A botnet is a network of internet-connected devices that have been infected with malicious software and placed under the control of an attacker. The compromised devices may include computers, smartphones, servers, routers, security cameras, smart TVs, and other connected devices.
The word botnet combines “bot,” referring to an infected device that can perform automated tasks, and “network,” because attackers typically control many compromised devices at the same time.
Botnets can remain unnoticed for long periods. A device may continue to work normally while secretly receiving commands from a criminal-controlled infrastructure. Once enough devices are infected, the attacker can use the network for activities such as distributed denial-of-service (DDoS) attacks, spam campaigns, credential theft, malware distribution, and other forms of cybercrime.
Understanding how botnets operate is important for both individuals and businesses because even a single poorly protected device can become part of a much larger malicious network.
What Is a Botnet?
A botnet is a collection of compromised devices controlled remotely by an attacker, often without the knowledge of the device owners.
The infected devices are commonly called bots or zombies. After malware gets installed, the compromised device can communicate with an attacker-controlled command infrastructure and receive instructions.
A botnet does not necessarily consist of identical devices. One network could contain Windows computers, Linux servers, smartphones, routers, IoT devices, and other internet-connected hardware.
The size of a botnet can range from a relatively small group of machines to hundreds of thousands or even millions of compromised devices.
The important distinction is that the device owner usually has no idea that their system is participating in the network.
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How Does a Botnet Work?
Although individual botnets use different techniques, their operation generally follows several stages.
1. Devices Become Infected
The process begins when attackers find a way to install malware on potential victims’ devices.
Common infection methods include:
- Phishing emails and malicious attachments
- Fake software updates
- Malicious websites or downloads
- Exploitation of unpatched software vulnerabilities
- Weak or reused passwords
- Compromised remote-access services
- Poorly secured IoT devices
- Malicious advertisements or applications
Once the malware successfully runs, the device may become part of the botnet.
2. The Malware Establishes Communication
After infection, the malware attempts to establish communication with infrastructure controlled by the attacker.
Depending on the botnet’s design, this communication may use a centralized command-and-control server or a more distributed system.
The malware may also attempt to maintain access after the device is restarted, allowing the attacker to continue controlling it.
3. The Bot Receives Instructions
The compromised device periodically communicates with the botnet’s command infrastructure to determine whether new instructions are available.
Commands could instruct infected devices to perform activities such as sending network requests, distributing malicious files, sending spam, or participating in a coordinated attack.
4. The Botnet Performs the Requested Task
Once instructions are received, thousands of compromised devices can perform the same activity simultaneously.
This is what makes botnets particularly dangerous. An attacker does not need to manually operate every infected machine. Instead, one command can potentially affect a large number of compromised systems.
5. Attackers Expand or Maintain the Network
Some botnets are designed to spread to additional devices. Others focus on maintaining access to existing systems.
Attackers may also update the malware remotely, change communication methods, or replace compromised infrastructure when security researchers discover it.
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How Are Botnets Controlled?
Botnets require some form of communication between infected devices and the infrastructure used by their operators. This is often referred to as command and control, or C2.
There are several approaches attackers may use.
Centralized Command-and-Control
In a centralized model, infected devices communicate with one or more servers controlled by the attacker.
The basic structure is relatively straightforward:
Attacker → Command Server → Infected Devices
The attacker sends instructions to the command server, and compromised devices retrieve or receive those instructions.
One weakness of this approach is that disrupting the command infrastructure can interfere with the botnet. Security researchers, hosting providers, domain registrars, and law enforcement agencies may attempt to identify and take down such infrastructure.
Peer-to-Peer Botnets
More sophisticated botnets may use peer-to-peer communication.
Instead of relying on one central server, infected devices can communicate with other compromised devices. This makes the network more distributed and potentially harder to disrupt.
A simplified structure looks like this:
Bot ↔ Bot ↔ Bot ↔ Bot
The exact architecture varies between botnets, but the basic idea is to reduce dependence on a single central point.
Other Communication Methods
Botnets can use different internet-based communication mechanisms depending on their design. Some have historically used protocols and services that make their traffic appear less unusual.
Modern defenders therefore look at more than just a single server address. They may examine unusual traffic patterns, domain activity, DNS requests, authentication events, and behavior occurring on the endpoint itself.
