Call Us! 1-800-383-5193
Call Us! 1-800-383-5193
Call Us! 1-800-383-5193

Need Help?

Emergency Response Time custom

Our experts have had an average response time of 11.06 minutes in March 2021 to fix urgent issues.

We will keep your servers stable, secure and fast at all times for one fixed price.

ACK flood DDoS attack – How does an ACK flood attack work?

by | Feb 24, 2021

Stuck with the ACK flood DDoS attack? We can help you.

When attackers attempt to overload a server with TCP ACK packets, it results in an ACK flood attack.

As part of our Server Management Services, we assist our customers with several DDoS attacks.

Today, let us see in detail the ACK flood DDoS attack and how it works.


ACK flood DDoS attack

An ACK flood denies service to other users by slowing down or crashing the target using junk data.

The targeted server has to process each ACK packet. It uses so much computing power that it is unable to serve legitimate users.


  • What is an ACK packet?

Data sent over the internet is broken up into smaller segments called packets.

The TCP protocol uses the packet header to tell the recipient the no. of packets present and the order they should arrive.

An ACK packet is any TCP packet that acknowledges receiving a message or series of packets. Technically, an ACK packet is a TCP packet with the “ACK” flag set in the header.

Device1                     ————->               Device2


ACK packets are part of the TCP handshake, a series of three steps that start a conversation between any two connected devices on the Internet.

  1. SYN
  2. SYN-ACK
  3. ACK

The three-way handshake starts by sending an SYN packet. The device at the other end of the connection replies with an SYN-ACK packet. Finally, the user’s laptop sends an ACK packet.

However, this is not the only time ACK packets are used. The TCP protocol requires that connected devices acknowledge they receive all packets in order.

Since an ACK packet is any TCP packet with the ACK flag, it can be part of a different message the source sends to the server. If we fill out a form and submit data to the server, the source can make one of those packets the ACK packet for the image.


  • How does an ACK flood attack work?

ACK flood attacks target devices that need to process every packet that they receive. Mostly firewalls and servers are targets for an ACK flood.

They are layer 4 DDoS attacks.

Legitimate and illegitimate ACK packets look essentially the same. Hence, it is difficult to stop them without using a content delivery network (CDN).

Although similar, packets in an ACK DDoS attack do not contain the main part of a data packet. In order to appear legitimate, they only have to include the ACK flag in the TCP header.


  • How does an SYN-ACK flood attack work?

In an SYN-ACK DDoS attack, the attacker floods the target with SYN-ACK packets. They are not part of a three-way handshake. The only purpose is to disrupt the target’s normal operations.

It is also possible for an attacker to use SYN packets in an SYN flood DDoS attack.


  • How does CDN stop ACK flood DDoS attacks?

The CDN proxies all traffic to and from a customer’s origin server. It does not pass along any ACK packets that are not associated with an open TCP connection.

This ensures that the malicious ACK traffic does not reach the origin server.

[Couldn’t figure out the attack? We’d be happy to assist]



In short, an ACK flood DDoS attack occurs when an attacker attempts to overload a server with TCP ACK packets. In order to prevent these attacks, our Support Techs suggest CDN.


Never again lose customers to poor server speed! Let us help you.

Our server experts will monitor & maintain your server 24/7 so that it remains lightning fast and secure.


var google_conversion_label = "owonCMyG5nEQ0aD71QM";


Submit a Comment

Your email address will not be published. Required fields are marked *

Privacy Preference Center


Necessary cookies help make a website usable by enabling basic functions like page navigation and access to secure areas of the website. The website cannot function properly without these cookies.

PHPSESSID - Preserves user session state across page requests.

gdpr[consent_types] - Used to store user consents.

gdpr[allowed_cookies] - Used to store user allowed cookies.

PHPSESSID, gdpr[consent_types], gdpr[allowed_cookies]


Statistic cookies help website owners to understand how visitors interact with websites by collecting and reporting information anonymously.

_ga - Preserves user session state across page requests.

_gat - Used by Google Analytics to throttle request rate

_gid - Registers a unique ID that is used to generate statistical data on how you use the website.

smartlookCookie - Used to collect user device and location information of the site visitors to improve the websites User Experience.

_ga, _gat, _gid
_ga, _gat, _gid


Marketing cookies are used to track visitors across websites. The intention is to display ads that are relevant and engaging for the individual user and thereby more valuable for publishers and third party advertisers.

IDE - Used by Google DoubleClick to register and report the website user's actions after viewing or clicking one of the advertiser's ads with the purpose of measuring the efficacy of an ad and to present targeted ads to the user.

test_cookie - Used to check if the user's browser supports cookies.

1P_JAR - Google cookie. These cookies are used to collect website statistics and track conversion rates.

NID - Registers a unique ID that identifies a returning user's device. The ID is used for serving ads that are most relevant to the user.

DV - Google ad personalisation

IDE, test_cookie, 1P_JAR, NID, DV, NID
IDE, test_cookie


These are essential site cookies, used by the google reCAPTCHA. These cookies use an unique identifier to verify if a visitor is human or a bot.