TCP ACK Flood DDoS Attack

What is a TCP ACK Flood DDoS Attack?

A TCP ACK Flood attack is a form of Denial-of-Service (DoS) or Distributed Denial-of-Service (DDoS) attack in which a large number of TCP packets with the ACK flag set are sent to the target system. The goal of the attack is to overload the bandwidth or resources of the target system or its firewall by sending an extremely large number of seemingly legitimate data packets. Such packets are difficult for the server to distinguish from legitimate connections, since ACK packets are typically part of active TCP sessions. Firewalls and stateful packet inspection systems are particularly targeted in this type of attack: the devices are forced to check the state of an existing connection for every received ACK packet, which can quickly lead to overload. As a result, legitimate users can no longer access servers or services.

How Does a TCP ACK Flood DDoS Attack Work?

A TCP ACK Flood exploits the TCP protocol by generating massive numbers of TCP packets with the ACK flag set and sending them to the target. Under normal conditions, a client sends an ACK packet as part of an established connection setup or to confirm receipt of data. In an ACK Flood, however, the attacker sends large numbers of such packets, often without a valid connection existing or from various spoofed sources. The process is as follows:

• The attacker generates a large number of TCP packets with the ACK flag set.
• These packets are sent to the target server at high speed, often using spoofed source addresses.
• Firewalls and other network security devices must check for every incoming ACK packet whether it belongs to an existing connection – this consumes considerable resources (stateful inspection).
• When such devices can no longer process the volume, legitimate connections are delayed or completely dropped. The server can also be additionally burdened by responding to the packets (e.g. with an RST if no connection exists).

The goal of the attack is not necessarily to exhaust server ports as in a SYN Flood, but rather to overload the infrastructure responsible for state tracking of TCP connections, in particular firewalls, load balancers, and other network appliances.

Typical Variants of a TCP ACK Flood Attack

• DDoS via Botnets

  In DDoS attacks using ACK Flood, attackers often use botnets to send large volumes of ACK packets from many different IP addresses. The distribution makes identifying and blocking sources considerably more difficult and can even push extensive firewall systems to their capacity limits.

• IP Address Spoofing

  Many ACK Floods use forged (spoofed) source addresses. The target system can therefore usually only trace and specifically block the packets with great difficulty, which increases the risk of a successful attack and complicates countermeasures.

• Flood on Firewalls and Filters

  Firewalls, Intrusion Detection Systems (IDS), and load balancers in particular are vulnerable to TCP ACK Floods since they rely on stateful inspection. They attempt to assign each ACK packet transaction to internal tracking, which can cause a system failure with large volumes.

Unlike SYN Flood attacks, which create open connections, ACK Floods aim to overwhelm existing infrastructure – especially stateful components – with requests so that they can no longer respond to legitimate traffic.

How to Protect Against a TCP ACK Flood DDoS Attack?

To protect against TCP ACK Flood DDoS attacks, various measures are available:

• Stateless Filtering

  Unlike classic firewalls that maintain a state for each connection, stateless filters can define rules that filter suspicious ACK packets at the network level without tracking the connection state. This allows large volumes of flood packets to be more effectively blocked before they overload a stateful mechanism.

• Rate Limiting and Traffic Shaping

  By limiting the maximum permissible volume of ACK packets per second per IP address, unexpectedly heavy traffic can be detected and throttled. Modern routers and firewalls offer extensive configuration options for this. ISPs also often help to contain flood attacks directly in the backbone network.

• Anomaly Detection and Automated Filtering

  Systems for automated detection of network anomalies can identify sudden and unexplained spikes in ACK packets and apply targeted blocking rules to the affected traffic. This prioritizes legitimate traffic and protects the infrastructure.

• Geo-Blocking and Blacklisting

  Suspicious sources, individual IP blocks, or various countries of origin can be temporarily blocked in exceptional cases to defuse the attack. However, this method should be used very selectively to avoid collateral damage to legitimate users.

How Tievolu Protects Your Network Against TCP ACK Flood DDoS Attacks

Tievolu PYRUS DDoS Protection

Tievolu detects and blocks TCP ACK Flood attacks with the in-house developed DDoS Protection "Tievolu PYRUS". All TCP traffic is continuously checked for anomalies and dangerous ACK Floods are throttled by intelligent filter rules. Even with high-volume attacks from many sources, a dynamic combination of automated pattern recognition and our specially developed Connection Inspection filtering prevents network overload. The Tievolu PYRUS filters are adjusted in real time and attacker sources are automatically blocked.

Tievolu Cloud Firewall

Alternatively, customers can activate an additional layer of protection against TCP ACK Flood attacks through our Cloud Firewall. In the Cloud Firewall, specific TCP filter rules can be configured that specifically capture only ACK packets. These rules can also be combined with various rate-limiting mechanisms to limit the number of incoming requests. Additionally, filters can be extended with ASN or geographic blocks (geo-blocking) to reduce suspicious or unwanted traffic at an early stage.