Mirai attacks spread after the malware’s source code became public in late September or early October 2016, allowing other operators to build their own botnets from it. Those botnets recruited internet-connected devices with default or hard-coded credentials, then used the infected devices to launch distributed denial-of-service (DDoS) attacks.
What Mirai did
Mirai was malware that searched the internet for IoT devices protected by factory-default or hard-coded usernames and passwords. When it found a vulnerable device, it installed itself and made the device part of a botnet: a group of compromised machines that an operator could direct through command-and-control infrastructure.
The system had several roles: a scanner found potential targets, a loader helped install the malware, and command-and-control and attack servers coordinated the bots and sent attack instructions. The compromised devices generated traffic toward a chosen victim, distributing the load across many sources. That is the basic mechanism of a distributed denial-of-service attack.
The scale of the wider IoT population was part of the context, not a measure of Mirai’s reach. Gartner forecast 6.4 billion connected things worldwide in 2016 and 20.8 billion by 2020; those were forecasts for connected devices, not counts of infected devices.
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What changed when the source code became public
In late September or early October 2016, a user calling themselves Anna-Senpai announced Mirai’s source code on the Hackforums community. Publishing it lowered the barrier to entry: other operators could reuse or modify the code rather than develop an IoT botnet from scratch.
| Before the public release | After the public release |
|---|---|
| The original code and infrastructure were controlled by one group. | Other operators could create separate botnets, alter their behavior, and operate separate infrastructure. |
| One operator’s botnet competed for vulnerable devices. | Copycat operators competed for the same limited pool of poorly secured devices. |
| Similar activity was more closely associated with the original operation. | Shared code and independent infrastructure made it harder to determine which group was behind an attack. |
KrebsOnSecurity described “dozens of copycat Mirai botnets,” and Cloudflare documented multiple independent infrastructures. The release therefore multiplied the number of potential operators; it did not mean that every later Mirai-related attack came from the original group.
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How attacks escalated in 2016
Mirai was publicly associated with major DDoS attacks on KrebsOnSecurity and French hosting provider OVH in September 2016. Internet Initiative Japan (IIJ), in a 2017 technical review, reported these historical peak estimates:
| Target | Reported peak | Source and qualification |
|---|---|---|
| KrebsOnSecurity | 665 Gbps | Internet Initiative Japan, 2017; a reported peak for the 2016 incident. |
| OVH | 1 Tbps | Internet Initiative Japan, 2017; a reported peak for the 2016 incident. |
These figures are historical estimates of attack traffic, not measurements of Mirai’s current capabilities or today’s threat landscape. Contemporary reporting after the code release also described a considerable increase in infected devices and linked Mirai to further DDoS activity.
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How a Mirai-derived botnet disrupted Dyn
In October 2016, a Mirai clone was used in an attack on Dyn, a provider of DNS services. DNS helps translate a website’s name into the network address browsers need to reach it. When Dyn’s service was disrupted, people had trouble reaching major destinations that relied on it, including Twitter, Netflix, and Reddit, for substantial periods.
The distinction between a clone and the original malware matters: later reporting describes Mirai clones and related infrastructures, rather than establishing that every disruption to Dyn came from the original Mirai binary. The incident showed how an attack on a supporting internet service could affect access to many familiar sites without those sites themselves being the direct target.
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What the reported bot count does—and does not—show
Anna-Senpai told KrebsOnSecurity: “With Mirai, I usually pull max 380k bots from telnet alone. However, after the Kreb [sic] DDoS, ISPs been slowly shutting down and cleaning up their act. Today, max pull is about 300k bots, and dropping.” This is the pseudonymous release author’s own account, not an independently verified census of infected devices. It should not be read as a definitive count of Mirai bots across all operators.
Why the leak made attribution harder
Once multiple groups could use related code while maintaining separate infrastructure, seeing Mirai-like behavior no longer identified a single operator. Investigators had to distinguish among copies, modifications, and independent botnets. The source-code release explains why Mirai use spread; it also helps explain why attributing a later attack to the original operators requires more than finding code associated with Mirai.
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