Denial of Service (DoS)
This class of attacks (DoS (Denial of Service, DoS) aims to breach the availability of the Web server. Usually attacks against denial of service being implemented at the network level, however, they can be sent and the application layer. By using the Web Applications An attacker can exhaust the critical resources of the system, or to exploit this vulnerability, leading to the cessation of operation of the system.
Typically, DoS attacks are aimed at the exhaustion of critical system resources such as processing power, memory, disk space or bandwidth communication channels. If some of the resources to reach the maximum load, the entire application will be unavailable.
Attacks may be directed to any of the components of Web-based applications, such as database server, authentication server, etc.
If the attack is carried out simultaneously with a large number of computers, then it means that we are talking about DDoS-attack. In some cases, a DDoS-attack causes legitimate action, for example, publishing a link to a website on a popular online resource. Large influx of users that exceed the load on the server which causes a denial of service.
See more information about DDoS attack in “Basics of DoS / DDoS attack Protection – How to Reduce the Risk“