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System Threats – Virus, Worms and Trojan horse

Guided By: Miss Binita Patel

Prepared by:

Vaibhavi Oza

Avani Panchal

Seminar Report on

System Threats – Virus, Worms and Trojan horse

AT

Hasmukh Goswami Collage of Engineering

Naroda - Dehgam Road, Vahelal - Dascroi.

Ahmedabad - 382330

Submitted By:

Oza Vaibhavi AlkeshBhai (24)

Panchal Avani (25)

Bachelors in Information Technology Semester – V

Year 2009 – 10

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Hasmukh Goswami College of Engineering Naroda - Dehgam Road , Vahelal - Dascroi. Ahmedabad - 382330

C E R T I F IC A T E

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Hasmukh Goswami College of Engineering

This is to certify that following student of Bachelors in Information Technology Semester – V has completed their Seminar on titled

System threats – virus, worms and Trojan horse satisfactory in partial requirement of Bachelors in Information Technology In the year 2009– 10.

|Roll No. |Name of the Student |

| 24 |Oza Vaibhavi AlkeshBhai |

| 25 | Panchal Avani |

Section Head – IT

Mr. Manthan Khopker

( Seminar Guide)

Miss Binita Patel

Date: -

Place: -

Hasmukh Goswami College of Engineering Naroda - Dehgam Road , Vahelal - Dascroi. Ahmedabad - 382330

Table of Content

|1 |Introduction to Virus | |

| |1.1 |Definition |1 |

| |1.2 |History of virus |2 |

| |1.3 |Virus Evolution |4 |

| |1.4 |Working of virus |6 |

| |1.5 |Types of virus |7 |

| |

|2 |Introduction to worm | |

| |2.1 |Definition |10 |

| |2.2 |History of worms |11 |

| |2.3 |Working of worms |13 |

| |2.4 |Types of worms |16 |

| |

|3 |Introduction to Trojan horse | |

| |3.1 |Definition |18 |

| |3.2 |History of Trojans |19 |

| |3.3 |Working of Trojan |20 |

| |3.4 |Types of Trojan |23 |

|4 |Difference between virus worm &Trojan| |

| |4.1 |Difference |26 |

ABSTRACT

The seminar we are going to present is on the Computer Virus, Worms and Trojan horse.

In today’s time every one uses computers and internet. The one who uses them must have come across to these words. People do know these words, but don’t know what actually they are and what the differences are between a Virus, a Worm & a Trojan horse.

In this seminar we are going to introduce you to what is a virus? What are the types of it? How do they work? How can you prevent your computer from those threats?

The same way we will tell about the Worms and Trojan horse. And also we would talk about the latest attack patterns, as you can notice there were no big attacks in the year2008 which can affect mass number of computers, but still they have affected the systems.

ACKNOWLEDMENT

I would like to take this opportunity to thank my institute for offering a course like Semiar to us, so that we can show our skills and can get the idea about how to handle presentations. And can be familiar about the things related to how to develop a project.

I would also like to thank our faculties Ms Binita Patel for providing us the guidelines whenever needed.

I would also like to thank our Head of the department Mr. Manthan Khopker for keeping an eye on us.

Oza Vaibhavi .

Panchal Avani.

1. Introduction to Virus:

1. Definition:

A computer virus, according to Webster's Collegiate Dictionary, is "a computer program usually hidden within another seemingly innocuous program that produces copies of itself and inserts them into other programs or files, and that usually performs a malicious action (such as destroying data)". Or you can say that,

A computer virus attaches itself to a program or file enabling it to spread from one computer to another, leaving infections as it travels. Like a human virus, a computer virus can range in severity: some may cause only mildly annoying effects while others can damage your hardware, software or files.

A computer virus shares some of these traits. A computer virus must piggyback on top of some other program or document in order to launch. Once it is running, it can infect other programs or documents. Obviously, the analogy between computer and biological viruses stretches things a bit, but there are enough similarities that the name sticks.

