In the very introduction to the paper, I describe information technologies and their great progress, which has influenced the better development of companies. Data security is very important for companies, and there are more and more tools to protect the company's information structures, so that data theft and misuse do not occur. The second part of the paper deals with computer security and the prevention of malicious actions in order to obtain user data. In order for the user to protect his data and unwanted intrusions into the system, he must use some of the security measures and procedures for data protection. The second part describes some categories of user security attacks. The third part describes some of the malicious programs, and their classification. The dangers to the user caused by such malware are also described. The fourth part refers to the attackers, and their motives, because not all attackers are malicious. Some system attackers try to help certain companies find vulnerabilities in their systems, and to protect themselves from real attackers at the right time. The most comprehensively described fifth part talks about the latest technologies for preventing and detecting the danger of intrusion into a computer system. The most used Snort system for prevention and detection of intrusions into the system, as well as its installation and use are described. The last part of the paper refers to the selection of companies' business with the cloud service, and the advantages that the cloud service itself brings. Finally, there is a list of references, lists of Internet sources, lists of images and finally a summary of this paper.