A sudden increase in a particularly dangerous type of distributed denial-of-service (DDoS) attack could portend big trouble for companies, according to VeriSign.

The company said that about 1,500 organisations worldwide were attacked earlier this year by unknown hackers who employed botnets and DNS servers to swamp networks with unmanageable torrents of data.

The attacks, which started on 3 January and ended in mid-February, were notable because they employed an especially devastating kind of DDoS attack, said Ken Silva, VeriSign's chief security officer.

Such an attack typically involves thousands of compromised zombie systems sending torrents of useless data or requests for data to targeted servers or networks - rendering them inaccessible for legitimate use.

In this case, attackers sent spoofed domain-name requests from botnets to DNS servers, which processed the requests and then sent replies to the spoofed victims, according to Silva.

"When the number of requests is in the thousands, the attacker could potentially generate a multigigabit flood of DNS replies" directed at the spoofed server, according to a description of such attacks on US-CERT website.

"This is known as an amplifier attack because this method takes advantage of misconfigured DNS servers to reflect the attack onto a target, while amplifying the volume of packets," the CERT description said.

Unlike typical DDoS attacks, the volume of data generated by amplifer attacks is greater by several orders of magnitude, Silva said. Though this type of attack is not especially new, the latest wave demonstrates how good hackers are getting at launching them.

"It really demonstrates how much horsepower these people have developed and how efficient they are at developing it," he said.