In 2016, every company, no matter the size, has an online presence. Whether that is a website, online store front, perhaps a mobile app, or a mail server, chances are your organization has some kind of system in production on the Internet.

It has long been established common practice to secure these types of services or devices via a firewall. A firewall functions exactly as the name implies and only exposes the necessary service ports to the outside world that were absolutely needed for the application to function. For example, you might configure the firewall to allow TCP port 80 and 443 which are common ports for web pages to be served on the Internet but block everything else.

Unfortunately, this creates a single point of failure in that all of that network traffic is now funneled through that firewall. Under normal circumstances, this is usually not a problem. You would “right-size” your firewall appliance to be able to handle your normal traffic load, probably allow for some head room for traffic growth over time, and then throw in a little extra head room in case of burst traffic. From there, you’d probably purchase a second firewall and cluster the two devices together for high availability if the application required it. This way, if one firewall were to become unresponsive, the other firewall would assume the traffic flow and your service would not be disrupted.

Then along came the BlackNurse Denial-of-Service (DoS) attack…

What is the BlackNurse Denial-of-Service Attack?

Note 1: The term “BlackNurse” is named after the team who discovered this type of attack: one was a blacksmith, and the other was a nurse.

Note 2: There have been a number of articles posted already that delve into the ICMP protocol and the different types of ICMP messages that can be generated and how these all work together to generate the BlackNurse Denial-of-Service attack. We won’t rehash detailed ICMP technical explanations here, but there are links for further reading at the end of this post if you’d like more information.

Before we delve into the BlackNurse attack, let’s quickly review a little bit of history, shall we?

At a high level, the BlackNurse DoS attack is a type of ping flood attack. Back in the days of grunge music in the 1990s, a common Denial-of-Service attack was to simply flood a target with the ping command (ICMP Echo packets – Type 8 Code 0 if you’re curious) and maxing out a host’s Internet connection with excessive data. You might remember the “ping of death,” which used malformed pings larger than 1500 bytes to crash devices.

In that era, many of us were on slow dial-up connections and most servers didn’t have 10 gigabit connections like they do today. The Internet was a much smaller place and didn’t quite have near the amount of attack mitigation technology available as it does today. Consequently, it was very easy for a malicious user with access to a relatively fast connection to flood a target with a small amount of traffic (by today’s standards), as most devices couldn’t handle a high amount of traffic in those days.

As technology evolved, switches, routers, firewalls, servers, and network interface controllers became able to generate and also accept much larger amounts of bandwidth. In the time since, we’ve also seen the advent of DDoS mitigation appliances, services such as those offered by traffic policers and other such technologies that have largely eliminated these types of basic attacks involving ping floods, the “ping of death,” and making other volumetric attacks easier to deal with.

Typically, volumetric attacks these days require several gigabits of traffic per second to bring a host offline and also usually involve multiple attack vectors. You’ve probably seen a few of these in the news over the past year or two. These usually involve a coordinated effort of multiple compromised hosts or botnets and generate huge amounts of traffic in order to take down a service. The most recent example in the news was the DDoS attack levied against This attack was reported to be operating at 1.1 Tbps at its peak.

Why is BlackNurse unique?

So if volumetric attacks require all of these resources in order to bring down servers and we have numerous tools at our disposal to mitigate volumetric attacks, then what makes the BlackNurse so special?

What makes BlackNurse unique is that it is able to bring down firewalls or other network devices with a small amount of traffic (think 15-20 Mbits per second generating 40-50,000 packets per second). Today, laptops and desktop computers can easily generate this amount of traffic on your typical broadband connection.

The way it works is that attackers have identified that a specially-crafted ping command generates a significantly higher amount of CPU usage on certain network devices, namely firewalls, when these devices respond to this type of ping command. This specially crafted ping command exploits how certain devices respond to ping commands that generate ICMP messages of Type 3. ICMP Type 3 messages are known as “Destination Unreachable” messages, which basically means that a host is not available or responding. Unfortunately, it is not possible to simply turn off or block “Destination Unreachable” responses to ping requests, as these are required for keeping hosts operating properly on an IPv4 network per RFC specifications—and for some vendors—required for IPSec and PPTP traffic to operate. For more information, please review the RFC specifications, specifically “RFC 1812 – Requirements for IPv4 Routers”.

Attackers discovered that only 15-20 Mbit of traffic with packet rates of 40,000 to 50,000 packets per second generated by this specially crafted ping command is able to bring these devices offline.

The attack is typically carried out when an attacker targets the WAN or public facing IP address of the firewall with a sustained stream of these ping commands until the firewall runs out of CPU cycles to process traffic. Devices behind the firewall are unable to communicate until the attack subsides.

As you can see, a single user can easily bring down a firewall with a modest amount of easily attainable resources at their disposal.

How is ServerCentral protecting customers against BlackNurse DoS attacks?

ServerCentral has taken several steps to prepare for and mitigate the BlackNurse DoS attack.

Our Network Engineering team has proactively implemented filters on all upstream connections to rate limit the maximum inbound rate of ICMP Type 3 Code 3 messages that are allowed towards our customers. These filters should not impact general day to day operations. All Managed and Colocation customers are covered by these changes.

Our Managed Services team has worked with our firewall vendors to identify potentially affected models used with our Managed Firewall service.

Juniper SRX: At this time, Juniper SRX models appear to not be affected but this is still pending additional research and verification with JTAC. However, we have worked with Juniper to proactively create a custom IDS filter that can mitigate the BlackNurse effects as an added precaution. This filter can be applied on demand.

Fortigate: At this time, Fortigate firewalls running FortiOS v5.2.x appear to not be affected. Some models running FortiOS v5.4.x appear to be affected. ServerCentral has standardized on the v5.2.x code branch and should not be affected. This is still pending additional research and verification.

Should the situation change with our firewall providers, we will update this post accordingly.

For users that have purchased our Managed DDoS Mitigation Service, the built-in behavior Denial-of-Service analysis engine will successfully detect and mitigate the BlackNurse attack during an attack.

For our colocation customers, we strongly urge you to contact your firewall vendors and confirm if your devices or configurations are susceptible to the BlackNurse DoS attack.

If you have any questions or if we can be of assistance, please contact us at your convenience.

Further Reading

Deft logo

2200 Busse Rd.
Elk Grove Village, IL 60007
+1 (312) 829-1111