Tievolu Tievolu TievoluA TCP SYN-ACK Flood is a specific variant of DDoS attacks in which target systems are flooded with a large number of TCP SYN-ACK packets. Unlike the classic SYN Flood, where numerous SYN packets are sent to create half-open connections, in a SYN-ACK Flood the attacker sends massive numbers of forged SYN-ACK responses to servers or endpoints. The goal of this attack is to trigger connection states, consume resources, or overload network devices such as firewalls and Intrusion Prevention Systems (IPS), as these must process or log every incoming SYN-ACK packet even though no connection was established.
Normally in the TCP protocol, after a SYN packet (connection request) the server responds with a SYN-ACK. The client confirms the connection with an ACK, and only then is the three-way handshake completed. In a SYN-ACK Flood, however, the attacker sends massive numbers of SYN-ACK packets, often with spoofed source and destination addresses, into the target network without any prior connection having been established. The behavior of the target towards these packets depends on the system:
Under high attack load, network devices or endpoints can become overwhelmed, leading to performance problems or even the failure of central security infrastructure. From an attacker's perspective, this attack is particularly attractive because in many networks filtering SYN-ACK packets is not explicitly provided for, and the attack can be carried out without stability checks on the other side.
As with other DDoS variants, the SYN-ACK Flood can also originate from a botnet. Thousands of infected devices synchronously send SYN-ACK packets to the target to overload firewalls, servers, and network hardware and disrupt legitimate operations.
SYN-ACK Floods are often used not against end users, but specifically against firewalls, load balancers, or Intrusion Detection/Prevention Systems, as these react particularly strongly to unexpected packets and are especially vulnerable to protocol table overflows or memory exhaustion attacks.
By forging the source address in the SYN-ACK packets, the attacker can make tracing more difficult and extend resource consumption to even more systems in the target network.
Unlike SYN Flood attacks, where servers reserve resources for incomplete connections, the SYN-ACK Flood focuses on the load caused by unusual, non-protocol-compliant packets and exploiting the reaction mechanisms of modern network security systems. The sheer volume of packets overburdens protocol tables, log files, and CPU resources, which in extreme cases can completely cripple operations.
To defend against TCP SYN-ACK Floods, various options are available:
Modern firewalls and Intrusion Prevention Systems should be able to detect and block SYN-ACK packets that appear without an associated prior SYN connection. This is achieved using connection and state tracking mechanisms ("stateful inspection").
By limiting the permissible number of SYN-ACK packets per time interval, the impact of the attack can be greatly mitigated. This limitation is particularly useful for SYN-ACK packets without an existing associated connection.
By monitoring incoming traffic and using systems that automatically detect patterns and anomalies (e.g. AI- or ML-based systems), SYN-ACK Floods can be quickly identified and countermeasures such as blackholing or scrubbing activated.
Our "Tievolu PYRUS DDoS Protection" detects SYN-ACK Floods based on characteristic packet anomalies in TCP traffic. Through connection tracking, SYN-ACK packets that appear outside a valid connection establishment can be effectively blocked. The protection works fully automatically and scales against both targeted and high-volume SYN-ACK Flood attacks. Configurable thresholds and intelligent filter mechanisms ensure that legitimate traffic is not affected.
Alternatively, customers can activate an additional layer of protection against TCP SYN-ACK Flood attacks through our Cloud Firewall. In the Cloud Firewall, specific TCP filter rules can be configured that specifically capture packets with the TCP flags SYN & ACK. 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.
Contact our expert Collin Schneeweiß today to protect your network from DDoS attacks and get your personal offer.
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