It happened last night, and for about six hours prevented some customers — no word on how many — from being able to use the Web. Cablevision says that it was a Distributed Denial of Service attack, which means that someone flooded its network with communications, causing it to slow down or grind to a halt. But the company says it has fixed the problem and is trying to find out what caused it. Cablevision has nearly 3M broadband customers, most concentrated in New York City and its suburbs. Here’s its release:

BETHPAGE, NY, November 30, 2011 — Cablevision Systems Corporation (NYSE: CVC) is alerting customers that it experienced a Distributed Denial of Service (DDoS) attack on the Optimum Online network during the evening hours Tuesday night. The company announced that the attack has been remediated and resolved. It began at approximately 6 p.m. Tuesday and was resolved shortly after midnight, when all service returned to normal.

The attack Tuesday night caused a disruptive increase in automated requests on a portion of the network, which impacted the ability of some customers to access certain websites and other Internet services.
DDoS attacks have been directed at several leading technology companies in recent months. Cablevision is continuing to investigate the cause of the attack.

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