What is an ACK Flood Attack?

Introduction

According to new DDoS (distributed denial-of-service) attack trend research, DDoS attacks increased by 56% from 2023 to 2024, accelerating in the second half of 2024 by 17%. Additionally, one of the top types of DDoS attacks threat actors use are ACK flood attacks.

ACK flood DDoS attacks disrupt the communication connection with a server by overwhelming the targeted server with bogus ACK packets. These attacks can appear to be coming from an amateur threat actor because they require little technical knowledge to execute, but beware. They are actually a growing challenge because they are difficult to detect and are often used as a smokescreen for more damaging, malicious activities.  

In this blog, we’ll dig into the different ways an ACK flood attack can occur, and the impact this type of DDoS attack can have on organizations. We’ll also offer guidance on how to detect and protect against this cyber threat.

ACK Flood DDoS Attacks 

An ACK flood attack is a type of denial-of-service attack that takes advantage of the Transmission Control Protocol (TCP) used by devices to exchange data, to flood a server or firewall with illegitimate traffic. As with all DDoS attacks, the goal of this attack type is to deny service to legitimate users by slowing down or crashing the server. But before we get into more detail about these attacks, it’s important to understand what ACK stands for and its role in the communication process.

What is an ACK Packet?

ACK is short for “acknowledgement”. As part of the TCP three-way handshake at Layer 4 of the OSI model, an ACK packet is sent to acknowledge receipt of a message. Under normal circumstances, a TCP connection is established, and communication is enabled by exchanging packets, following a three-step process:

1. The client sends a SYN (synchronize) packet to the server to request to open a connection.
2. The server responds with a SYN/ACK packet to acknowledge the request by the client which creates a half-open connection.
3. The client acknowledges the SYN/ACK by sending an ACK packet to the server to complete the connection. At this point, the connection is opened for a certain period of time so that data can be exchanged. For example, if you were trying to access your bank’s website, the website would load, and you could start using services like online bill pay, deposits, and transfers.

How ACK Flood Attacks Work 

In an ACK flood attack, the threat actor abuses step 3 of the process and overwhelms the server with ACK packets. Often working through a botnet (a network of machines that have been hijacked for malicious purposes) the threat actor sends massive volumes of packets from spoofed IP addresses to maximize the impact of the attack. There are several aspects to ACK packets that make them attractive to threat actors.

1. ACK messages aren’t just sent to open the connection. Each time an image is uploaded, or a user completes a form and submits it, an ACK packet is sent.
2. Any TCP packet that has an ACK flag in the header is considered an ACK packet. This makes it easy for threat actors to send ACK packets that just contain the ACK flag in the header without having to create any content. This also makes it difficult for targeted organizations to detect illegitimate ACK packets.
3. Firewalls and servers need to process every packet, not just the initial ACK packet to open the connection.

Because of these characteristics and the ability for threat actors to use botnets to launch these attacks at scale, network bandwidth and resources can become saturated quickly with bogus ACK packets.

SYN-ACK Flood Attacks 

A SYN-ACK flood attack is a variation on an ACK flood attack. The difference is that the attacker either floods a network with SYN-ACK packets or floods numerous servers with SYN packets, triggering a large number of SYN-ACK responses in return. In either case, servers are unavailable to respond to new, legitimate packets which results in denial-of-service.

The Impact of ACK Flood Attacks

There are multiple ways that ACK flood attacks and SYN-ACK flood attacks can negatively impact networks, servers, and organizations.

Effects on Network Performance

• Network congestion. Overall performance of the network suffers due to the volume of ACK or SYN-ACK packets.
• Delayed responses. Legitimate users trying to gain access to services may experience slow response time.

Impact on Server Resources

• Resource strain. The server’s memory and processing power are consumed by a flood of bogus requests leading to system slowdowns.
• System outages. The server may crash and become entirely unavailable. Legitimate users find it difficult, if not impossible, to access applications, data, and ecommerce sites.

Damage to the Business

• Increased operational costs. Mitigation and dealing with the aftermath of an attack can require significant resources.
• Reputational damage. Slow response times and service outages can cause users and customers to lose trust in the service.
• Lost revenue. Because legitimate users have trouble accessing services, organizations can lose sales in the short term, and in the long run experience high levels of customer churn.
• Secondary attacks. Threat actors often use ACK flood attacks as smokescreens to disguise their real intentions. While the security team is distracted addressing the denial of service, the threat actor shifts their focus to execute potentially more damaging attacks such as data breaches and ransomware.

Mitigating ACK Flood DDoS Attacks

ACK flood attacks are difficult to detect because they closely mimic legitimate traffic and use botnets or spoofed IP addresses to bypass traditional defenses. However, there are several best practices and tools security teams can use to mitigate and prevent ACK flood attacks, including:

Network architecture and redundancy
Use load balancers and failover mechanisms that distribute traffic across different servers and cloud resources to help mitigate the impact of an attack and assist with continuity of service while IT teams investigate and respond. Additionally, leverage Content Delivery Networks (CDNs) to distribute your website’s content across multiple servers and locations. CDNs can absorb and distribute traffic, minimizing the impact of DDoS attacks on a single server.

Rate-based limiting and filtering rules
Implement rate limiting by setting a threshold of the number of packets that can be sent to and from the server at any one time. Reduce the impact of an attack by using filtering to setup rules in tools such as load balancers, firewalls, and intrusion prevention systems, to detect and block packets that exceed normal rates.

User education and awareness
Periodically provide training sessions to educate employees on the dangers of DDoS attacks and how to recognize and respond to potential attacks.

Network security and threat detection
To protect against secondary attacks that may occur, apply network segmentation to limit movement of traffic between devices and to other parts of the network. Use tools including endpoint detection and response and intrusion detection/prevention systems to detect and block anomalous traffic.

DDoS protection software
Use a DDoS protection solution coupled with real-time threat intelligence to mitigate the gamut of DDoS attacks, including ACK flood and SYN-ACK flood attacks, and stay ahead of emerging threats. The most effective solutions will allow you to maintain uninterrupted service availability even in the midst of a DDoS attack and protect you against follow-on malicious activity including data leakage, ransomware attacks, and other threats to your operations.

Conclusion

ACK flood DDoS attacks require little technical knowledge to execute but can pack a powerful punch, particularly when launched using botnets. Threat actors can very quickly slow down or completely disrupt the ability to exchange data, access websites, and interact with online services we rely on, like commerce, banking, and media. They can also use these attacks as smokescreens for more damaging, secondary attacks. 

By flooding networks with ACK or SYN-ACK packets, these types of DDoS attacks can negatively impact network performance and server resources. Network congestion, performance, and response time can degrade service as can consumption of server resources and system outages. Ultimately, businesses that experience an ACK or SYN-ACK flood attack can face increased operational costs, reputational damage, and revenue loss.

There are several best practices and technologies to mitigate and prevent ACK flood attacks, including load balancing and redundancy, rate limiting and filtering rules, user education, and network security and threat detection.  

However, ACK flood attacks are extremely deceptive, often use botnets to maximize their impact, and compound the damage with additional malicious activity. So, organizations should also consider implementing an advanced DDoS protection solution with a proven track record. DDoS protection coupled with intelligence to stay ahead of emerging and evolving threats, provides uninterrupted service availability even in the midst of a DDoS attack. Comprehensive DDoS protection can also defend against follow-on attacks that can harm your business. Visit our threat intelligence research center for more information on DDoS defense in depth.