Botnet Attacks: How attackers build and use them?

Botnet Attacks Explained: What They Are and How to Detect and Prevent Them

Botnets are behind some of the largest and most disruptive cyberattacks in history. From knocking major websites offline to silently mining cryptocurrency on your laptop, botnet infrastructure powers a significant portion of global cybercrime. And the scale is growing. Modern botnets can include millions of infected devices, spanning home computers, smartphones, routers, and smart home gadgets.

In this guide, you will learn exactly what a botnet is, how attackers build and use them, what to look for if you suspect a device is infected, and what steps you can take to protect yourself or your organization.

What Is a Botnet?

A botnet is a collection of compromised, internet-connected devices that an attacker controls remotely, usually without the device owners realizing anything is wrong. The name combines two words: robot and network.

Each infected device is called a bot (sometimes a "zombie"). The person controlling the botnet is called a botmaster or herder. The attacker issues instructions through a command and control (C2) server, and every bot in the network executes those instructions  quietly, in the background, while the owner goes about their normal activity.

Bots, Zombies, and Herders: The Key Terms Explained

• Bot / Zombie: A device that has been compromised by botnet malware and is under remote attacker control.
• Botmaster / Herder: The attacker or threat actor who operates the botnet.
• C2 Server (Command and Control): The infrastructure the botmaster uses to issue instructions to the bots.
• Botnet: The full network of infected devices under a single attacker's control.

How Is a Botnet Different from Malware, a Virus, or Ransomware?

People often use these terms interchangeably, but they describe different things. Here is a quick comparison:

The key point: a botnet is not a single piece of malware. It is a persistent, coordinated infrastructure that attackers use to execute many different types of attacks at scale.

How Does a Botnet Work?

Building and operating a botnet follows a predictable lifecycle. Understanding it helps you recognize the warning signs at each stage.

Step 1: Infection

The attacker first needs to get malware onto a device. This typically happens through:

• Phishing emails with malicious attachments or links.
• Drive-by downloads from compromised or malicious websites.
• Exploiting unpatched software vulnerabilities in operating systems or applications.
• Infected USB drives or pirated software.
• Weak or default credentials on routers and IoT devices.

Once the malware executes, it runs silently in the background. The device owner typically notices nothing unusual at this stage.

Step 2: Connection to the Attacker

After infection, the malware reaches out to the attacker's infrastructure to announce itself and await instructions. This outbound connection, called beaconing, is one of the detectable behavioral signatures of botnet activity. Beaconing typically happens on a regular schedule, mimicking normal traffic to avoid detection.

Step 3: Command and Control (C2)

The bot is now under the attacker's control. The botmaster can issue commands across the entire network simultaneously, directing thousands or millions of devices at once. C2 communication can happen over HTTP, IRC, or more evasive channels like encrypted peer-to-peer traffic.

Step 4: Malicious Activity Execution

With the network established, the botmaster puts the devices to work. This might mean sending spam, launching a DDoS attack, mining cryptocurrency, stealing credentials, or any combination of activities. The infected device is doing the heavy lifting, and the victim is paying the electricity bill.

Centralized vs. Peer-to-Peer Botnets

Modern sophisticated botnets increasingly use P2P or hybrid architectures, making them significantly harder for defenders and law enforcement to dismantle.

What Are Botnets Used For?

Botnets are a general-purpose attack platform. Once an attacker controls a large enough network of devices, they can monetize it in many ways:

• DDoS Attacks: Flooding a target website, server, or network with traffic to take it offline. Botnets make this trivially scalable.
• Spam Campaigns: Sending billions of phishing or malware-laced emails using infected devices as proxy senders, avoiding spam filters tied to known bad IPs.
• Malware Distribution: Using botnet infrastructure to spread additional malware, ransomware, or other botnet infections.
• Credential Stuffing: Testing stolen username/password pairs against banking, email, or retail sites at a massive scale.
• Account Takeover and Fraud: Exploiting compromised accounts for financial theft or resale on dark web markets.
• Cryptojacking: Silently using the infected device's CPU or GPU to mine cryptocurrency for the attacker, increasing device operating costs for the victim.
• Data Theft: Capturing keystrokes, browser credentials, or sensitive documents from infected machines.
• Proxy Abuse / Bandwidth Theft: Routing the attacker's own traffic through victim devices to obscure their real identity or location.

