Excel Formula Returning Results Only If Certain Criteria Are Nmet Computer Viruses Made Easy

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Computer Viruses Made Easy

i virus

1 Definition — What is malicious code?

Malicious code refers to any instruction or set of instructions that performs a suspicious action without the user’s consent.

2 Definition – What is a computer virus?

A computer virus is a type of malicious code. It is a set of instructions (ie a program) that is both self-replicating and infectious that mimics a biological virus.

3 Program Viruses and Boot Sector Infectors

Viruses can first be classified based on what they infect. Viruses that infect user programs such as games, word processors (Word), spreadsheets (Excel), and DBMS (Access) are called program viruses. Viruses that infect boot sectors (explained later) and/or master boot records (explained later) are called boot sector infectors. Some viruses belong to both groups. All viruses have three functions: reproduce, infect, and deliver the payload. Let’s look at program viruses first.

3.1 How do program viruses work?

A program virus must attach itself to other programs to survive. This is the main characteristic that distinguishes a virus from other forms of malicious code: it cannot exist by itself; It is parasitic on another program. The program that the virus attacks is called the host program. When a virus-infected program is executed, the virus is also executed. The virus now performs its first two functions simultaneously: reproduction and infection.

After executing the infected program, the virus takes control from the host and starts looking for other programs on the same or other disks that are not currently infected. When it finds one, it copies itself to an uninfected program. Later, it may start looking for more programs to infect. After the transition is complete, control returns to the host program. When the host program terminates, it and possibly the virus are removed from memory. The user will be completely unaware of what just happened.

A variation on this method of infection involves leaving the virus in memory even after the host has died. The virus will now remain in memory until the computer is turned off. From this position, the virus can infect programs to its heart’s content. The next time a user boots his computer, he may unknowingly execute one of his infected applications.

As soon as the virus is in memory, there is a risk that the virus’s third function may be invoked: deliver the payload. This activity can be anything the virus creator wants, such as deleting files, or slowing down the computer. A virus can remain in memory until the computer is turned off, delivering its payload. It can modify data files, damage or delete data files and programs, etc. It can wait patiently for you to create data files with word processors, spreadsheets, databases, etc. Then, when you exit the program, the virus can modify or. Delete new data files.

3.1.1 Transition process

A program virus usually infects other programs by placing a copy of itself at the end of the intended target (host program). It modifies the first few instructions of the host program so that when the host executes, control passes to the virus. Afterwards, control returns to the host program. Making the program read-only is ineffective protection against viruses. Viruses can gain access to read-only files simply by disabling the read-only attribute. After the transition the read-only feature will be restored. Below, you can see the operation of the program before and after it is infected.

Before infection

1. Instructions 1

2. Instruction 2

3. Instruction 3

4. Direction n

Completion of the program

After infection

1. Go to Virus Instructions 1

2. Host program

3. Host Instructions 1

4. Host Instructions 2

5. Host Instructions 3

6. Host instructions n

7. Termination of Host Program

8. Virus Program

9. Virus Instructions 1

10. Virus Instruction 2

11. Virus Instructions 3

12. Virus Directive n

13. Go to host instruction 1

14. Termination of Virus Program

3.2 How does a boot sector infector work?

On hard disks, track 0, sector 1 is known as the master boot record. The MBR contains a program as well as data describing the hard disk being used. A hard disk can be divided into one or more partitions. The first area of ​​the partition containing the OS is the boot area.

A boot sector infector program is slightly more advanced than a virus, as it attacks an area of ​​the disk that is normally restricted to the user. To understand how a Boot Sector Infector (BSI) works, one must first understand something called the boot-up process. This sequence of steps begins when the power switch is pressed, thereby turning on the power supply. The power supply starts the CPU, which in turn executes the ROM program known as the BIOS. The BIOS tests the system components, and then executes the MBR. The MBR locates and executes the boot sector that loads the operating system. The BIOS does not check what is programmed in track 0, sector 1; It just goes there and executes it.

To prevent the following diagram from getting too large, boot area will refer to both the boot area and the MBR. A boot sector infector moves the contents of the boot sector to a new location on the disk. It places itself in the original disk location. The next time the computer boots, the BIOS will go into the boot sector and execute the virus. The virus is now in memory and can remain there until the computer is turned off. The first thing a virus does is execute the program in its new location, the boot sector. This program will then load the operating system and everything will continue as normal except that there is a virus in memory. The boot-up process, before and after the viral infection, can be seen below.

