Denial-of-service (DoS) attacks have long been considered the blunt wooden club of online hazards -- a multigigabit stream of shock and awe.

Yet increasingly the noisy attacks are being used to hide more subtle infiltrations of a target's network. A number of financial institutions, for example, have been targeted by distributed denial-of-service (DDoS) attacks immediately following a wire transfer, according to security firms familiar with the cases. The attacks, generated by computers infected with the DirtJumper DDoS malware, attempt to disrupt any response to the fraudulent transfer of funds, which are usually in the six-figure dollar range, according to a report by Dell Secureworks published in April.

"The analogy is signal jamming," says Kevin Houle, director of threat intelligence for managed security provider Dell SecureWorks. "To the extent that you can use the DDoS attack to do cause chaos electronically, to prevent access to particular systems during an attack, the tactic has proved successful."

While DirtJumper has focused on causing chaos immediately following money transfers, the technique could be generalized to other attack scenarios. A variation of the attack has been used by Iranian hacktivists groups to disrupt the online operations of U.S. financial institutions by hiding more subtle application-layer attacks within larger packet floods. And South Korean companies were flooded with data while malware deleted information on organizations' servers.

"Your goal is to sow confusion," says Vann Abernethy, a senior product manager at NSFOCUS, a DDoS mitigation firm. "A DDoS attack is designed to get your IT department to run around like their hair is on fire."

[While distributed denial-of-service attacks topping 100 Gbps garner the headlines, they are not the threat that should worry most companies. See Large Attacks Hide More Subtle Threats In DDoS Data.]

In addition, noisy DDoS attacks could attract more attackers, says Terrence Gareau, principal security architect for Prolexic, a DDoS mitigation firm. A very public attack could convince other groups to attempt their own operations in the chaos, he says.

"If it's a very public attack, then there is a high probability that other opportunistic attackers could take part as well," Gareau says. "Opportunistic criminals will say, wow they are under a DDoS attack, so let's look at the network and see what changes have been made."

Companies need to structure their response group to handle a large infrastructure attack, but not be blinded by the influx of alerts to their system. Like magicians, the goal of the attackers is to force the security staff to only pay attention to a distraction to keep them from discovering the actual trick.

"You almost have to have a team that deals with the infrastructure attack, and a separate group that goes into hypervigilance to find any other attacks coming in," says NSFOCUS's Abernethy.

A third-party provider, which can use intelligence from attacks on other customers to more quickly identify new attacks, can help eliminate much of the inbound attack traffic, dialing down the volume of alerts that the security team has to process. The level of alerts seen by a security team during a DoS attack can increase by an order of magnitude. Filtering them out at the edge of the Internet can greatly reduce the impact on a business' network and employees.

"If you don't have to have all those alerts on your network, you can pay attention to what matters," Prolexic's Gareau says. "Using a third-party mitigation provider can significantly reduce the noise."

Yet attacks that use a variety of traffic and techniques in a short time period can cause problems for DoS mitigation firms, says Lance James, head of intelligence for Vigilance, a threat information firm that is now part of Deloitte.

"They are not perfect," James says. "We still see major banks going down. But they do well against long-period-term DDoS attacks."

While DirtJumper, also known as Drive, is not the only botnet that is used for combined attacks, it is a popular one. DirtJumper has a half dozen ways of attacking infrastructure, including flooding Web sites with GET requests and POST requests, targeting infrastructure with two types of IP floods, and using UDP packets to slow down networks.

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