Two categories of viruses, macro viruses and worms, are especially common today.

Computer viruses are never naturally occurring; they are always man-made. Once created and released, however, their spread is not directly under human control.

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1.2 History:

Traditional computer viruses were first widely seen in the late 1980s, and they came about because of several factors.

The first factor was the spread of personal computers (PCs). Prior to the 1980s, home computers were nearly non-existent or they were toys. Real computers were rare, and they were locked away for use by "experts." During the 1980s, real computers started to spread to businesses and homes because of the popularity of the IBM PC (released in 1982) and the Apple Macintosh (released in 1984). By the late 1980s, PCs were widespread in businesses, homes and college campuses.

The second factor was the use of computer bulletin boards. People could dial up a bulletin board with a modem and download programs of all types. Games were extremely popular, and so were simple word processors, spreadsheets and other productivity software. Bulletin boards led to the precursor of the virus known as the Trojan horse. A Trojan horse is a program with a cool-sounding name and description. So you download it. When you run the program, however, it does something un cool like erasing your disk. You think you are getting a neat game, but it wipes out your system. Trojan horses only hit a small number of people because they are quickly discovered, the infected programs are removed and word of the danger spreads among users.

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Floppy disks were factors in the spread of computer viruses.

The third factor that led to the creation of viruses was the floppy disk. In the 1980s, programs were small, and you could fit the entire operating system, a few programs and some documents onto a floppy disk or two. Many computers did not have hard disks, so when you turned on your machine it would load the operating system and everything else from the floppy disk. Virus authors took advantage of this to create the first self-replicating programs.

Early viruses were pieces of code attached to a common program like a popular game or a popular word processor. A person might download an infected game from a bulletin board and run it. A virus like this is a small piece of code embedded in a larger, legitimate program. When the user runs the legitimate program, the virus loads itself into memory and looks around to see if it can find any other programs on the disk. If it can find one, it modifies the program to add the virus's code into the program. Then the virus launches the "real program." The user really has no way to know that the virus ever ran. Unfortunately, the virus has now reproduced itself, so two programs are infected. The next time the user launches either of those programs, they infect other programs, and the cycle continues.

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If one of the infected programs is given to another person on a floppy disk, or if it is uploaded to a bulletin board, then other programs get infected. This is how the virus spreads.

The spreading part is the infection phase of the virus. Viruses wouldn't be so violently despised if all they did was replicate them selves. Most viruses also have a destructive attack phase where they do damage. Some sort of trigger will activate the attack phase, and the virus will then do something -- anything from printing a silly message on the screen to erasing all of your data. The trigger might be a specific date, the number of times the virus has been replicated or something similar.

1.3 Virus Evolution:

As virus creators became more sophisticated, they learned new tricks. One important trick was the ability to load viruses into memory so they could keep running in the background as long as the computer remained on. This gave viruses a much more effective way to replicate themselves. Another trick was the ability to infect the boot sector on floppy disks and hard disks. The boot sector is a small program that is the first part of the operating system that the computer loads. It contains a tiny program that tells the computer how to load the rest of the operating system. By putting its code in the boot sector, a virus can guarantee it is executed. It can load itself into memory immediately and run whenever the computer is on. Boot sector viruses can infect the boot sector of any floppy disk inserted in the machine, and on college campuses, where lots of people share machines, they could spread like wildfire.

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In general, neither executable nor boot sector viruses are very threatening any longer. The first reason for the decline has been the huge size of today's programs. Nearly every program you buy today comes on a compact disc. Compact discs (CDs) cannot be modified, and that makes viral infection of a CD unlikely, unless the manufacturer permits a virus to be burned onto the CD during production. The programs are so big that the only easy way to move them around is to buy the CD. People certainly can't carry applications around on floppy disks like they did in the 1980s, when floppies full of programs were traded like baseball cards. Boot sector viruses have also declined because operating systems now protect the boot sector.

