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What is a Denial-of-Service Attack?

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You may have heard news reports about popular websites such as CNN, Amazon and Yahoo! being taken down by a DoS attack, but have you ever wondered what DoS means?

This common tech term stands for “denial-of-service,” where an attacker attempts to prevent legitimate users from accessing a website entirely or slowing it down to the point of being unusable. The most common and obvious type of DoS attack occurs when an attacker “floods” a network with useless information.

When you type a URL for a particular website into your browser, you are sending a request to that site’s computer server to view the page. The server can only process a certain number of requests at once, so if an attacker overloads the server with requests, it can’t process your request. The flood of incoming messages to the target system essentially forces it to shut down, thereby denying access to legitimate users.

A distributed denial-of-service (DDos) attack is one where a site is attacked, but not by just one person or machine. DDos are attacks on a site by two or more persons or machines. These attacks are usually done by cybercriminals using botnets (remote computers that are under their control), to bombard the site with requests. Cybercriminals create botnets by infecting a collection of computers—sometimes hundreds or thousands—with malware that gives them control of the machines, allowing them to stage their attack.

There is also an unintentional DoS where a website ends up denied, not due to a deliberate attack by a single individual or group of individuals, but simply due to a sudden enormous spike in popularity. This can happen when an extremely popular website posts a prominent link to a second, less well-prepared site, for example, as part of a news story. The result is that a significant proportion of the primary site’s regular users–potentially hundreds of thousands of people—click that link in the space of a few hours, having the same effect on the target website as a DDoS attack. When Michael Jackson died in 2009, websites such as Google and Twitter slowed down or even crashed.1

While this can be an inconvenience to you, as you may not be able to complete transactions or access your banking site, there’s no real danger for you. But unbeknownst to you, your computer or mobile device could be part of the botnet that is causing a DDos attack.

To make sure you’re not part of a DDos attack:

  • Pay attention if you notice that your Internet connection is unusually slow or you can’t access certain sites (and that your Internet connection is not down)
  • Make sure you have comprehensive security installed on all your devices, like McAfee LiveSafe™ service
  • Be careful when giving out your email address, clicking on links and opening attachments, especially if they are from people you don’t know
  • Stay educated on the latest tactics that hackers and scammers use so that you’re aware of tricks they use

1“Web slows after Jackson’s death”. BBC News

Robert Siciliano is an Online Security Expert to McAfee. He is the author of 99 Things You Wish You Knew Before Your Mobile was Hacked! Disclosures.

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