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DDoS Attack Trends for 2021 Q2

Cloudflare automatically detects and mitigates DDoS attacks across its global network using its autonomous edge DDoS detection and mitigation engine. This report includes the DDoS insights and trends as observed on our network.



DDoS attack trends for 2021 Q2

Recent weeks have witnessed massive ransomware and ransom DDoS (Distributed Denial of Service) attack campaigns that interrupted aspects of critical infrastructure around the world, including one of the largest petroleum pipeline system operators, and one of the world's biggest meat processing companies. Earlier this quarter, more than 200 organizations across Belgium, including the government and parliament websites and other services, were also DDoS'd.

And when most of the United States were celebrating Independence Day on July 4, hundreds of US companies were hit by a ransomware attack demanding 70 million USD in Bitcoin. Attackers known to be affiliated with REvil, a Russian ransomware group, exploited multiple previously unknown vulnerabilities in IT management software. The targets included schools, small public-sector bodies, travel and leisure organizations, and credit unions, to name a few. While the threat of ransomware and ransom DDoS is not new (read our posts on ransomware and ransom DDoS from 2021 Q1), the latest attacks on Internet properties ranging from wineries, professional sports teams, ferry services and hospitals has brought them from just being background noise to front page headlines affecting our day-to-day lives. In fact, recent attacks have propelled ransomware and DDoS to the top of US President Biden's national security agenda.

The DDoS attack trends observed over Cloudflare's network in 2021 Q2 paint a picture that reflects the overall global cyber threat landscape. Here are some highlights of DDoS attack trends observed in 2021 Q2.


Application-layer DDoS attacks

Application-layer DDoS attacks, specifically HTTP DDoS attacks, are attacks that usually aim to disrupt an HTTP server by making it unable to process legitimate user requests. If a server is bombarded with more requests than it can process, the server will drop legitimate requests or even crash resulting in performance penalties or a denial of service event for legitimate users.

DDoS activity per market industry

When we analyze attacks, we calculate the 'DDoS activity' rate, which is the percentage of attack traffic out of the total traffic (attack + clean). This allows us to normalize the data points and avoid biases towards, for example, a larger data center that naturally handles more traffic and therefore also more attacks.

In 2021 Q2, Consumer Services was the most targeted industry followed by Government Administration and Marketing & Advertising.

DDoS activity per source country

To understand the origin of the HTTP attacks we observed over Cloudflare's network, we look at the source IP address of the client generating the attack HTTP requests. Unlike network-layer attacks, source IPs cannot be spoofed in HTTP attacks. A high DDoS activity rate in a given country indicates large botnets operating from within.

China and the US remain in the first and second places, respectively, with regards to the percentage of DDoS activity originating from within their territories. In China, more than 7 out of every 1,000 HTTP requests were part of an HTTP DDoS attack, while in the US almost 5 out of 1,000 HTTP requests were part of an attack.

DDoS activity per target country

In order to identify which countries the targets of the DDoS attacks resided in, we break down the DDoS activity by our customers' billing countries. Note that Cloudflare does not charge for attack traffic and has pioneered providing unmetered and unlimited DDoS protection since 2017. By cross-referencing the attack data with our customers' billing country, we can identify which countries were attacked the most.

Data observed in 2021 Q2 suggest that organizations in the US and China were the most targeted by HTTP DDoS attacks. In fact, one out of every 200 HTTP requests destined to US-based organizations was part of a DDoS attack.


Network-layer DDoS attacks

While application-layer attacks strike the application (Layer 7 of the OSI model) running the service end users are trying to access, network-layer attacks target network infrastructure (such as in-line routers and other network servers) and the Internet link itself.

The chart above shows the distribution of network-layer DDoS attacks in 2021 Q2.

Distribution of attacks by size (packet rate and bit rate)

There are different ways of measuring the size of a L3/4 DDoS attack. One is the volume of traffic it delivers, measured as the bit rate (specifically, gigabits-per-second). Another is the number of packets it delivers, measured as the packet rate (specifically, packets-per-second). Attacks with high bit rates attempt to saturate the Internet link, while attacks with high packet rates attempt to overwhelm the servers, routers or other in-line hardware appliances.

