user’s name.
  • The FTP Trojan opens port 21 (the port for FTP transfer) and lets the attacker connect to your computer using File Transfer Protocol (FTP).
The security software disabler Trojan is designed to stop or kill security programs such as anti-virus software, firewalls, etc., without you knowing it.
  • Spyware :
  • Spyware programs explore the files in an information system.
  • Information forwarded to an address specified in Spyware.
  • Spyware can also be used for investigation of software users or preparation of an attack.
Trapdoor : Secret undocumented entry point to the program.
  • An example of such feature is so called back door, which enables intrusion to the target by passing user

authentication methods.

  • A hole in the security of a system deliberately left in place by designers or maintainers. 
  • Trapdoor allows unauthorized access to the system.
  • Only purpose of a trap door is to "bypass" internal controls.  It is up to the attacker to determine how this circumvention of control can be utilized for his benefit.
Types of Trapdoor



Virtually undetectable.




hardware flaws.

Worms :
  • program that spreads copies of itself through a


  • Does irrecoverable damage to the computer system.
  • Stand-alone program, spreads only through network.
  • Also performs various malicious activities other than spreading itself to different systems e.g., deleting files.
  • Attacks of Worms:
  • Deleting files and other malicious actions on systems.
  • Communicate information back to attacker e.g., passwords, other proprietary information.
  • Disrupt normal operation of system, thus denial of service attack (DoS) – due to re-infecting infected system.
  • Worms may carry viruses with them.
Means of spreading Infection by Worms :
  • Infects one system, gain access to trusted host lists on infected system and spread to other hosts.
  • Another method of infection is penetrating a system by guessing passwords.
  • By exploiting widely known security holes, in case, password guessing and trusted host accessing fails.

e.g., A well-known example of a worm is the ILOVEYOU

worm, which invaded millions of computers through

e-mail in 2000.

VIRUSES – More Description

Desirable properties of Viruses :

  • Virus program should be hard to detect by

anti-virus software.

  • Viruses should be hard to destroy or deactivate.
  • Spread infection widely.
  • Should be easy to create.
  • Be able to re-infect.
  • Should be machine / platform independent, so that it can spread on different hosts.
Detecting virus infected files/programs :
  • Virus infected file changes – gets bigger.
  • Modification detection by checksum :

> Use cryptographic checksum/hash function

e.g., SHA, MD5.

> Add all 32-bit segments of a file and store the sum

(i.e., checksum).

Identifying Viruses :
  • A virus is a unique program.
  • It as a unique object code.
  • It inserts in a deterministic manner.
  • The pattern of object code and where it is inserted provides a signature to the virus program.
  • This virus signature can be used by virus scanners to identify and detect a particular virus.
  • Some viruses try to hide or alter their signature:
  • Random patterns in meaningless places.
  • Self modifying code – metamorphic, polymorphic viruses.
  • Encrypt the code, change the key frequently.
Places where viruses live :
  • Boot sector
  • Memory resident
  • Disk – Applications and data stored on disk.
  • Libraries – stored procedures and classes.
  • Compiler
  • Debugger
  • Virus checking program infected by virus – unable to detect that particular virus signature.
Effect of Virus attack on computer system
  • Virus may affect user’s data in memory – overwriting.
  • Virus may affect user’s program – overwriting.
  • Virus may also overwrite system’s data or programs – corrupting it – disrupts normal operation of system.
  • “Smashing the Stack” – Buffer overflow due to execution of program directed to virus code.
Preventing infection by malicious software :
  • Use only trusted software, not pirated software.
  • Test all new software on isolated computer system.
  • Regularly take backup of the programs.
  • Use anti-virus software to detect and remove viruses.
  • Update virus database frequently to get new virus signatures.
  • Install firewall software, which hampers or prevents the functionality of worms and Trojan horses.
  • Make sure that the e-mail attachments are secure.
  • Do not keep a floppy disk in the drive when starting a program, unless sure that it does not include malicious software, else virus will be copied in the boot sector.
  • Trojan Horse. Retrieved Nov 8, 2003 from website:
  • Staffordshire University, Information & Security Team (Jun 8,

2002). Information Systems Security Guidelines. Retrieved

Nov 10, 2003 from website:

  • M.E.Kabay, Norwich University, VT (2002). Malicious Software. Retrieved Nov 9, 2003 from website:

  • Computer Emergency Response Team (CERT), Information Security (Jul 2, 2002). Malicious Software – general. Retrieved Nov 10, 2003 from


references cont
References Cont...
  • Rutgers, New Jersey (Oct 10, 2003). Trojan Horses. Retrieved Nov 10, 2003 from website:
  • Dr. Roger R. Schell, Monterey CA (Apr 24, 2000). Malicious Software.

Retrieved Nov 11, 2003 from website:

  • Edward F. Gehringer. Computer Abuse – Worms, Trojan Horses, Viruses. Retrieved Nov 12, 2003 from website:

  • Computer Viruses. Retrieved Nov12, 2003 from website:

  • Program Security. Retrieved Nov 12, 2003 from website: