DoS technique lets a single laptop take down an enterprise firewall

ICMP Type 3 Code 3 packets can overload firewalls, researchers warn.

A new low-bandwidth denial-of-service technique can clog enterprise firewalls.
Credit: Gerd Altmann / Pixabay

At a time when the size of distributed denial-of-service attacks has reached unprecedented levels, researchers have found a new attack technique in the wild that allows a single laptop to take down high-bandwidth enterprise firewalls.

The attack, dubbed BlackNurse, involves sending Internet Control Message Protocol (ICMP) packets of a particular type and code. ICMP is commonly used for the ping network diagnostic utility, and attacks that try to overload a system with ping messages—known as ping floods—use ICMP Type 8 Code 0 packets.

BlackNurse uses ICMP Type 3 (Destination Unreachable) Code 3 (Port Unreachable) packets instead and some firewalls consume a lot of CPU resources when processing them.

According to experts from the Security Operations Center of the Danish telecom operator TDC, it would take from 40,000 to 50,000 ICMP Type 3 Code 3 packets a second to overload a firewall. This is not a large number of packets and the bandwidth required to generate them is 15Mbps to 18Mbps, which means that BlackNurse attacks can be launched from a single laptop.

“The impact we see on different firewalls is typically high CPU loads,” the TDC Security Operations Center (SOC) said in a technical report. “When an attack is ongoing, users from the LAN side will no longer be able to send/receive traffic to/from the Internet. All firewalls we have seen recover when the attack stops.”

TDC SOC tested the attack successfully against Cisco Adaptive Security Appliance (ASA) firewalls in default configurations. Cisco’s own documentation recommends that users allow ICMP Type 3 messages.

“Denying ICMP unreachable messages disables ICMP Path MTU discovery, which can halt IPSec and PPTP traffic,” the company warns in its user guidelines.

Some firewalls from Palo Alto Networks, SonicWall and Zyxel Communications are also affected, but only if they’re misconfigured or if certain protections are not turned on.

“Palo Alto Networks Next-Generation Firewalls drop ICMP requests by default, so unless you have explicitly allowed ICMP in a security policy, your organization is not affected and no action is required,” Palo Alto said in a blog post in response to TDC SOC’s report.

Customers who need to allow ICMP requests can follow best practices for DoS protection to mitigate this attack, the company said. This involves enabling ICMP Flood and ICMPv6 Flood in their firewall’s DoS protection profile.

Denial of service attacks are typically about generating more traffic than the target’s Internet bandwidth can take. BlackNurse is unusual in this respect, because it cannot be stopped by provisioning additional bandwidth.

“On firewalls and other kinds of equipment a list of trusted sources for which ICMP is allowed could be configured,” the TDC SOC experts advise. “Disabling ICMP Type 3 Code 3 on the WAN interface can mitigate the attack quite easily. This is the best mitigation we know of so far.”

That said, there are many devices out there that are configured to accept ICMP traffic from the Internet. The TDC SOC has identified 1.7 million of them in Denmark alone.

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