What Can Botnets Be Used For?
Botnets are not limited to one type of cyberattack. Once attackers control a large collection of devices, they can use that infrastructure for different purposes.
Some common criminal uses include:
- Launching DDoS attacks
- Sending large volumes of spam
- Distributing malware
- Stealing credentials
- Conducting automated fraud
- Generating fraudulent advertising activity
- Scanning networks for vulnerable systems
- Attempting to spread the botnet to additional devices
- Abusing compromised computing resources
The exact purpose depends on the botnet and the goals of its operators.
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Types of Botnet Attacks
Botnets can support several different types of attacks. Some target individual users, while others are designed to overwhelm websites, networks, or online services.
1. DDoS Attacks
A distributed denial-of-service attack is one of the most well-known uses of a botnet.
In a DDoS attack, a large number of compromised devices generate traffic or requests toward a target. The objective is to consume available resources and make a website, application, server, or network difficult or impossible for legitimate users to access.
Because traffic comes from many different devices and locations, blocking a single source may not be enough.
2. Spam and Phishing Campaigns
Botnets can also be used to send enormous volumes of unwanted messages.
Compromised machines may send spam emails or other malicious communications containing phishing links, fraudulent offers, or malware.
Using thousands of infected devices can make large-scale campaigns more difficult to block than messages coming from a single source.
3. Credential Theft
Some botnet malware includes functionality for stealing information from compromised devices.
Depending on the malware, attackers may attempt to obtain usernames, passwords, browser information, session data, or other valuable information.
Stolen credentials can subsequently be used for account compromise and other criminal activities.
4. Malware Distribution
A botnet can become a delivery mechanism for additional malicious software.
An infected device might be instructed to download another payload, potentially turning an initial infection into a more serious security incident.
This approach can also allow attackers to modify the capabilities of compromised systems over time.
5. Cryptomining Abuse
Attackers may use compromised computers or servers to perform cryptocurrency mining without the owner’s permission.
Mining can consume substantial CPU or GPU resources, resulting in higher electricity usage, reduced system performance, increased hardware load, and unexpected resource consumption.
6. Brute-Force and Automated Attacks
A large botnet provides attackers with a substantial pool of machines from which automated login attempts or other malicious activity can be launched.
Compromised devices may also be used to scan internet-facing systems for weak configurations or vulnerable services.
7. IoT-Based Attacks
Internet of Things devices can be attractive targets because many are continuously connected to the internet.
Poorly secured cameras, routers, DVRs, smart appliances, and other connected devices may become part of a botnet if they contain exploitable vulnerabilities or use weak credentials.
This is particularly concerning because owners may not regularly inspect these devices for suspicious activity.
Why Are Botnets Difficult to Detect?
One reason botnets can remain active for a long time is that the infected device may not show obvious signs of compromise.
The malware may operate quietly in the background while the computer continues performing normal tasks.
However, some warning signs can include:
- Unexpected system slowdowns
- Unusually high CPU or memory usage
- Increased network activity
- Unknown applications or processes
- Unexpected outbound connections
- Browser behavior that suddenly changes
- Security software detecting suspicious files
- Unexplained internet or bandwidth consumption
- Devices restarting or behaving unusually
None of these symptoms alone proves that a device is part of a botnet. Similar behavior can also be caused by legitimate applications, hardware problems, or other types of malware.
How to Protect Against Botnets
Preventing a botnet infection is generally easier than dealing with a compromised network later. A few practical security measures can significantly reduce the risk.
Keep Software Updated
Install security updates for operating systems, browsers, applications, routers, and other connected devices.
Software updates frequently address vulnerabilities that attackers could otherwise exploit.
Do not ignore update notifications for long periods, particularly when an update includes security fixes.
Use Strong, Unique Passwords
Avoid using the same password across multiple accounts.
Use long and unique passwords or passphrases, and consider using a reputable password manager to manage them.
For routers, cameras, and IoT devices, change default administrator credentials before putting the device into regular use.
Enable Multi-Factor Authentication
Where available, enable multi-factor authentication (MFA), particularly for email, cloud services, administrator accounts, financial accounts, and other important services.
