What Is a Botnet? How They Work and How to Avoid Being Part of One

What Is a Botnet?

A botnet is a network of internet-connected devices — computers, smartphones, routers, smart home devices, or any networked hardware — that have been infected with malware and brought under the covert control of an attacker. The infected devices in a botnet are called "bots" or "zombies," and the person controlling them is called the "botmaster" or "bot herder."

The defining characteristic of a botnet is that the device owners are typically completely unaware that their device has been compromised or that it is being used to conduct malicious activities. Your computer could be part of a botnet right now, quietly sending spam emails or participating in a cyberattack while you browse the web, and you might never notice.

How Botnets Are Built

Building a botnet involves several stages:

Stage 1: Malware Distribution

Botnet malware spreads through a variety of vectors:

• Phishing emails with malicious attachments or links
• Drive-by downloads from compromised or malicious websites that exploit browser vulnerabilities
• Infected USB drives or other physical media
• Exploit kits that scan for unpatched vulnerabilities in browsers and plugins
• Malicious applications in third-party app stores or disguised as legitimate software
• IoT vulnerabilities — many routers, cameras, and smart devices ship with default credentials and unpatched firmware, making them trivially easy to compromise

Stage 2: Infection and Persistence

Once the malware executes on a device, it establishes persistence — modifying system settings, registry keys, or startup processes to ensure it survives reboots. It then typically downloads additional components and connects to command-and-control infrastructure.

Stage 3: Command and Control (C2)

Infected devices need a way to receive instructions from the botmaster. Traditionally, bots connected to centralized C2 servers. Modern botnets increasingly use peer-to-peer (P2P) architectures, where bots communicate with each other in a distributed network, making them much harder to disrupt — there is no single server to take down.

Botnets also commonly use legitimate platforms for command distribution: Twitter, Discord, GitHub, and even Google Docs have been used to hide C2 commands in plain sight.

Stage 4: Operation

Once established, the botnet can be operated by its creator or rented to third parties for various malicious purposes.

What Botnets Are Used For

The criminal utility of a botnet derives from scale — even a simple attack becomes devastating when executed simultaneously from thousands or millions of compromised devices:

Distributed Denial of Service (DDoS) Attacks The most well-known botnet use. Thousands to millions of bots simultaneously send traffic to a target server or network, overwhelming its capacity and taking it offline. DDoS attacks using large botnets can take down major websites, financial institutions, or critical infrastructure for hours or days. The Mirai botnet, which hijacked millions of IoT devices in 2016, took down major portions of the internet by targeting Dyn, a major DNS provider.

Spam Distribution Email spam campaigns require enormous sending volume to be effective — legitimate email providers block servers that send unusual volumes. By distributing the sending load across thousands of compromised devices, botmasters can send billions of spam messages while evading detection and avoiding being traced.

Credential Stuffing Botnets are used to automate large-scale credential stuffing attacks — automatically trying username/password combinations from leaked databases against target websites. Distributing the attempts across thousands of IPs evades rate limiting and IP-based blocking.

Cryptocurrency Mining Mining certain cryptocurrencies requires significant computational power. By covertly using the collective CPU/GPU resources of botnet members, botmasters can mine cryptocurrency at no cost to themselves while victims pay the electricity bill and suffer reduced device performance.

Click Fraud Advertisers pay for ad clicks. Botnets can generate fraudulent ad clicks at massive scale, defrauding advertisers and enriching botmasters who collect per-click revenue from ad networks they participate in.

Data Theft Sophisticated botnets include keyloggers, screenshot tools, and credential-harvesting capabilities that steal banking information, passwords, and other sensitive data from infected devices and transmit it to the botmaster.

Famous Botnets in History

Mirai (2016): Infected hundreds of thousands of IoT devices — cameras, routers, DVRs — using their default credentials. Used to launch some of the largest DDoS attacks in history. Its source code was released publicly, spawning dozens of variants that continue to infect IoT devices today.

Zeus (2007-ongoing variants): One of the most prolific banking credential-stealing botnets. Zeus infected millions of computers and stole hundreds of millions of dollars before law enforcement action disrupted its operations. Its source code leak led to many descendants still active today.

Emotet (2014-2021, returned 2022): Originally a banking trojan, Emotet evolved into the world's most sophisticated malware distribution infrastructure, providing "malware-as-a-service" to other criminal organizations. It was taken down in a coordinated international law enforcement operation in January 2021, re-emerged in late 2021, and was disrupted again in 2022.

Conficker (2008): At its peak, infected an estimated 9-15 million computers worldwide, including systems in government, military, and critical infrastructure. Despite its scale, its operators never launched the devastating attack it was feared capable of.

How to Tell If Your Device Is Part of a Botnet

Signs that may indicate botnet infection:

• Unexplained slowdown in device performance
• Higher than usual network activity when the device is idle
• Unusual data usage spikes on your internet plan
• Emails in your sent folder that you did not write
• Security software disabled or unable to update
• Browser redirects or unexpected pop-ups
• Overheating (particularly on laptops and IoT devices) due to cryptocurrency mining

Note that botnets are specifically designed to be stealthy. Absence of obvious symptoms does not confirm that a device is clean.

How to Protect Your Devices

Keep all software updated. Most botnet infections exploit known, patched vulnerabilities. Timely updates are the most effective defense.

Use strong, unique passwords. Change the default credentials on all routers, IoT devices, and home network equipment immediately. Default credential databases are publicly available and actively used by botnet operators.

Install reputable security software. On Windows, Microsoft Defender Antivirus is adequate for baseline protection. Malwarebytes Premium adds effective malware-specific scanning. Ensure real-time protection is active.

Be cautious with email attachments and links. Phishing remains the most common initial infection vector. Never open unexpected attachments or click links in unsolicited emails.

Segment your home network. Place IoT devices (smart TVs, cameras, doorbells) on a separate guest network, isolating them from computers containing sensitive data.

Check your router's security settings. Disable remote administration, change default admin credentials, and keep the router's firmware updated. Most home routers are woefully insecure in their default configurations.

Botnets represent one of the most persistent challenges in cybersecurity precisely because they weaponize ordinary users' devices without those users' knowledge. Maintaining basic security hygiene — updates, strong passwords, security software, and email vigilance — keeps your devices from becoming unwilling participants in others' crimes.