A Distributed Denial of Service (DDoS) attack is a coordinated attempt to make an online service, application, or infrastructure unavailable to its legitimate users by overwhelming it with excessive traffic from multiple compromised sources.

In the context of IBM z/OS environments, a DDoS attack can target traditional mainframe services (TN3270, FTP, HTTP/S, MQ), overload the TCP/IP stacks of the mainframe, or saturate front-end routers and switches, thereby interrupting business-critical workloads and transactions.

How does a DDoS attack work?

DDoS attacks usually follow a systematic process:

  • The attacker compromises many devices (bots) or uses amplification techniques to generate vast volumes of traffic.
  • These devices send simultaneous network or application-level requests to the target system.
  • The target’s capacity (bandwidth, CPU, memory, session tables) becomes saturated, resulting in degraded response times or outright unavailability.
  • In an IBM z/OS environment, critical components such as the Communications Server TCP/IP stack, transaction gateways (CICS/IMS), front-end TN3270 emulators, MQ channels or HTTP servers may be overwhelmed.
  • Once the backbone processing is impacted, even redundant or high-availability setups (e.g., LPARs in a sysplex) can be affected due to shared resources (network, I/O, system coupling).

In some cases, DDoS attacks might be combined with logic-bomb style payloads, where an insider trigger leads to a Sysplex-wide failure. For more information on this, read our article on logic bomb attacks in IBM z/OS environments here

Main objectives of a DDoS attack

DDoS attacks can serve several purposes, including:

  • Financial extortion: demanding ransom to stop the attack.
  • Hacktivism: ideological or political motives to disrupt service.
  • Competitive disruption: affecting business rivals or industry services.
  • Defense testing: probing your environment to measure defence capability.
  • Diversion tactics: distracting security teams while another attack (e.g., logic bomb, malware insertion) is carried out across your infrastructure.

Types of DDoS attacks

Below is a summary of the different types of DDoS attacks. These attacks vary in scale, targeting methods, and impact, and understanding them is key to implementing effective mitigation strategies.

Type of Attack Targeting Method Example of Attack
Volumetric Attacks Overwhelm network bandwidth UDP floods, ICMP floods, DNS amplification
Protocol Attacks Exploit protocol weaknesses SYN flood, Ping-of-Death, Smurf attacks
Application-Layer Attacks Mimic legitimate traffic HTTP floods, CICS Web Gateway abuse, MQ API overloading

Volumetric Attacks

Volumetric attacks aim to saturate your available network capacity. Attack vectors include large UDP floods, ICMP floods, amplification via DNS/NTP reflectors. In mainframe-centric architectures, such attacks may flood the z/OS TCP/IP stacks or saturate the front-end network routers that serve the sysplex, leading to resource exhaustion and service disruption.

Protocol Attacks

Protocol attacks exploit weaknesses in the transport or network protocols. Examples: SYN floods, Ping-of-Death, Smurf attacks. On IBM z/OS, attackers might target the Communications Server’s connection management tables, cause excessive session initiation, or force repeated TCP resets, thereby bringing down legitimate connectivity to TN3270 terminals or CICS/IMS gateways.

Application-Layer Attacks

Application-layer DDoS attacks are sophisticated and stealthy: they mimic legitimate user traffic but target the application logic itself. In z/OS environments, application-layer vectors might include flooding a CICS Web Gateway, overloading an HTTPD server on z/OS, or abusing MQ REST/API endpoints. Because the service appears legitimate, these attacks often bypass traditional volumetric filters and require advanced detection logic.

How to defend against DDoS attacks

Defending against DDoS attacks in hybrid enterprise-mainframe environments requires layered strategies tailored to your z/OS ecosystem:

  1. Network perimeter protection
    • Use perimeter firewalls, IDS/IPS, load-balancers and cloud-based scrubbing services to absorb volumetric floods early.
    • Ensure that front-end routers and switches connected to your mainframe sysplex are hardened and monitored for unusual traffic patterns.
  2. Mainframe (z/OS) configuration hardening
    • Configure the IBM z/OS Communications Server with connection limits, timeout thresholds, and session queue alerts.
    • Apply RACF or equivalent access controls to restrict TCP/IP service endpoints (e.g., TN3270, FTP, HTTP, MQ).
    • Use IPSec or TLS to protect terminal and remote access traffic, reducing the surface for blind floods.
  3. Traffic monitoring and anomaly detection
    • Leverage SMF/TCPIP records, NetView/OMVS tracking, or third-party analytics integrated with SIEM for early detection of unusual volume or session patterns.
    • Monitor the health of sysplex coupling links, shared I/O channels, LPAR CPU/memory loads for signs of latent DDoS impact.
  4. Incident response and recovery planning
    • Define clear escalation paths and thresholds for triggering mitigation (e.g., diverting traffic, isolating LPARs, throttling incoming sessions).
    • Conduct DDoS simulation drills that include mainframe services and sysplex failover to validate resilience.
    • Factor in attacker tactics involving both DDoS and logic-bomb style triggers across the mainframe environment.

DDoS resilience in mainframe-driven ecosystems

Modern financial institutions rely on hybrid infrastructures where the mainframe remains a core pillar of operations. A single DDoS event targeting z/OS services can disrupt thousands of transactions per second, compromise service-level agreements and regulatory compliance obligations (e.g., under the Digital Operational Resilience Act (DORA)).

Developing true DDoS resilience is therefore not just a network-security exercise: it must be embedded into the mainframe architecture, operational monitoring and incident-response frameworks. At Bsecure – The Mainframe & Security Company, we enable organizations to protect their critical IBM z/OS environments by combining continuous risk assessment, resilience testing and automated control validation leveraging our DataPASS platform.

For further reading, check our article on logic bombs in mainframe z/OS sysplex environments: https://www.go2bsecure.com/blog/logic-bomb-attack-mainframe-zos/