Security Experts:

Nitol Botnet Fuels 8.7 Gbps Layer 7 DDoS Attack

A recent layer 7 distributed denial of service (DDoS) attack managed to break all previous known records in terms of bandwidth consumption, peaking at 8.7 gigabits per second (Gbps), cyber security solutions provider Imperva warns.

Layer 7 DDoS attacks – or HTTP floods – rely on crippling servers by depleting their resources at the application layer, and not the network layer, which means they aren’t usually large in terms of packet volume. With many web applications capped at hundreds of requests per second (RPS), even small attacks can take down unprotected servers.

While network layer DDoS attacks can easily peak at over 150 gigabit-per-second (Gbps), and some even go beyond the 500 Gbps mark, application layer attacks use much less bandwidth. Although they have been growing in terms of RPS rates as of late, and even surpassed 250,000 RPS in the third quarter of 2015, layer 7 attacks remained well below 500Mbps.

In this context, the newly observed HTTP POST flood attack, which peaked at the high rate of 163,000 RPS, broke all records in terms of bandwidth footprint by reaching 8.7 Gbps. This was not only a record for an application layer attack, but also the largest the security company “had ever seen or even heard about up until that point,” Imperva’s Igal Zeifman notes in a blog post.

The attack targeted a Chinese lottery website and was launched from a botnet powered by a Nitol malware variant, which was accessing the target website disguised as a Baidu spider. The attack originated from roughly 2,700 IP addresses, over 2,500 of which are located in China, the researchers said.

By analyzing the malicious POST requests, they also discovered that the attackers used a script that randomly generated large files and the attempted to upload (POST) them to the server. Thus, they created a massive HTTP flood of extremely large content-length requests that initially appeared as legitimate.

The issue, Imperva says, is that no one expects an application layer DDoS attack to be that large, which makes mitigation difficult, especially when it comes to hybrid DDoS protection solutions. These solutions rely on an off-premise service for network tier threats and on a customer-premises equipment (CPE) to counter application tier attacks. 

While Imperva is somwhat picking on hybrid DDoS protection offerings, it's important to note that Imperva offers cloud-based DDoS protection via its Incapsula unit.

To successfully mitigate a Layer 7 attack, a CPE would require an uplink higher than the attack’s size, namely 10Gbps in the case of a 9Gbps attack. A smaller uplink would result in denial of service, since the connection gets clogged with DDoS requests that cannot be identified until they establish a connection with the appliance.

Because application layer traffic can be filtered only after the TCP connection has been established, a multi-gigabit application layer assault is an unforeseen threat, Imperva’s researchers say. Such attacks can succeed where much larger network layer attacks would fail, and organizations are advised to upgrade their uplink to ensure it can at least counter attacks below 10 Gbps.

Related: Britain's HSBC Recovers from Massive DDoS Attack

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