The BlackNurse Denial of Service (DoS) attack is a low-bandwidth, high-impact denial of service technique that exploits the ICMP Type 3 Code 3 (Destination Unreachable - Port Unreachable) messages to overwhelm target systems. Unlike traditional volumetric DDoS attacks that rely on massive traffic volumes, BlackNurse uses relatively low traffic rates to cause significant CPU load on firewalls and network devices, leading to service degradation or outages. The attack targets the availability of network infrastructure by exploiting how certain firewalls and routers process ICMP error messages, causing them to consume excessive resources and become unresponsive. This technique is notable for its efficiency, requiring less bandwidth to achieve disruption compared to typical flood attacks. The campaign was publicly documented around 2016, with no specific affected product versions identified, indicating a broad potential impact on network devices that inadequately handle ICMP error processing. Although no known exploits in the wild are reported, the threat remains relevant due to the fundamental nature of the ICMP protocol and the widespread use of vulnerable network equipment. The low severity rating reflects the attack's limited scope and the requirement for the attacker to send ICMP packets to the target network, but the potential for service disruption remains significant, especially for organizations relying heavily on affected firewall or router models.

For European organizations, the BlackNurse DoS attack poses a risk primarily to network availability and operational continuity. Critical infrastructure providers, financial institutions, and enterprises with stringent uptime requirements could experience service interruptions if their network devices are susceptible. The attack's low bandwidth requirement makes it accessible to a wider range of threat actors, including those with limited resources, increasing the risk of opportunistic attacks. Disruption of firewall or router functionality can lead to temporary loss of access to internal or external services, impacting business operations and potentially causing financial and reputational damage. Additionally, organizations with remote or cloud-based services relying on vulnerable network equipment could face cascading effects, affecting end-users and partners. The attack does not directly compromise confidentiality or integrity but can be used as a smokescreen for other malicious activities during downtime.

To mitigate the BlackNurse DoS threat, European organizations should implement specific measures beyond generic DoS protections: 1) Review and update firewall and router firmware to the latest versions that include patches or improvements in ICMP error message handling. 2) Configure network devices to limit the rate of ICMP Type 3 Code 3 messages processed or to drop excessive ICMP error packets from untrusted sources. 3) Employ intrusion prevention systems (IPS) or next-generation firewalls capable of detecting and blocking anomalous ICMP traffic patterns associated with BlackNurse. 4) Conduct targeted testing and monitoring of network device CPU utilization under ICMP traffic to identify susceptibility. 5) Implement network segmentation and strict ingress filtering to reduce exposure of critical devices to unsolicited ICMP traffic. 6) Collaborate with ISPs to filter or rate-limit suspicious ICMP traffic upstream where possible. 7) Maintain robust incident response plans that include detection and mitigation of low-bandwidth DoS attacks. These tailored actions will help reduce the attack surface and improve resilience against this specific DoS vector.