Denial of Service attacks aren't technically "hacks," since they can be done without breaking into any systems. Typically, DoS attacks overwhelm a website's servers by flooding them with requests. That makes websites unreachable or unresponsive.
To bring down bigger sites, attackers will sometimes use botnets, or large numbers of infected computers, to send requests all at once. There's not much that Web companies can do to prevent that kind of attack except to get more servers.
"DoS attacks are very difficult to defend against," said Paul Asadoorian, CEO of security consultancy PaulDotCom Enterprises. "But attackers are not stealing any information when they do a DoS."
Large, organized crime syndicates have been launching sophisticated attacks for decades.
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