Infection from boot sector viruses and executable viruses is still possible. Even so, it is a lot harder, and these viruses don't spread nearly as quickly as they once did. Call it "shrinking habitat," if you want to use a biological analogy. The environment of floppy disks, small programs and weak operating systems made these viruses possible in the 1980s, but that environmental niche has been largely eliminated by huge executables, unchangeable CDs and better operating system safeguards.

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1.4 Working of computer virus:

A file virus attaches itself to a file or the usually an executable application (e.g. a word processing program or a DOS program). In general, file viruses don't infect data files. However, data files can contain embedded executable code such as macros, which may be used by virus or Trojan writers. Recent versions of Microsoft Word are particularly vulnerable to this kind of threat. Text files such as batch files, postscript files, and source code which contain

commands that can be compiled or interpreted by another program are

potential targets for malware (malicious software), though such malware

is not at present common.

Boot sector viruses alter the program that is in the first sector

(boot sector) of every DOS-formatted disk. Generally, a boot

sector infector executes its own code (which usually infects the boot

sector or partition sector of the hard disk), then continues the PC

boot up (start-up) process. In most cases, all write-enabled floppies

used on that PC from then on will become infected.

Multipartite viruses have some of the features of both the above

types of virus. Typically, when an infected *file* is executed, it

infects the hard disk boot sector or partition sector, and thus

infects subsequent floppies used or formatted on the target system.

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1.5 Types of computer virus:

Macro viruses: A macro is a piece of code that can be embedded in a data file. Some word processors (e.g., Microsoft Word) and spreadsheet programs (e.g., Microsoft Excel) allow you to attach macros to the documents they create. In this way, documents can control and customize the behavior of the programs that created them, or even extend the capabilities of the program. For example, a macro attached to a Microsoft Word document might be executed every time you save the document and cause its text to be run through an external spell-checking program.

A macro virus is a virus that exists as a macro attached to a data file. In most respects, macro viruses are like all other viruses. The main difference is that they are attached to data files (i.e., documents) rather than executable programs. Many people do not think that viruses can reside on simple document files, but any application that supports document-bound macros that automatically execute is a potential haven for macro viruses. By the end of the last century, documents became more widely shared than diskettes, and document-based viruses were more prevalent than any other type of virus. It seems highly likely that this will be a continuing trend.

Stealth viruses: A stealth virus is one that, while active, hides the modifications it has made to files or boot records. It usually achieves this by monitoring the system functions used to read files or sectors from storage media and forging the results of calls to such functions.

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This means that programs that try to read infected files or sectors see the original, uninfected form instead of the actual, infected form. Thus the virus's modifications may go undetected by antivirus programs. However, in order to do this, the virusmust be resident in memory when the antivirus program is executed, and the antivirus program may be able to detect its presence.

Polymorphic viruses: A polymorphic virus is one that produces varied but operational copies of itself. This strategy assumes that virus scanners will not be able to detect all instances of the virus. One method of evading scan-string driven virus detectors is self-encryption with a variable key. More sophisticated polymorphic viruses vary the sequences of instructions in their variants by interspersing the decryption instructions with "noise" instructions (e.g., a No Operation instruction, or an instruction to load a currently unused register with an arbitrary value), by interchanging mutually independent instructions, or even by using various instruction sequences with identical net effects (e.g., Subtract A from A, and Move 0 to A). A simple-minded, scan-string based virus scanner would not be able to reliably identify all variants of this sort of virus; in this case, a sophisticated scanning engine has to be constructed after thorough research into the particular virus.

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Boot sector viruses: Boot sector viruses infect or substitute their own code for either the DOS boot sector or the Master Boot Record (MBR) of a PC. The MBR is a small program that runs every time the computer starts up. It controls the boot sequence and determines which partition the computer boots from. The MBR generally resides on the first sector of the hard disk. Since the MBR executes every time a computer is started, a boot sector virus is extremely dangerous. Once the boot code on the drive is infected, the virus will be loaded into memory on every startup. From memory, the boot virus can spread to every disk that the system reads. Boot sector viruses are typically difficult to remove, as most antivirus programs cannot clean the MBR while Windows is running. In most cases, it takes bootable antivirus disks to properly remove a boot sector virus.

