IDG News Service — A week-long DDoS attack that launched a flood of traffic at an Asian e-commerce company in early November was the biggest such incident so far this year, according to Prolexic, a company that defends websites against such attacks.

The distributed denial-of-service attack consisted of four consecutive waves launched from multiple botnets between Nov. 5 and Nov. 12, 2011, Prolexic said.

It estimated that up to 250,000 computers infected with malware participated in the attack, many of them in China.

At the height of the attack, those computers made 15,000 connections per second to the target company's e-commerce platform, swamping it with up to 45 Gbps of traffic, Prolexic said. It declined to name the company, one of its clients, citing a confidentiality agreement.

The reason for the attack is unknown, but a disgruntled user or a competitor performing industrial sabotage are two of the possibilities, said Prolexic CTO Paul Sop.

"Sometimes we also see a state-sponsored or state-complicit attack because of large amounts of out-of-country Internet payments for these e-commerce transactions. The state does not collect taxes for them and in some countries these e-commerce transactions are targeted," he said.

Rival DDoS mitigation vendor Arbor Networks didn't have information on this particular attack, but said that Prolexic's description is consistent with data collected recently.

The size of the attack described by Prolexic is plausible, and is just above what Arbor saw in the third quarter, said Jose Nazario, Arbor's senior manager of security research.

While this DDoS event might be the biggest so far this year, it is far from the largest of all time. The biggest attack observed by Arbor in 2010 peaked at over 100 Gbps, Nazario said.

Prolexic also said it had observed incidents exceeding 100 Gbps in the past. While attacks are typically less powerful now, their frequency has increased, the company said.

There are still thousands of botnets capable of taking out 99 percent of the websites on the Internet, said Sop.

Today's attackers prefer to use the combined power of smaller botnets instead of building large ones, he said. "If you run a huge monster botnet it is more likely to catch the eye of security professionals and law enforcement. Attackers know they can stay under the radar if their botnets are less than 50,000 in size."

This year, most of DDoS traffic came out of Asia, but the problem remains a global one. For example, last week Prolexic recorded a number of attacks that originated in Eastern Europe.

However, when counting the number of infected computers that participate in DDoS attacks, China and the U.S. take the top spots, the company said.