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Redazione RHC : 12 November 2025 22:24
Many years have passed since the SQL Slammer worm spread uncontrollably on January 25, 2003.
It was one of the fastest-spreading malware in history , and with this article we want to retrace those moments, to understand technically how it worked and what happened that day many years ago.
At that time, cybersecurity concepts were not as widespread as they are today, and many people found themselves completely unprepared and disoriented in dealing with this cyber pandemic.
If you worked in IT in 2003, you’ll remember what you were doing when Slammer entered your life. It was a Saturday, January 25, 2003, when the SQL Slammer worm was launched in the early hours of the morning.
By the time many of us woke up and heard about it, Slammer had already taken down most of the world’s SQL servers and networks, including bank ATM networks, which were completely out of service.
Newspapers and print media didn’t reach the newsstands, and if your company used personal computers at the time, you likely heard about them.
SQL Slammer was an incredible piece of malware measuring just 376 bytes . It attempted to connect to every computer it could, searching for the MS-SQL UDP port 1434. It didn’t care whether the computer it was connecting to was running SQL Server or not.
Until then, many researchers thought the “try an exploit to see if the flaw actually exists” approach was highly inefficient. Why would you waste time testing an exploit if there wasn’t an MS-SQL server, or perhaps one did exist and was up to date?
Slammer proved the experts wrong and laid the foundation for the mass scanning that is now commonplace.
In fact, Slammer attempted to connect to every reachable computer on the network to find a vulnerable one. This didn’t just include SQL servers, but any workstation running unpatched versions of Microsoft’s SQL product.
SQL Slammer spread to tens of thousands of computers within the first hour.
Within hours, its code had infected most of the unpatched servers on the Internet, and would continue to do so until late that evening. When Slammer identified a vulnerable SQL server, it propagated to rerun a copy of itself on the newly infected server.
We don’t know today who wrote it or for what purpose, but we know that it was an extremely versatile and lightweight code.
Hypotheses include an experiment by a super-smart hacker who wanted to break into a specific SQL server amid all the noise the malware was generating, or a mischievous hacker who didn’t know what he was doing when he launched it. Until then, no other malware had ever spread like that and caused so much damage so quickly.
It was an immediate paradigm shift.
The two biggest signs of a SQL Slammer infection were interconnected applications stopping working or the entire network collapsing due to all the bandwidth the malware was using.
The surprising thing about Slammer (something we have seen many times in modern times) was that the patch for the exploited vulnerability had been available for almost six months.
In fact, security researcher David Litchfield and his company, NGSSoftware, discovered the bug in SQL Server in May 2002, responsibly alerting Microsoft, who released the patch in July 2002.
SQL Slammer taught us that critical vulnerability patches needed to be applied as quickly as possible.
The world from then on could no longer be lulled into a false sense of security simply because hackers and malware authors did not take full advantage of the exploits they generated.
This meant that the time between the patch release and the exploit had to start being measured in seconds.
Slammer ultimately caused no intentional damage other than crashing the SQL server and collapsing network bandwidth.
It didn’t infect files, delete data, collect passwords, or do any of the other sneaky things that almost all malware these days does by default.
Those were different times than today, where cybercrime is no longer a game, but a multi-billion dollar industry.
That said, always apply security patches as soon as possible.
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