TCP DDoS Reflection Attacks on the Rise

Radware has found itself in 2019 defending against what it calls a "steady" rise in TCP DDoS reflection attacks. Since low-bandwidth TCP reflection attacks were thought in the past not to be able to generate enough amplification compared to the more used UDP-based reflection method, this is a definite change in attacker strategy.

The company has issued a report, TCP Reflection Attacks, which reveals what it is finding.

In a TCP SYN-ACK reflection attack, an attacker will send a spoofed SYN packet (that has the original source IP replaced by the victim’s IP address) to a wide range of random or pre-selected reflection IP addresses. If the victim does not respond with the last ACK packet the reflection service expects from it, the service will continue to retransmit the SYN-ACK packet giving amplification if there are multiple services involved.

In October, Radware found that sizable amounts of TCP SYN requests from what appeared to be legitimate sources have appeared and indicated an ongoing blacklist spoofing or reflection DDoS campaign. Eurobet was DDoSed in such a manner.

This follows an April TCP-based DDoS attack on financial institutions that experienced a wave of attacks that could be observed externally, independently of the targeted network. These institutions became the target of multi-vector campaigns that produced bursts of high bandwidth and throughput with both UDP and TCP protocols. Such high volumes of TCP in a DDoS situation had not really been seen previously.

Later in October, a multi-vector campaign appeared which targeted the financial and telecommunication industry in Turkey and exhibited the same patterns seen in previous TCP-based campaigns. Multi-vector style attacks were becoming the new normal.

Carpet bombing (attacks that use multiple vectors of attack to target a specific device or service) has been used in a number of notable attacks against South African ISPs. It can be leveraged to increase the hit rate onto the victim’s services and devices that do not respond with RST or ICMP unreachable/host prohibited messages. In the case of the South African ISP attacks, it was used to sidestep DDoS mitigation and target ISP customers.

Radware says that currently, "Over the last 30 days, Radware has observed a number of criminal campaigns that have been abusing the TCP implementation by performing TCP reflection attacks against large corporations. The attacks not only impacted the targeted networks, but also disrupted reflection networks across the world, creating a fallout of suspected SYN-flood attacks by many businesses."

Trying to reset the affected TCP ports didn’t seem to work as mitigation. They seemed to have tried that one. The report says that, "most of the targeted networks did not respond properly to spoofed requests using RST packets. This would have disabled the TCP retransmit amplification. As a result, the TCP reflection attacks had a major impact on the targeted network and also the reflectors used around the world."

The method gums up the legitimate functions of the TCP reflection networks when it being abused for attack purposes, but there is also a secondary effect. The IP addresses of the reflectors are noticed by other networks (usually enterprise ones) and assumed to be in some faulty state because of their irregular behavior. As a response the enterprise may automatically blacklist that IP address in the future, thereby crippling itself on future TCP reflection.

This attack vector is not new, but has been seriously underutilized in the past. While it’s not something an untrained threat actor can use with ease thus far, some other actor may offer the technique as a service. It should be seriously accounted for in an enterprise’s security posture.

— Larry Loeb has written for many of the last century's major "dead tree" computer magazines, having been, among other things, a consulting editor for BYTE magazine and senior editor for the launch of WebWeek.