BlackNurse DoS Attacks Can Bring Down Large Servers Using a Single Laptop

BlackNurse is a revolutionary technology in the field of cyber attacks. Hackers only need one laptop and minimal data to perform a DoS (denial of service) attack. The BlackNurse attacks target Cisco, SonicWall, Palo Alto and Zyxel firewalls. This method requires small resources to bring down large servers offline.

The Security Operations Center of Danish telecom operator TDC did research on the BlackNurse attacks and wrote a report, detailing their technological aspects and their severity. The researchers highlighted that the method uses low bandwidth Internet Control Message Protocol (ICMP). They explained that BlackNurse “is capable of doing a denial of service to well-known firewalls”. The unusual aspect is that a hacker needs a simple device and a small amount of data to initiate an attack.

The TDC experts shared their observations on the method: “The BlackNurse attack attracted our attention, because in our anti-DDoS solution we experienced that even though traffic speed and packets per second were very low, this attack could keep our customers’ operations down. This even applied to customers with large internet uplinks and large enterprise firewalls in place. We had expected that professional firewall equipment would be able to handle the attack.”

The BlackNurse technology utilizes ICMP Type 3 Code 3 “port unreachable” messages to attack a server. The goal of these messages is to overload the firewall’s CPU. As the research team wrote: “Based on our test, we know that a reasonable sized laptop can produce approx. a 180 Mbit/s DoS attack with these commands”. This leads to the conclusion that a laptop has enough resources to exert the amount of CPU which would put the targeted server in a DoS state.

The researchers explained how the BlackNurse method performs DoS attacks using a low bandwidth connection of 15 to 18 Mbps. “This is to achieve the volume of packets needed which is around 40 to 50K packets per second. It does not matter if you have a 1 Gbit/s Internet connection. The impact we see on different firewalls is typically high CPU loads. When an attack is ongoing, users from the LAN site will no longer be able to send/receive traffic to/from the Internet. All firewalls we have seen recover when the attack stops.”

The TDC team managed to determine which devices are vulnerable to the BlackNurse attacks. The list is as follows:

Cisco ASA 5506, 5515, 5525 (default settings)
Cisco ASA 5550 (legacy) and 5515-X (latest generation)
Cisco Router 897 (can be mitigated)
Some unverified Palo Alto
SonicWall (misconfiguration can be changed and mitigated)
Zyxel NWA3560-N (wireless attack from LAN side)
Zyxel Zywall USG50

The security specialists summed up their findings by pointing out that a certain type of device is most vulnerable to BlackNurse attacks. “We see the Cisco ASA firewall 55xx series to have the biggest problems. Even if you deny all ICMP traffic to the firewalls, they still suffer from the DOS attack, with as little as 4Mbit of traffic.”

TDC listed mitigations and SNORT IDS rules to assist users in detecting BlackNurse attacks. Another source of advice people can use is a post on GitHub, published by a security engineer for OVH. The technician provided a proof-of-concept (PoC) code which allows users to check if their device is vulnerable to BlackNurse attacks.

Independent software developers NETRESEC also made a contribution to the research efforts on the BlackNurse technology. They issued a blog post, titled “The 90’s called and wanted their ICMP flood attack back”. The publication outlines the risk of granting permission for ICMP unreachable message Type 3 while acknowledging TDC’s report. There is a conflict between the Cisco ASA 5500 manual, which recommends giving permission, and the analysis of TDC, which advises denying “ICMP Type 3 messages sent to the WAN interface of Cisco ASA firewalls to prevent the BlackNurse attack.”

Palo Alto also addressed TDC’s findings. To help users combat against the BlackNurse attacks, they issued an advisory and list of recommendations post.

You can add the diary entry of the SANS Internet Storm Center in your list of advisory notes regarding the BlackNurse attacks. The publication offers solutions to help users deal with the threat.

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