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2. Introduction to Worms:

2.1 Defination:

A worm is a computer program that has the ability to copy itself from machine to machine. Worms use up computer time and network bandwidth when they replicate, and often carry payloads that do considerable damage.

A worm usually exploits some sort of security hole in a piece of software or the operating system. For example, the Slammer worm (which caused mayhem in January 2003) exploited a hole in Microsoft's SQL server.

Worms normally move around and infect other machines through computer networks. Using a network, a worm can expand from a single copy incredibly quickly.

When the worm is launched, it opens a back door into the computer, adds the infected machine to a botnet and installs code that hides itself. The botnets are small peer-to-peer groups rather than a larger, more easily identified network. Experts think the people controlling Storm rent out their micro-botnets to deliver spam or adware, or for denial-of-service attacks

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2.2 History of computer worm:

Malware with self-replicating capability has been an issue in the world of computing for several years, dating back to the first self-replicating code created by Ken Thompson in 1984. Over the past few years, both worms and viruses have become major problems, mainly due to widespread use of the internet. This wide open platform enables these infections to spread rapidly with no geographic restrictions. Worms in particular are becoming more sophisticated as malicious coders have learned from their mistakes and successes as well.

[pic][pic][pic]In this article, we will take brief glance at the history of computer worms and how they have impacted the current state of computing.

Early Infections

Self-replicating applications date back to the early days of the Unix operating system. Ken Thompson's code was essentially a compiler modification that manipulated login procedures and the compiler itself. The conventional virus became a common plague in the era of the Apple II system. This infection moved rather slowly, yet provided the means of distributing some of the most known viruses, such as Chernobyl and Michelangelo.

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The first Internet infection that required no human intervention to propagate was the Morris Worm, discovered in 1988 and released by Robert Morris. It spread very quickly, infecting a number of vulnerable computers in a matter of hours. The Morris Worm infected various machines and also used multiple exploits including buffer overflows, debugging routines in mail components, password sniffing, and other streams of execution to improve its ability to attack other computers.

Although released on accident, the benign concept doesn't really apply to the Morris Worm, as it had a significant amount of impact because of the bug in its code. When re infecting a computer, there remained the possibility that ssthe new infection would be persistent, allowing other worms to run and terribly impact system performance. However, this caused the worm to be noticed instantly, and therefore, quickly contained.

Modern Worms

Active computer worms have returned to prominence in recent times. The first one to cause an eruption was Code Red. This infection proved how quickly a simple self-replicating program could spread via the internet's current infrastructure. Code Red exploited a buffer flow condition in the Microsoft IIS (Internet Information Server). It was able to propagate quickly because of the "always on" nature of IIS and many versions of the Windows operating system. Code Red was also equipped with scanning capabilities that improved its throughput and gave it the ability to elude numerous IP address security features.

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one should also create a strategy to elude worm exploits. While there is no perfect solution, there are many steps that can be taken to prevent damage and reduce the spread of infection. Anti-virus software and firewalls are a must these days, two powerful weapons that will keep you one step ahead of a worm outbreak. It is also critical to conduct routine backups of your data as these infections can easily corrupt or completely overwrite existing files. When it comes to the disruption of worms and other malware, it's much better to be safe than sorry.

2.3 Working of computer worms:

To understand the working of the worm we will see working of some of the worms that how they attacked and how dangerous they can be.

Worms normally move around and infect other machines through computer networks. Using a network, a worm can expand from a single copy incredibly quickly. The Code Red worm replicated itself more than 250,000 times in approximately nine hours on July 19, 2001.

