Security Experts:

Large DNS Text Records Used to Amplify DDoS Attacks: Akamai

Attackers are increasingly leveraging large Domain Name System (DNS) TXT records in an effort to amplify the impact of their distributed denial-of-service (DDoS) attacks, Akamai's Prolexic Security Engineering and Research Team (PLXsert) warned on Tuesday.

In October, researchers noticed that cybercriminals were magnifying DNS amplification and reflection attacks by crafting large TXT records. These DNS TXT records provide text information, which is either human readable or machine readable, to sources outside a domain.

According to the advisory published by PLXsert, several campaigns observed last month used pieces of text taken from White House press releases to flood servers with junk traffic and overwhelm them.

The attackers appear to be using a tool called "DNS Flooder" in their operations. In the October attacks, the TXT records came from the domain, PLXsert said.

"Malicious requests for can be observed in the wild on an ongoing basis. These   requests attempt to use open resolvers as intermediate victims to reflect attack traffic back to a target. For the most part, the usefulness of these malicious domains drops off after a few days as server admins begin to block off the requests," the advisory reads.

This isn't the first time this technique has been used to amplify DDoS attacks. In the past, attackers leveraged the method against the Internet Systems Consortium (ISC) and several United States Government websites.

However, the latest attacks are different because the malicious actors have crafted TXT records to provide the largest response size possible.

The largest DDoS attack observed by PLXsert last month peaked at 4.3 Gbps. The primary targets of the campaigns were organizations in the entertainment (75%), education (12.5%), and high-tech consulting (12.5%) industries. The attacks lasted roughly between 7 and 17 hours, the advisory shows.

Akamai advises organizations to defend themselves against such attacks by using cloud-based protection services.

"DNS reflection attacks can be blunted at the network edge. An access control list (ACL) would suffice but only in cases where available bandwidth exceeds attack size," explained Bill Brenner, Senior Program Manager for Editorial, Information Security Group at Akamai. "Some DNS servers will attempt to retry the response using TCP, but when the request is sent to the target host, no transfer will occur and the attempt will fail."

In an advisory published last month, PLXsert reported seeing a spike in reflection and amplification DDoS attacks abusing communication protocols found on Universal Plug and Play (UPnP) devices.

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Eduard Kovacs (@EduardKovacs) is a contributing editor at SecurityWeek. He worked as a high school IT teacher for two years before starting a career in journalism as Softpedia’s security news reporter. Eduard holds a bachelor’s degree in industrial informatics and a master’s degree in computer techniques applied in electrical engineering.