Before infection

1. Press the power switch

2. The power supply starts the CPU

3. The CPU executes the BIOS

4. BIOS test components

5. BIOS executes the boot sector

6. Boot Sector Load OS

After infection

1. Press the power switch

2. The power supply starts the CPU

3. The CPU executes the BIOS

4. BIOS test components

5. BIOS executes the boot sector

6. BSI executes original boot sector program at new location

7. The original boot sector program loads the OS (the BSI remains in memory when the boot-up process is complete)

BSI = Boot Sector Infector

4. Stealth viruses

Another way of classifying viruses relates to the way they hide within the host, and applies to program and boot sector viruses. A regular virus infects a program or boot sector and then resides there. A special type of virus, known as a stealth virus, encrypts itself when it hides inside another program or the boot sector. However, an encrypted virus is not executable. Therefore, the virus leaves a small tag hanging out that is never encrypted. When the host program or boot sector is executed, the tag takes control and decodes the rest of the virus. A fully decoded virus can either perform its infecting and reproduction functions or perform its delivered payload function, depending on how the virus is written.

An advanced form of stealth virus is a polymorphic stealth virus, which uses a different encryption algorithm each time. The tag, however, must not be encrypted in any way. Otherwise, it will not be executable and the rest of the virus will be unable to decode.

5 logic bombs

Viruses are often programmed to wait until a certain condition is met before delivering their payload. Such situations include: after it reproduces a certain number of times, when the hard disk is 75% full, etc. These viruses are known as logic bombs because they wait until a logical condition is true before delivering the payload.

5.1 Time Bomb

The term time bomb is used to refer to a virus that waits until a certain date and/or time before delivering its payload. For example, some viruses close on Friday 13th, April 1st, or October 31st. The trigger date for the Michelangelo virus was March 6. Waiting until a certain date and/or time before delivering the payload means that a time bomb is a special type of logic bomb (discussed earlier) because waiting for a date/time means the virus is waiting for a logical condition to become true. There is considerable overlap in these regions that describe viruses. For example, a particular virus can be a program virus, and a polymorphic stealth virus. Another virus can be boot sector infector, stealth virus and time bomb. Each term refers to a different aspect of the virus.

II in more malicious code

1 Trojan Horses

A Trojan horse is an independent program and a form of malicious code. It is not a virus but a program that pretends one thing but actually does something else. The user is misled by the name of the program which lures unsuspecting users to run it, and once executed, a piece of malicious code is invoked. Malicious code can be a virus, but it doesn’t have to be. It can simply be a few instructions that are neither infectious nor self-replicating but provide some sort of payload. A Trojan horse from the DOS days was SEX.EXE that was intentionally infected with viruses. If you found a program with this name on your hard disk, would you run it? When the program was loaded, some interesting images appeared on the screen to distract you. Meanwhile, an embedded virus was infecting your hard disk. After some time, the third function of the virus scrambled the FAT (File Allocation Table) of your hard disk, which means you cannot access any of your programs, data files, documents, etc.

A Trojan horse can find its way onto your hard disk in different ways. The most common include the Internet.

– It can download without your permission while you are downloading something else.

– It may download automatically when you visit certain websites.

– This can be an attachment to the email.

As mentioned earlier, the filename of the Trojan horse lures unsuspecting users to run it. If the Trojan horse is an attachment to the email, the subject line of the email can also be written to entice the user to run it. For example the subject line “You’ve won $5 million!” may be And the file name of the attachment might be “million dollar winner.exe”.

2 insects

Worms are not viruses. Rather, it is a form of malicious code that reproduces and delivers a payload but is not infectious. It is an independent program that exists on its own like a Trojan horse or any regular program. A virus cannot exist on its own. Worms do not infect programs, but they do reproduce, and are usually propagated using Trojan horse technology.

3 Deliver the payload – what can malicious code do?

– Display messages or graphics on the screen, such as lots of crabs that eat and destroy what they find. This very old virus was called Crabs.

– Demanding that the user perform a certain function such as pressing a certain sequence of keys before allowing normal operation to resume. An example of this is the Cookie Monster virus, in which Cookie Monster will appear on your screen and demand a cookie before returning control of your computer to you. You must reply by typing a cookie. A few minutes later, he reappears and demands another cookie.

– Causing the computer and/or mouse to lock up and become inactive until the system is rebooted.

– Redefining the keyboard (press r and ak, etc.).

– To operate the computer at a fraction of its regular speed.

– Deleting one or more files from the computer.

– The contents of data files are altered or corrupted (subtly or otherwise), often in a way that is almost undetectable to the user until a much later date. For example, malicious code can move the decimal point in a spreadsheet budget file, or change the first word of each paragraph in a word processor file to “gotcha!” can change in

III Preventive maintenance

The best way to avoid becoming a victim of a virus attack is to prevent your system from contracting viruses. By taking simple, precautionary measures, you can reduce the chances of your system ever becoming infected.

– Install antivirus software. I recommend Avast Free Antivirus. It’s free, has comprehensive protection and works well.

– Only visit websites you trust

– Make a backup of your data

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