The Code Red worm slowed down Internet traffic when it began to replicate itself, but not nearly as badly as predicted. Each copy of the worm scanned the Internet for Windows NT or Windows 2000 servers that did not have the Microsoft security patch installed. Each time it found an unsecured server, the worm copied itself to that server. The new copy then scanned for other servers to infect. Depending on the number of unsecured servers, a worm could conceivably create hundreds of thousands of copies.

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The Code Red worm had instructions to do three things:

Replicate itself for the first 20 days of each month

Replace Web pages on infected servers with a page featuring the message "Hacked by Chinese"

Launch a concerted attack on the White House Web site in an attempt to overwhelm it .Upon successful infection, Code Red would wait for the appointed hour and connect to the domain. This attack would consist of the infected systems simultaneously sending 100 connections to port 80 of (198.137.240.91).

The U.S. government changed the IP address of to circumvent that particular threat from the worm and issued a general warning about the worm, advising users of Windows NT or Windows 2000 Web servers to make sure they installed the security patch. .

A worm called Storm, which showed up in 2007, immediately started making a name for itself. Storm uses social engineering techniques to trick users into loading the worm on their computers. So far, it's working -- experts believe between one million and 50 million computers have been infected

When the worm is launched, it opens a back door into the computer, adds the infected machine to a botnet and installs code that hides itself.

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2.4 Types of computer Worms:

Email Worms

Spreading goes via infected email messages. Any form of attachment or link in an email may contain a link to an infected website. In the first case activation starts when the user clicks on the attachment while in the second case the activation starts when clicking the link in the email.

Known methods to spread are :

- MS Outlook services

- Direct connection to SMTP servers using their own SMTP API

- Windows MAPI functions

This type of worms is known to harvest an infected computer for email addresses from different sources.

- Windows Address Book database [WAB]

- MS Outlook address book

- Files with appropriate extensions will be scanned for email like strings

Be aware that during spreading some worms construct new sender addresses based on possible names combined with common domain names. So, the sender address in the email doesn't need to be the originator of the email.

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Instant Messaging Worms

The spreading used is via instant messaging applications by sending links to infected websites to everyone on the local contact list. The only difference between these and email worms is the way chosen to send the links.

Internet Worms

Nasty ones. These ones will scan all available network resources using local operating system services and/or scan the Internet for vulnerable machines. Attempt will be made to connect to these machines and gain full access to them.

Another way is that the worms scan the Internet for machines still open for exploitation i.e. not patched. Data packets or requests will be send which install the worm or a worm downloader. If succeeded the worm will execute and there it goes again!

IRC Worms

Chat channels are the main target and the same infection/spreading method is used as above - sending infected files or links to infected websites. Infected file sending is less effective as the recipient needs to confirm receipt, save the file and open it before infection will take place.

File-sharing Networks Worms

Copies itself into a shared folder, most likely located on the local machine. The worm will place a copy of itself in a shared folder under a harmless name. Now the worm is ready for download via the P2P network and spreading of the infected file will continue.

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3. Trojan Horse:

3.1 Definition:

A Trojan horse is a computer program which carries out malicious operations without the user's knowledge. The name "Trojan horse" comes from a legend told in the Iliad (by the writer Homer) about the siege of the city of Troy by the Greeks.

Legend has it that the Greeks, unable to penetrate the city's defences, got the idea to give up the siege and instead give the city a giant wooden horse as a gift offering.

The Trojans (the people of the city of Troy) accepted this seemingly harmless gift and brought it within the city walls. However, the horse was filled with soldiers, who came out at nightfall, while the town slept, to open the city gates so that the rest of the army could enter.

Thus, a Trojan horse (in the world of computing) is a hidden program which secretly runs commands, and usually opens up access to the computer running it by opening a backdoor. For this reason, it is sometimes called a Trojan by analogy to the citizens of Troy.

A Trojan horse may, for example:

steal passwords;

copy sensitive date;

carry out any other harmful operations;

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3.2 History of Trojan Horse:

The name Trojan horse is a bit different so as it has a tell is bounded with it. It was called so because of a Greek tale.