In many cases, botnet operators rent out access to their network, a model known as Botnet-as-a-Service (BaaS), to other cybercriminals who pay per device or per attack.

Warning Signs: Is Your Device Part of a Botnet?

Most botnet infections are designed to be invisible. But infected devices often show subtle behavioral changes over time. Watch for these warning signs:

1. Unusual slowness: The device is processing background tasks for the attacker.

2. High CPU or RAM usage even when idle or running simple applications.

3. Increased network activity at unexpected hours, especially late at night.

4. Strange outbound connections to unfamiliar IP addresses or foreign servers.

5. Device overheating without an obvious cause.

6. The battery is draining faster than normal on laptops or smartphones.

7. Unusual DNS requests or traffic to known malicious domains.

8. Unexplained crashes, reboots, or instability.

9. Unfamiliar processes running in the background (visible in Task Manager or Activity Monitor).

10. Router behaving strangely, with slow speeds, unexpected reboots, and unfamiliar connected devices.

No single symptom confirms a botnet infection, but a cluster of these signs warrants investigation.

How to Check Whether Your Device Is Infected

For Home Users

Check running processes:

Open Task Manager (Windows) or Activity Monitor (macOS). Look for unfamiliar processes consuming significant CPU or memory. Search for any unknown process name before ending it.

Review your router's connected devices: 

Log in to your router's admin panel and review the list of connected devices. If you see devices you do not recognize, investigate immediately.

Monitor outbound network traffic:

Use a tool like GlassWire (Windows) or Little Snitch (macOS) to see which apps and processes are sending data outside your network and to where.

Run a reputable malware scanner:

Tools like Malwarebytes, Windows Defender, or your endpoint protection software can detect many known botnet agents. Run a full system scan.

Check for beaconing behavior:

Repeated, timed outbound connections at regular intervals, especially to the same unfamiliar IP, are a red flag. Your router logs or a network monitoring tool can reveal these patterns.

Look up your IP reputation:

Services like MXToolbox or AbuseIPDB can tell you whether your IP address has been flagged as part of a botnet or sending spam.

For Business and IT Teams

Review endpoint detection and response (EDR):

Alerts for behavioral anomalies consistent with C2 beaconing (regular outbound connections, unusual process trees, injected code).

Analyze network flow data (NetFlow / IPFIX):

For unusual outbound traffic volume, patterns, or destinations, especially to known malicious IPs or unusual geographies.

Query DNS logs:

For repeated lookups to unfamiliar or algorithmically generated domain names (a technique called Domain Generation Algorithms, or DGAs, used by some botnets to evade detection).

Cross-reference threat intelligence feeds against observed IPs, domains, and file hashes.

Check for lateral movement indicators:

A bot may attempt to spread across your internal network using credential theft, SMB exploitation, or remote administration tools.

Escalate to your SIEM or SOC:

If multiple endpoints show correlated anomalous behavior, Coordinated C2 communication is often detectable at the network level even when hidden at the endpoint level.

How to Remove a Botnet Infection?

If you have confirmed or strongly suspect an infection, act methodically. Speed matters, but so does doing this in the right order.

Step-by-Step Remediation Guide

1. Isolate the device immediately: Disconnect it from your network, unplug the Ethernet, and turn off Wi-Fi. This prevents the botnet from receiving new instructions and limits lateral spread.
2. Do not use the infected device to change passwords, access banking, or communicate sensitive information until it is clean.
3. Run a full malware scan using an up-to-date endpoint protection tool or a dedicated malware removal scanner. Boot from external media or safe mode if necessary for deeper scanning.
4. Remove identified malware: Follow the tool's guidance to quarantine and delete detected threats. Do not assume a single scan is sufficient  run at least two different tools.
5. Patch your operating system and all applications: Many botnet infections exploit known vulnerabilities. Ensure all software is fully updated before reconnecting.
6. Update or replace firmware on routers, cameras, and IoT devices. Factory reset smart home devices that cannot be reliably scanned, and configure them with strong, unique credentials before reconnecting.
7. Change all passwords that may have been exposed or accessible on the infected device, in email, in banking and work accounts, and on social media. Assume they are compromised.
8. Enable multi-factor authentication (MFA) on all accounts immediately.
9. Consider a clean reinstall: For severe or persistent infections, the safest course of action is to wipe the device and reinstall the operating system from a known-clean source.
10. Reconnect carefully and monitor: After remediation, watch for signs of reinfection, especially unusual outbound traffic. If the behavior returns, the infection may have persisted or re-entered via another vector.

