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EternalBlue: A retrospective on one of the biggest Windows exploits ever

It's been more than three years since EternalBlue; let's do a recap.

EternalBlue: A retrospective on one of the biggest Windows exploits ever

Many stories are floating around on the internet about computer viruses and worms that made an enormous impact on the world. From legends like the blistering fast SLAMMER worm from 2003 or the I LOVE Bug worm that set the stage for the fishing email notoriety, we see today. But the story behind EternalBlue has it all. Leaked straight out of the NSA's toolbox, it was a key asset for the well-known WannaCry ransomware to wreak havoc on the world. Some will even argue that it led to a full-on "Cyber War"...

Before we start, this story is way too complex to fully cover in a short blog. We are skipping a lot, so go out and read more if you are interested.

The beginning

From 2016 to 2017, the InfoSec community was turned upside down by the so-called "Shadow Brokers". This unknown, mysterious group was responsible for five major leaks containing classified hacking tools, collected and archived by the NSA. The NSA has an unprecedented catalog of unknown exploits and backdoors due to decades of hoarding. But instead of notifying the software vendor about these vulnerabilities, they keep these to themselves. These exploits are valuable weapons for the NSA to protect the United States of America and its citizens (controversial, but a topic for another day).

On April 14, 2017, the Shadow Brokers released their fifth leak containing multiple exploits. Infamous EternalBlue was one of them.

So what does EternalBlue do exactly?

Great question! EternalBlue takes advantage of a vulnerability seen in Microsoft's implementation of the SMB protocol. SMB stands for Server Message Block. It is primarily used in Windows for shared folders. Like most of these protocols, it makes communication between two nodes on a network possible via request-response messages (gross over-simplification).

However, Microsoft made a critical error in the way these request messages where handled. By sending a carefully made request to port 445 (which was by default open) containing more data than expected, an attacker was able to set a "buffer overflow" in motion. Making the execution of unsigned (and thus malicious) code possible.


A month before the leak, something remarkable happened. The NSA contacted Microsoft to inform them about EternalBlue, presumably because the NSA knew something was about to go down. Microsoft got to work immediately and mysteriously delayed its Patch Tuesday because of a "last-minute issue". They eventually released a security bulletin on March 14, the now infamous: MS17-010. Unfortunately, such a wide-spread piece of software like Windows has a prolonged update adaption rate. This proved, once again, to be true...


On May 12, 2017, the WannaCry (powered by EternalBlue) ransomware attack took the world by storm, infecting and encrypting 230,000 computers in over 150 countries. Russia, Ukraine, India, and Taiwan where affected the most, according to Kaspersky Lab. Huge factories from Nissan and Renault came to a screeching halt. NHS hospitals in the UK where severely affected by the attack and where locked out of essential medical equipment. Some estimated that economic losses could be in the hundreds of millions. North Korea is the main suspect for the attack. Two North Korean hackers have been expedited.


Not much later, on June 27, 2017, Ukraine got stomped by another vicious cyberattack. Armed with EternalBlue, the malware called "NotPetya" was able to cause significant damage. The attack was mainly targeted towards Ukrainian banks, government bodies, and state-owned organizations. Think of airports, railways, and telecom companies. Even the radiation monitoring system at the Chernobyl Nuclear Power Plant went offline. Because the attack seemed to be aimed at paralyzing the Ukrainian state rather than for monetary motives, Russia is the main suspect. Although Ukrainian authorities and the CIA spoke out, Russia denies any involvement.

Present day

Scary stuff, right? Should you still be worried about EternalBlue in 2020? Well, the exploit indeed lives up to its name. According to Trend Micro, in 2019, two years after WannaCry broke loose, 73,763 detections were made of specific malware samples known to use EternalBlue. Luckily, Microsoft's MS17-010 patch has reached most home users. But organizations are notoriously slow with updating their hardware. Therefore, EternalBlue is still a famous attack today.

But what can you do, you might ask? If we can take one thing from all of this, we should stop using SMB over the internet. EternalBlue is undoubtedly not the last SMB related exploit. Blue's younger brother, SMBleed (CVE-2020-1206), is a new vulnerability and works very similar. It was only patched by Microsoft in June of this year.

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Bas Palinckx

Written by Bas Palinckx

A software engineer interested in Cyber security.

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