A Trojan horse derives its name from the Trojan War. Legend has it that King Odysseus built a [pic]Trojan Horse as a gift to the city of Troy to signify surrender. He then ordered the Greek army to retreat and left the ‘gift’ outside [pic]the city gates. However it turned out that the Horse had more than 40 soldiers hidden in its belly. Once inside [pic]the city of Troy, these soldiers snuck out and opened the gates for their fellow soldiers who went on to attack the unsuspecting city.

So as the same way a Trojan horse come into your computer as a ‘gift’ or you can say a non harmful package of software but once you run it you will know what actually it was.

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3.3: Working of Trojan horse:

Trojans work similar to the client-server model. The attacker deploys the client to connect to the server, which runs on the remote machine when the remote user unknowingly executes the Trojan on the machine. 

The typical protocol used by most Trojans is the TCP/IP & UDP protocol. It will usually try to remain in a stealth mode, or hidden on the computer. When Trojan is activated, the server starts listening on default or configured ports for incoming connections from the attacker. It is usual for Trojans to also modify the registry and/or use some other auto starting method.

When the remote machine is on a network with dynamically assigned IP address or when the remote machine uses a dial-up connection to connect to the internet in that case Trojans can configure the features like mailing the victim’s IP, as well as messaging the attacker via instant messaging application or Internet Relay Chat (IRC). DSL users on the other hand, have static IPs so the infected IP is always known to the attacker.

Most of the Trojans use auto-starting methods so that the servers are restarted every time the remote machine reboots or starts. This is also notified to the attacker. Some of the well known system files targeted by Trojans are Autostart Folder, Win.ini, System.ini, Wininit.ini, Winstart.bat, Autoexec.bat, & Config.sys.

 

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Modes of Transmission

Trojan can infect the target system with different modes of Transmission. Common transmission mode is as follows:

➢ Instant Message

➢ IRC (Internet Relay Chat)

➢ Attachments

➢ Physical Access

➢ Browser and E-mail Software Bugs

➢ NetBIOS (File Sharing)

 

Instant message

People can also get infected while chatting / talking / video messaging over any Instant Messenger Application. It is a risk that the user undertakes when it comes to receiving files no matter from whom or where it comes. 

IRC

In Internet Relay Chat, the threat comes from exchange of files no matter what they claim to be or where they come from. It is possible that some of these are infected files or disguised files. 

Attachments

Any attachment, even if it is from a known source should be screened as it is possible that the source was infected earlier and is not aware of it.

 

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Physical Access

Physical access to a target machine is perhaps the easiest way for an attacker to infect a machine.

 

Browser and E-mail Software Bugs

Having outdated applications can expose the system to malicious programs such as trojans without any other action on behalf of the attacker.

 

NetBIOS (File Sharing)

If port 139 is opened, the attacker can install trojan.exe and modify some system file, so that it will run the next time the system is rebooted.

 

To block file sharing in Windows version, navigate to:

Start–>Settings–>Control Panel–>Network–>File and Print Sharing and uncheck the boxes there.

 

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3.4 Types of Trojan horse:

Trojan horses are broken down in classification based on how they breach systems and the damage they cause.

 

The seven main types of Trojan horses are: 

➢ Remote Access Trojans

➢ Data Sending Trojans

➢ Destructive Trojans

➢ Proxy Trojans

➢ FTP Trojans

➢ Denial-of-service attack (DoS) Trojans

➢ Security software disabler Trojans

 

Remote Access Trojans

The attacker gains full control over the systems that the Trojan infects, and gains full access to files, private conversations, accounting data and so on. The remote access Trojans acts as a server, and listens on a port that is not supposed to be available to the internet. Attacker in the same network located behind the firewall can easily access the Trojans. Example: Back Orifice and, NetBus.