MFA adds another layer of protection if a password is exposed.
Be Careful With Email Attachments and Links
Do not open unexpected attachments or click suspicious links simply because a message appears urgent.
Check the sender and destination carefully. Attackers often use convincing messages to persuade people to install software, disclose credentials, or visit malicious websites.
Secure IoT Devices
IoT devices should not be overlooked when creating a security strategy.
Change default passwords, install available firmware updates, disable unnecessary services, and review remote-access settings.
If a device no longer receives security updates, replacing it may be safer than leaving a vulnerable product connected to the internet.
Use Security Software
Modern endpoint security tools can detect many forms of malicious software and suspicious behavior.
Keep antivirus or endpoint protection enabled and updated. Businesses should consider centralized endpoint monitoring so security teams can investigate unusual activity across multiple systems.
Secure Your Router
A router is an important part of a home or business network.
Use a strong administrator password, install firmware updates, disable unnecessary remote administration, and review connected devices periodically.
Changing the default Wi-Fi credentials and using modern wireless security standards can also reduce opportunities for unauthorized access.
Monitor Network Activity
Businesses can benefit from monitoring outbound traffic and unusual communication patterns.
Unexpected connections to unfamiliar destinations, unusual volumes of traffic, or devices communicating at unusual times can provide useful clues during an investigation.
Network monitoring is particularly valuable for organizations with large numbers of computers, servers, and IoT devices.
Back Up Important Data
Regular backups will not necessarily prevent a botnet infection, but they can reduce the impact of many malware incidents.
Keep important backups separate from ordinary user systems where possible, and periodically verify that the backups can actually be restored.
What Should You Do If You Suspect a Botnet Infection?
If you believe a device may be compromised, do not simply ignore the warning signs.
For a personal computer, start by disconnecting the device from the internet if doing so will not create another safety or operational problem. Run a trusted security scan and install pending security updates.
If malware is detected, follow the security software’s recommended removal process. For serious infections, professional assistance or a complete system rebuild may be appropriate.
For businesses, the response should generally involve isolating the affected system, preserving relevant security information, investigating other potentially compromised devices, resetting exposed credentials, and determining how the initial infection occurred.
Simply removing one malicious file may not be enough if attackers still have another method of accessing the environment.
Botnet vs. Malware: What’s the Difference?
The terms botnet and malware are related but do not mean the same thing.
Malware is a broad term for malicious software, including viruses, worms, trojans, spyware, ransomware, and other harmful programs.
A botnet is a network of compromised devices that can be remotely coordinated for malicious purposes.
In simple terms, malware can infect a device, while a botnet can organize many infected devices into a remotely controlled network.
Final Thoughts
Botnets turn ordinary internet-connected devices into resources that attackers can control remotely. Their strength comes from scale: one compromised computer may not seem significant, but thousands of compromised devices can generate substantial computing power, network traffic, or automated activity.
The best defense starts with basic security practices. Keep software and firmware updated, use strong unique passwords, enable multi-factor authentication, secure IoT devices, avoid suspicious downloads and links, and monitor unusual activity.
For organizations, endpoint protection, network monitoring, access controls, vulnerability management, and an established incident-response process can provide additional layers of defense.
Botnets continue to evolve, but the fundamental security principle remains the same: reducing unnecessary vulnerabilities and detecting suspicious behavior early can make it much harder for attackers to turn your devices into part of a malicious network.
FAQs
Q1. What is a botnet?
A botnet is a group of internet-connected devices infected with malware and remotely controlled by an attacker without the owners’ knowledge.
Q2. How does a botnet work?
A botnet infects devices with malware and connects them to attacker-controlled infrastructure. The compromised devices can then receive commands and perform coordinated tasks.
Q3. How are botnets controlled?
Botnets can be controlled through command-and-control infrastructure, including centralized servers or distributed peer-to-peer networks.
Q4. What are common botnet attacks?
Common uses include DDoS attacks, spam campaigns, credential theft, malware distribution, unauthorized cryptomining, and automated attacks against vulnerable systems.
Q5. How can I protect my device from botnets?
Keep software updated, use strong unique passwords, enable multi-factor authentication, secure IoT devices, avoid suspicious links and downloads, and use reliable security software.
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