The distribution of attacks by their size (in bit rate) and month is shown below. As observed in the chart, all attacks over 300 Gbps were observed in the month of June.

In terms of bit rate, attacks under 500 Mbps constituted a majority of all DDoS attacks observed in 2021 Q2.

Similarly, looking from the lens of packet rate, nearly 94% of attacks were under 50K pps. Even though attacks from 1-10M pps constituted only 1% of all DDoS attacks observed, this number is 27.5% higher than that observed in the previous quarter, suggesting that larger attacks are not diminishing either -- but rather increasing.

Note that while attacks under 500 Mbps and 50K pps might seem 'small' compared to other headline-making large attacks, they are often sufficient to create major disruptions for Internet properties that are not protected by an always-on, automated cloud-based DDoS protection service. More so, many organisations have uplinks provided by their service providers with a bandwidth capacity smaller than 1 Gbps. Assuming their public-facing network interface also serves legitimate traffic, DDoS attacks smaller than 500 Mbps are often capable of taking down exposed Internet properties.

Distribution by attack duration

Cloudflare continues to see a large percentage of DDoS attacks that last under an hour. In Q2, over 97% of all DDoS attacks lasted less than an hour.

Short burst attacks may attempt to cause damage without being detected by DDoS detection systems. DDoS services that rely on manual analysis and mitigation may prove to be useless against these types of attacks because they are over before the analyst even identifies the attack traffic.

Alternatively, the use of short attacks may be used to probe the cyber defenses of the target. Load-testing tools and automated DDoS tools, that are widely available on the dark web, can generate short bursts of a SYN flood, for example, and then follow up with another short attack using a different attack vector. This allows attackers to understand the security posture of their targets before they decide to launch larger attacks at larger rates and longer durations - which come at a cost.

In other cases, attackers generate small DDoS attacks as proof and warning to the target organization of the attacker's ability to cause real damage later on. It's often followed by a ransom email to the target organization, demanding payment to avoid suffering an attack that could more thoroughly cripple network infrastructure.

This highlights the need for an always on, automated DDoS protection approach. DDoS protection services that rely on manual re-routing, analysis and mitigation may prove to be useless against these types of attacks because they are over before the analyst can even identify the attack traffic.

Distribution of attacks by attack vectors

An attack vector is the term used to describe the method that the attacker utilizes in their attempt to cause a denial of service event.

As observed in previous quarters, attacks utilizing SYN floods and UDP-based protocols remain the most popular methods by attackers.

What is a SYN flood attack? It's a DDoS attack that exploits the very foundation of the TCP protocol. A stateful TCP connection between a client and a server begins with a 3-way TCP handshake. The client sends an initial connection request packet with a synchronize flag (SYN). The server responds with a packet that contains a synchronized acknowledgment flag (SYN-ACK). Finally, the client responds with an acknowledgment (ACK) packet. At this point, a connection is established and data can be exchanged until the connection is closed. This stateful process can be abused by attackers to cause denial of service events.

By repeatedly sending SYN packets, the attacker attempts to overwhelm a server or the router's connection table that tracks the state of TCP connections. The router replies with a SYN-ACK packet, allocates a certain amount of memory for each given connection, and falsely waits for the client to respond with the final ACK. Given a sufficient number of connections occupying the router's memory, the router is unable to allocate further memory for legitimate clients, causing the router to crash or preventing it from handling legitimate client connections, i.e., a denial of service event.

Emerging threats

Emerging threats included amplification DDoS attacks that abuse the Quote of the Day (QOTD) service which increased by 123% QoQ. QOTD was defined in RFC-865 (1983) and can be sent over either the UDP or TCP protocols. It was originally designed for debugging and as a measurement tool, with no specific syntax for the quote. The RFC does however recommend the use of ASCII characters and to limit the length to 512 characters.

