This thesis deals with the topic of security of information systems. As part of the work, information on information systems, security of information systems and methods of protection of information security are presented. Information security is a set of measures and techniques used to control and protect all data handled. In information security, the word vulnerability refers to a weakness in a system that allows an attacker to compromise the confidentiality, integrity, availability, access control, and consistency of the system or its data and applications. Vulnerabilities are the result of errors or flaws in system design. In a broader sense, they can also be the result of technological limitations. When large amounts of data or information are stored electronically, they are vulnerable to many types of threats. The lack of security of access or also the lack of security in the way of communicating and transferring information on the Internet can be dangerous to the extent that hackers can take this information.