Security is a degree of protection of property from danger, damage, loss or criminal activity. The fundamental elements of security are: authentication, authorization, auditing, confidentiality, integrity and availability. Flaws in the development of software solutions undermine the security of web application. The most frequent attacks are: SQL injection attatck, cross-site scripting attack (XSS attack), cross-site request forgery attack (CSRF attack), and broken authentication and session management. For successful protection against attacks, it is necessary to develop mechanisms that will validate, filter, and protect the data that users exchange with web application.