For businesses: Remediation should follow your incident response plan. Contain the affected segment, preserve forensic evidence before wiping, and notify the relevant stakeholders in accordance with your breach notification policy.

How to Prevent Botnet Infections

Prevention is significantly less costly and disruptive than remediation. The following controls address the most common botnet infection vectors:

For organizations, layering these controls defense in depth significantly raises the cost and difficulty of a successful botnet infection and limits its impact if one occurs.

Real-World Botnet Examples

Understanding real botnets puts the threat in concrete terms.

Mirai (2016)

Mirai remains one of the most influential botnets in history. It targeted internet-facing IoT devices, routers, IP cameras, and DVRs by scanning for devices still using factory-default credentials. At its peak, Mirai infected over 600,000 devices. In October 2016, it was used to launch a DDoS attack against Dyn, a major DNS provider, taking down large portions of the internet across the US and Europe, including Twitter, Netflix, Reddit, and GitHub. The key lesson: default credentials on IoT devices are an existential risk at scale.

GameOver Zeus (2011–2014)

GameOver Zeus (GOZ) was a sophisticated, P2P botnet based on the Zeus banking trojan. It was estimated to have infected between 500,000 and 1 million machines globally and was used primarily for financial fraud, capturing banking credentials and facilitating fraudulent wire transfers. GOZ was also the distribution vehicle for the CryptoLocker ransomware. A coordinated international law enforcement takedown in 2014 disrupted the network, though the operators behind it were not immediately apprehended. The key lesson: peer-to-peer architecture makes botnets highly resilient to takedown attempts.

Emotet (2014–Present)

Emotet started as a banking trojan but evolved into one of the most dangerous and modular botnets ever documented. It operated primarily through malicious email campaigns, delivered malicious Word documents with macro-enabled payloads, and was frequently used to deploy secondary infections, including TrickBot and Ryuk ransomware. Europol led a major disruption operation in January 2021, but Emotet infrastructure re-emerged by late 2021.

The key lesson: botnets can evolve, survive disruption, and act as delivery platforms for other malware, making them highly dangerous secondary threats.

Frequently Asked Questions About Botnets

Is a botnet a virus?

A: Not exactly. A botnet uses malware to infect devices, and that malware may include virus-like components, but a botnet specifically refers to the network of infected, remotely controlled devices. The malware is the tool; the botnet is the infrastructure it creates.

Can smart home devices like cameras or thermostats be infected?

A: Yes, and this is one of the most rapidly growing attack surfaces. IoT devices running embedded operating systems with no user-visible security controls and often shipped with identical default credentials are prime targets. The Mirai botnet demonstrated just how devastating IoT botnet attacks can be.

Can a botnet steal my passwords?

A: Yes. Many botnet agents include keylogging or credential-harvesting modules. They can capture usernames and passwords entered into browsers, banking portals, or other applications on the infected device and silently transmit them to the attacker.

What is the difference between a botnet and a DDoS attack?

A: A botnet is the infrastructure of a network of infected, controlled devices. A DDoS (Distributed Denial of Service) attack is one of the uses of that infrastructure. Not all botnets are used for DDoS, and while DDoS attacks can technically be launched from other means, botnets are the most common and scalable tool for doing so.

Conclusion

Botnets represent one of the most scalable and versatile threats in the modern cybersecurity landscape. They are not exotic, nation-state-level attacks reserved for governments and large enterprises; they affect home users, small businesses, hospitals, and critical infrastructure alike. Any internet-connected device, from a laptop to a smart lightbulb, is a potential recruit.

The good news is that the protective measures are well understood and largely accessible. Keeping devices patched, using strong credentials with MFA, training users to recognize phishing, and monitoring network traffic will make your environment significantly harder to compromise. And if you suspect an infection, acting quickly, isolating the device, and working through a methodical remediation process limits the damage substantially.

➤ For a deeper understanding of related threats and defenses, explore our resources on:

• DDoS attacks
• Malware prevention
• Endpoint security
• Network monitoring best practices