 

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Data Sending Trojans

Data Sending Trojans provide hackers with passwords or other confidential data such as credit card numbers and audit sheets. This Trojans look for particular information in certain locations.Example:  Badtrans.B email virus

 Destructive Trojans:

The sole purpose of the Destructive Trojans is to delete files on the target system. Destructive Trojans are generated on the basis of a fixed time and data much like the logic bomb. Example: dll, .ini, or .exe files.

 Proxy Trojans:

Proxy Trojans convert the user’s computer into a proxy server. This makes the computer accessible to the entire world or only the specified attacker. The attacker has full control over the user’s system, and can also launch attacks on other systems from the affected user’s network. Generally it is used for Telnet, ICQ or IRC in order to purchase goods using stolen credit cards, other illegal activities.

 FTP Trojans:

FTP Trojans are used for FTP transfers and allowing the attackers to connect to the victim’s system via FTP.

 

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Denial-of-Service (DoS) Attack Trojans:

This type of Trojans empowers the attacker to start a distributed Denial of Services (DDoS) attack, if there are a fair number of victims on the network at that specific time. Example: WinTrinoo, CNN, E*Trade 

Security Software Disablers:

These are designed to disturb the functions of anti-virus software or firewalls. After these programs are disabled, the hacker can easily attack the victim’s system. 

 Hazards of Trojan

A botnet also known as a zombie army is a number of Internet computers that, although their owners are unaware of it, have been set up to forward transmissions including spam or viruses to other computers on the Internet such computer is referred to as a zombie - in effect, a computer “robot” or “bot” that serves the wishes of some master spam or virus originator.

 An increasing number of home users have high speed connections for computers that may be inadequately protected. A zombie or bot is often created through an Internet port that has been left open and through which a small Trojan horse program can be left for future activation. At a certain time, the zombie army “controller” can unleash the effects of the army by sending a single command, possibly from an Internet Relay Channel (IRC) site.  

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4. Diffarance between virus, worm & Trojan horse:

Most of us don't make a real difference between worm, virus and Trajan Horse or refer to a worm or Trojan Horse as a virus.

All of us know all are malicious programs that can cause very serious damage to PC. Exist differences among the three, and knowing those differences can help you to better protect your computer from their often damaging effects.

A computer virus is a program or piece of code that is loaded onto your computer without your knowledge and runs against your wishes. Some viruses cause only mildly annoying effects while others can damage your hardware, software or files. Almost all viruses are attached to an executable file, which means the virus may exist on your computer but it cannot infect your computer unless you run or open the malicious program. A virus cannot be spread without a human action, (such as running an infected program) to keep it going. People continue the spread of a computer virus, mostly unknowingly, by sharing infecting files or sending e-mails with viruses as attachments in the e-mail.

A worm is a program or algorithm that replicates itself. A worm has the capability to travel without any help from a person from PC to PC and have ability to replicate itself on your system, so rather than your computer sending out a single worm, it could send out hundreds or thousands of copies of itself, creating a huge devastating effect.

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A Trojan Horse is a destructive program that "working" as a benign application ( (like changing your desktop, adding silly active desktop icons) or can cause serious damage by deleting files and destroying information on your system. Trojans are also known to create a back door on your computer that gives malicious users access to your system, possibly allowing confidential or personal information to be compromised. Trojans do not reproduce by infecting other files nor do they self-replicate.

Added into the mix, we also have what is called a blended threat. Blended threats combine the characteristics of viruses, worms, Trojan Horses, and malicious code with server and Internet vulnerabilities. By using multiple methods and techniques, blended threats can rapidly spread and cause widespread damage. Characteristics of blended threats include: causes harm, propagates by multiple methods, attacks from multiple points, and exploits vulnerabilities. This combination of method and techniques means blended threats can spread quickly and cause widespread damage. Characteristics of blended threats include: causes harm, propagates by multiple methods, attacks from multiple points and exploits vulnerabilities. They are considered to be the worst risk to security since the inception of viruses, as most blended threats require no human intervention to propagate.

Therefore is very important to protect your PC with a good anti-virus software installed on your system with the latest fixes for new viruses, worms, and Trojan horses.

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