Akamai’s Security Intelligence Response Team (SIRT) recently issued an advisory warning for a new spate of distributed denial-of-service (DDoS) attacks leveraging the Trivial File Transfer Protocol (TFTP).
As of April 20, 2016, Akamai had mitigated 10 attacks that were using this method. It reported most of the campaigns consisted of multivector attacks that included TFTP reflection. The security firm also found indications that this method may have been integrated into at least one site offering DDoS-as-a-service.
Why the Trivial File Transfer Protocol?
According to Akamai, TFTP has been around for a long time. It was intended to be used for file transfers of firmware and configuration files, typically for networked devices. However, the simple design of the protocol omits features such as authentication and directory listing capabilities.
The attack engages TFTP servers connected to the internet. It makes a default request for a file, and the victim TFTP server returns data to the requesting target host as a result of this request regardless of a file name mismatch. It spends time performing a wasted effort, which is just what the attackers want to happen.
TFTP only sends out data in specific block sizes and requires acknowledgment of each block being received. Akamai explained that because the target of the DDoS attack will never acknowledge the data being exfiltrated, only the first block is sent. This mitigates the potential of higher amplification based on single requests.
Some observed attacks had several data blocks attached to them, which could have greatly increased how much wasted effort the TFTP server expended.
SecurityWeek noted that Akamai’s Jose Arteaga said a weaponized version of the TFTP attack script started circulating in March, around the same time the media began highlighting research into this attack.
A Simple Mitigation Technique
In its advisory, Akamai recommended a few mitigation techniques. “For those hosting TFTP servers, assess the need to have UDP port 69 exposed to the internet. This should be firewalled and only allowed to trusted sources,” it said.
Intrusion detection systems can also help flag suspicious activities on the network.