Malicious code poses a real danger to modern systems. Today, most systems do not operate in isolation; they are more likely to be connected to and sometimes even dependent on other systems. Therefore, an attack on one of the network's systems constitutes a potential attempted attack on any other system with which it interacts. Therefore, it is inevitable that any networked or internet-connected computer will face malicious code attacks at some point. Businesses lose billions of dollars every year to malicious code attacks. Responding to the attack and restoring all data on computers is a time-consuming and expensive task. It is much better to try to prevent it by organizing and maintaining effective defenses. However, it is important to keep in mind that there is no single general solution to prevent all attacks. Attackers are constantly looking for new ways to exploit system vulnerabilities and discover new ones. This is why organizations must not only defend against existing attack methods, but also try to predict and prevent new attack techniques. This means that computer and network security is a never-ending challenge and expense. Malicious code, also called malware, installed on a computer allows an attacker to control that machine. Therefore, malware can be defined as “a set of instructions that execute on your computer and force your system to do something that an attacker wants to do.” It is more common to see malicious code implemented in binary executable files. However, it can be implemented in almost any computer language. The attackers used a large number of scripting languages, word processing macro languages ​​and a middle of paper...... e Trojan, users should be familiar with all programs that would normally run run on their computer. systems. Once there is an addition or change, it is easy to notice and investigate. Another technique often used by attackers is to use wrapper programs, which combine both malicious code and original code. Once executed on the victim's computer, the program first installs the malicious code and then executes the original code. Works CitedErbschloe, Michael “Trojans, Worms, and Spyware. A Computer Security Professional's Guide to Malicious Code", Elsevier Inc, 2005Nazario, Jose "Internet Worm Defense and Detection Strategies", Artech House Computer Security Library, 2004Skoudis, Ed, Zeltser, Lenny "Malware: Fighting Malicious Code", Prentice Hall, 2003Zesheng Chen, Lixin Gao, Chuanyi Ji « On the effectiveness of defense systems against worms”