What Is an ACK Flood DDoS Attack?

Introduction to ACK Flood DDoS Attacks

An ACK flood DDoS attack is a specific type of Distributed Denial of Service (DDoS attack). This attack overwhelms a target system with numerous TCP ACK packets, which confirm successful data transmission. Attackers exploit these packets to drain resources and disrupt legitimate activity. Operating at Layer 4 of the OSI model, it is stealthy, efficient, and hard to mitigate. Learn why DDoS attacks are dangerous—particularly when they take advantage of fundamental TCP features such as ACK packets.

This article will explore the mechanics of an ACK flood DDoS attack, its effects on systems, how it contrasts with other TCP-based floods, and strategies that can prevent or lessen the impact of such attacks.

How the ACK Flag Works in TCP Communication

Before diving in to learn more about the attack, it’s essential to understand how TCP communication operates and the role of ACK packets. Understanding how DDoS attacks work often starts with analyzing legitimate networking behavior.

How the TCP Three-Way Handshake Works

The handshake establishes a dependable connection between two devices before any data is exchanged.

Step	Packet	Description
1	SYN	Client requests to initiate a connection
2	SYN-ACK	The server responds, recognizing the request
3	ACK	The client approves and completes the handshake

What Happens in an ACK Flood DDoS Attack?

An ACK Flood DDoS attack bypasses the general TCP initiation by sending a large number of TCP ACK packets to the target system without any previous handshake. These packets force the system to allocate resources to what appears to be a valid connection, consuming processing power, memory, and bandwidth.

This overload disrupts normal operations and can lead to complete service outages.

Key Characteristics of ACK Flood DDoS Attacks

Common targets of DDoS attacks include web servers, online applications, and infrastructure elements such as load balancers and firewalls, all of which can be affected by an influx of TCP ACK packets.

How Does an ACK Flood DDoS Attack Work?

In a standard TCP session, ACK packets indicate a smooth data transfer between a client and server. However, in an ACK flood attack, this process is exploited to bombard a target system.

Here’s how an ACK flood works:

Attackers create a large number of spoofed ACK packets that seem legitimate but do not belong to any established TCP session.
These counterfeit packets are sent quickly and in large quantities to the target server or firewall.
The server, striving to function as intended, inspects and processes these ACK packets despite their presence being meaningless. As a result, legitimate traffic experiences delays or drops, resulting in service degradation or outages.

Effects of an ACK Flood DDoS Attack on Your Infrastructure

An ACK flood attack can lead to various effects on the targeted victim infrastructure:

CPU Drain: Servers squander processing resources by analyzing and filtering unnecessary ACK packets.
Memory Depletion: Stateful firewalls and load balancers can become saturated with session tracking entries, potentially blocking new connections.
Network Saturation: The attack uses up precious bandwidth, pushing out legitimate traffic.
User Downtime: End users might experience sluggish response times, connection issues, or total service outages.

How to Detect an ACK Flood Attack

Modern DDoS protection systems are increasingly dependent on behavioral analytics to differentiate malicious ACK floods from standard traffic. These systems analyze patterns such as:

ACK Flood DDoS Attack vs. SYN Flood Attack

Both attacks utilize TCP, yet they vary in their methods and effects.

Feature	ACK Flood DDoS Attack	SYN Flood DDoS Attack
Packet Used	ACK [Acknowledgement]	SYN [Synchronisation]
Session Type	Appears as mid-session	Initiates handshake
Resource Impact	CPU and bandwidth exhaustion	Server state exhaustion (half-open connections)
Target	Firewalls, servers	Servers, network stacks

Sophisticated systems utilize AI to detect DDoS attacks by analyzing the actions of these packet types and detecting irregularities.

Real-World Example of an ACK Flood DDoS Attack

ACK floods can be incredibly destructive, particularly when carried out by a vast botnet. In April 2024, an unprecedented ACK flood reached a staggering 840 million packets per second, severely impacting segments of internet infrastructure. Although the assault stemmed from more than 5,000 IP addresses, the majority of traffic was funneled through a select few network chokepoints, highlighting the concentrated power of such attacks.

This underscores the critical requirement for proactive DDoS mitigation and intelligent traffic management.

Why Are ACK Flood Attacks Hard to Detect?

ACK packets are a standard component of TCP communications. They:

Conventional firewalls or intrusion prevention systems struggle to differentiate malicious ACKs from legitimate ones. As a result, ACK flood DDoS attacks are especially stealthy and resistant to basic defense strategies.

AI-driven threat systems are increasingly utilizing behavioral analytics in DDoS protection to identify these subtle attack signatures in real time.

How to Prevent and Mitigate ACK Flood DDoS Attacks

To prevent an ACK flood, it is essential to implement layered defense strategies. These should integrate network design, traffic analysis, and advanced techniques filtering.

1. Network Redundancy and Scalability

These are common ways to stop a DDoS attack before it interrupts your environment.

2. Use of Intelligent Packet Filtering

3. DDoS Protection and Detection Systems

Modern DDoS protection tools are capable of:

4. Content Delivery Networks (CDNs)

CDNs help mitigate floods by:

Key Takeaways on ACK Flood DDoS Attacks

Aspect	Details
Attack Type	TCP-based Layer 4 DDoS attack
Mechanism	Floods server with ACK packets
Target	Firewalls, servers, and Layer 4 devices
Symptoms	Slowed performance, outages, high CPU usage
Detection	Difficult due to seemingly valid TCP headers
Defense	Packet filtering, rate limiting, scalable architecture

Comprehending how DDoS attacks work at the protocol level is essential for implementing effective detection and DDoS mitigation solutions.

Why ACK Floods Demand Advanced Defense

The ACK flood DDoS attack quietly disrupts online services by exploiting TCP’s reliability mechanism for harmful purposes. Unlike volumetric or application-layer attacks, ACK floods focus on the server’s processing layer, making them more challenging to identify while still being equally—if not more- profoundly disruptive and dangerous.

Organizations need to invest in scalable, redundant infrastructure, advanced DDoS protection, and intelligent traffic monitoring to defend against such attacks. Learn why DDoS attacks are dangerous, particularly when traditional signatures fail to expose the threat. Proactive defense, ongoing analysis, and behavioral analytics in DDoS protection remain the most reliable methods to guarantee service availability and network resilience against modern ACK flood threats.

How Prophaze Helps Mitigate ACK Flood DDoS Attacks

Prophaze provides advanced protection against ACK Flood DDoS attacks through its AI-powered WAF and real-time behavioral analytics. By continuously monitoring TCP traffic patterns, Prophaze can detect and block irregular ACK packets—often used in these stealthy Layer 4 attacks—before they disrupt your infrastructure. Its cloud-native, scalable architecture ensures uninterrupted application performance, even during high-volume DDoS attempts. Whether the attack is distributed, subtle, or aggressive, Prophaze delivers intelligent mitigation that keeps your systems secure and resilient.