Denial of Service Attacks

A denial of service attack (DoS attack) floods a server or online service with so many requests that genuine users cannot use it. The attacker is not trying to steal anything: the aim is simply to make the service unavailable.

How the attack works

A server can only handle a certain number of requests at once. Each request, such as a page load, a search or a login attempt, uses a slice of the server's processing power and memory. In a DoS attack, a huge number of fake requests arrives at the same time, often sent by software rather than by a person. The server tries to answer every one, so its resources are used up on fake traffic. Real customers trying to buy a ticket or check their account get a slow page, a timeout, or nothing at all.

If the flood is big enough, the service can crash and stay offline until staff restart it or block the traffic. The attacker never needs a password or any inside knowledge: the attack works by volume alone.

Why a server is the target

The target is a server or service used by many people, such as a school's learning platform or a cinema's booking site. Taking one personal laptop offline would achieve almost nothing, because only its owner is affected. Knocking out a shared service causes real disruption, and one attack can affect thousands of people at once.

What the attacker gains

The purpose is disruption. A shop whose site goes down on a sale day loses sales while customers give up and go elsewhere, and it may have to pay staff to fix the problem. Repeated outages make customers lose trust, so the company's reputation suffers too. Because the aim is to stop the service working rather than to take data, a DoS attack has a different purpose from attacks such as data interception and theft.