In this thesis I have described website vulnerabilities. The goal of the thesis is to educate and inform website developers about the importance of online security. To this end I have dedicated a chapter describing the top 10 website vulnerabilities according to OWASP. I described the main aspects of security and how to properly prevent system hijacks and data theft. The emphasis was given on how to properly generate user accounts, especially passwords – their saving, usage and resetting. I have described and developed a web application for user registration and logging by using Laravel, a web framework. I have chosen this framework because it is built systematically and because it offers basic level of protection out of the box. I have described how to prevent specific system intrusions and data theft. I have included examples of the test web application which clearly illustrates proper procedures of authentication, form submission, form security and the Laravel setup. In practical web developement it is very important that the developer is aware of the differences between the production and the test environment.