What Is a SYN Flood Attack?

A SYN flood attack abuses a particular component within the TCP 3-way handshake: by sending lots of connection initiation (SYN) requests, the attacker forces the victim server to keep a port open for each fake connection. This leads to server resources being tied up in waiting for the handshake to complete, eventually causing the system to become unresponsive to legitimate users.

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What Is a SYN Flood Attack?

How Does a SYN Flood Attack Work?

To understand how a SYN flood attack works, let’s examine the standard TCP three-way handshake.

  1. When a client wants to wind up a connection to a specific server, it sends a SYN packet, signaling its intent to start communication.
  2. The server replies with a SYN-ACK packet, confirming the receipt of the SYN and indicating its readiness to complete the connection
  3. The client replies with an ACK, completing the handshake and establishing the connection.

This entire back-and-forth occurs before the device starts transferring new packets of information.

However, to abuse this protocol in a SYN flood attack, the attacker forces the server to keep its connection open by simply withholding the final ACK.

As a result, each incoming SYN request requires the server to allocate memory and connection slots for connections that never complete. When the server is flooded with abusive SYN packets, these resources are rapidly depleted.

This means that legitimate users are denied access to the service as the server becomes unable to handle additional connection attempts.

Over time, this can lead to a complete denial of service…

Beyond the direct impact on system resources, SYN flood attacks can cause significant stress fractures across broader operations. For businesses with online operations or critical infrastructure, these disruptions can lead to:

  • Financial losses
  • Reputational damage
  • Critical infrastructure failings

Thanks to the ease with which SYN flood attacks can be spun up, it’s common to see them deployed as part of a larger attack strategy and weakening the target’s defenses – creating a critical combination of disrupted service and data exfiltration.

SYN Flood Detection

Detecting a SYN flood attack requires careful monitoring of network traffic. One of the most effective ways to detect such an attack is by monitoring traffic patterns to identify unusual spikes in the ratio of SYN packets to the number of completed handshakes.

Even basic network monitoring tools like Wireshark offer a way to filter connections by unacknowledged SYN packets. Manual traffic filtering can put admins on the backfoot, as well, as these attacks need to be prevented before the damage is done.

This is why a lot of SYN flood attack detection is handled automatically by Intrusion and Detection Systems (IDSs).

They monitor normal traffic types and behaviors, and can therefore spot the statistical anomalies created by SYN floods in the nick of time. Less advanced models may rely on signature-based detection, where predefined attack characteristics are recognized in incoming traffic.

This allows the system to identify and respond in real-time, which makes a SYN flood highly detectable.

SYN Flood Attack Mitigation Options

IP-based blocking alone won’t work against SYN flood attacks, since attackers are wary enough to use spoofed IP addresses – therefore hiding the attack behind the facade of different devices.

This isn’t the most advanced form of SYN flood distribution, either – spoofed IP addresses can be traced back to the source via an Internet Service Provider. If an attacker relies on a botnet for these spoofed IP addresses, there’s essentially no way to easily trace the attack to a source.

To mitigate SYN flood attacks, several strategies can be employed, each designed to reduce the server’s vulnerability and maintain service during an attack.

SYN Cookies

One of the most widely used methods is SYN cookies, a technique that prevents the server from allocating resources for a connection until the entire TCP handshake is complete. Instead of reserving memory for each SYN request, the server encodes the state information of the connection into the SYN-ACK packet it sends to the client.

If the client responds with a valid ACK, the server verifies the cookie and completes the connection.

Increased Backlog

Another approach is to increase the size of the backlog queue, allowing the server to handle more incomplete connections at once. While this can provide temporary relief during a low-level SYN flood attack, it’s not a long-term solution.

After all, it still allows a large enough attack to overwhelm the server.

Rate Limiting

Rate limiting is another effective strategy, which involves setting a maximum limit on the number of SYN packets the server can process in a given time frame.

By controlling the flow of incoming connection requests, rate limiting prevents the server from being overwhelmed by a sudden surge in traffic. Similarly, reducing the number of SYN-ACK retries that the server allows can also minimize the amount of time the server waits for the final ACK before releasing resources.

This allows the server to free up connection slots more quickly and continue processing new requests.

Load Balancers

For larger-scale protection, load balancers or distributed server architectures can help mitigate the effects of a SYN flood. By distributing incoming traffic across multiple servers, load balancers reduce the likelihood that a single server will become overwhelmed.

This approach not only improves resilience against SYN flood attacks but also enhances overall system performance and availability.

This sits well with wider IPS tools, as they can move legitimate traffic over to unaffected servers.

Choose Attack Prevention with Check Point Quantum

Check Point Quantum enables organizations to adopt a prevention-first approach across their network security stance. Its SandBlast network protection defends against a wide range of cyberattacks, including:

  • SYN flood attacks
  • Ransomware
  • Trojans
  • Phishing attempts

Offering seamless integration with existing infrastructure, it offers configuration with fully automated policies, ensuring robust security without compromising business productivity or agility. In the face of SYN flood attacks, Check Point Quantum offers rapid alerts and automated mitigation – the multi-layered defense strategy that maintains network resilience even when handling well-funded attacks.

Find out more and discover how Check Point Quantum can prevent SYN flood attacks with a demo.

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