In a Denial-of-Service (DoS) attack, attackers take advantage of many hosts across the Internet, which they had previously compromised, to launch a brute-force attack that starves the target of its essential resources.
The objective of a DoS attack is to deprive organizations of access to the services or resources. If a website is hit by a DoS attack, millions of users are denied access to the website. As such, DoS attacks do not aim at intruding a network or information theft. By preventing authorized and legitimate access, DoS attacks cause aggravation and cost to the target customer. Distributed Denial-of-Service (DDoS) uses multiple compromised systems to launch an attack. This amplifies the effect of an attack.
Some of the DoS attacks are hard to defend because DoS packets appear to be normal packets. Most DoS attacks use spoofing and flooding techniques to impact network infrastructure.
The degraded service and lost business from a DoS attack leads to staggering costs both during and after an attack. For an e-commerce site like eBay or Buy.com, one day of disruption due to a DoS attack can result in huge revenue loss. The SQL Slammer worm, a DoS attack that made mission-critical Microsoft® SQL servers inaccessible, cost corporations billions of dollars worldwide. Beyond the immediate costs, the lasting effects of a successful DoS attack include lost customers, loss of faith in the service’s dependability, and damage to the corporate brand.