Furthermore, we've seen a 107% increase QoQ in UDP Portmap and Echo attacks -- all of which are really old attack vectors. This may indicate attackers digging up old methods and attack tools to try and overcome protection systems.

As we've seen in previous quarters, the adoption of the QUIC protocol continues to increase. Consequently, so do attacks over QUIC, or more specifically floods and amplification attacks of non-QUIC traffic in places where we'd expect to see QUIC traffic. In 2021 Q2, these types of attacks increased by 109% QoQ. This continued trend may indicate that attackers are attempting to abuse the QUIC-designated ports and gateways into organizations' networks -- searching for vulnerabilities and security holes.


DDoS activity by Cloudflare data center country

In 2021 Q2, our data centers in Haiti observed the largest percentage of network-layer DDoS attack traffic, followed by Brunei (almost 3 out of every 100 packets were part of an attack) and China. Note that when analyzing network-layer DDoS attacks, we bucket the traffic by the Cloudflare edge data center locations where the traffic was ingested, and not by the source IP. The reason for this is that, when attackers launch network-layer attacks, they can spoof the source IP address in order to obfuscate the attack source and introduce randomness into the attack properties, which may make it harder for simple DDoS protection systems to block the attack. Hence, if we were to derive the source country based on a spoofed source IP, we would get a spoofed country. Cloudflare is able to overcome the challenges of spoofed IPs by displaying the attack data by the location of Cloudflare's data center in which the attack was observed. We're able to achieve geographical accuracy in our report because we have data centers in over 200 cities around the world.

Network-layer DDoS Attacks by Country


A note on ransomware and ransom DDoS - a growing global threat

The last few weeks have seen a resurgence of ransom-driven cyber threats: ransomware and ransom DDoS (RDDoS).

So what is ransomware and ransom DDoS, and how are they different?

Ransomware is malicious software that encrypts an organization's systems and databases, rendering them inaccessible and unusable. Malware is usually introduced into an organization's systems via phishing emails -- tricking employees to click on a link or download a file. Once the malware is installed on the employee's device, it encrypts the device and can propagate to the entire network of the organization's servers and employee devices. The attacker will demand money, usually in the form of Bitcoin, in exchange for decrypting the organization's systems and granting them access back to their systems.

Unlike a ransomware attack, a ransom DDoS attack does not encrypt a company's systems; it aims to knock them offline if the ransom is not paid. What makes ransom DDoS attacks even more dangerous is that they do not require the attacker to gain access to a business's internal systems to execute the attack. However, with a strong DDoS protection strategy in place, a ransom DDoS attack has little to no effect on businesses.

Ransomware and ransom DDoS threats are impacting most industries across the globe --- the financial industry, transportation, oil and gas, consumer goods, and even education and healthcare.

Entities claiming to be 'Fancy Lazarus', 'Fancy Bear', 'Lazarus Group', and 'REvil' are once again launching ransomware and ransom-DDoS attacks against organizations' websites and network infrastructure unless a ransom is paid before a given deadline. In the case of DDoS threats, prior to the ransom note, a small DDoS attack is usually launched as a form of demonstration. The demonstration attack is typically over UDP, lasting roughly 30-120 minutes.

The ransom note is typically sent to the common group email aliases of the company that are publicly available online such as [email protected], [email protected], [email protected], [email protected], [email protected], etc. In several cases, it has ended up in spam. In other cases, we've seen employees disregard the ransom note as spam, increasing the organization's response time which resulted in further damage to their online properties.

Cloudflare's recommendation for organizations that receive a threat or ransom note:

  1. Do not panic, and we recommend you do not pay the ransom: Paying ransom only encourages and funds bad actors. There's also no guarantee that you won't be attacked again anyway.

  2. Contact local law enforcement: Be ready to provide a copy of the ransom letter you received and any other logs or packet captures.

  3. Activate an effective DDoS protection strategy: Cloud-based DDoS protection can be quickly onboarded in the event of an active threat, and with a team of security experts on your side, risks can be mitigated quickly and effectively.

Here's a short video by Cloudflare CTO, John Graham-Cumming addressing the threat of ransom DDoS attacks.