What Is a Botnet?

A botnet is a network of compromised computers or devices, controlled remotely by cybercriminals without the owners’ knowledge.

A botnet is a network of internet-connected devices, such as computers, servers, or Internet of Things (IoT) devices, that have been infected with malicious software and are controlled remotely by an attacker, often without the knowledge of the device's owner. These compromised devices, also known as "bots" or "zombies," are typically coordinated by a central command system that directs their collective actions.

Botnets can be used for a wide range of illegal activities, including launching large-scale distributed denial-of-service (DDoS) attacks, sending vast quantities of spam emails, or mining cryptocurrencies. The malicious operator can manage the botnet to exploit the processing power of the infected devices, disrupt services, or steal sensitive data, making botnets a significant tool for cybercriminals in large-scale cyberattacks.

Botnets can be highly resilient and difficult to detect, as they often operate in the background without noticeably affecting the performance of individual devices. Their decentralized and distributed nature makes them a potent and ongoing cybersecurity threat.

How Does a Botnet Work?

A botnet operates through a series of coordinated steps that allow cybercriminals to gain control over multiple devices and use them for malicious purposes. Here's how a typical botnet works:

Infection. The process starts with the attacker spreading malicious software (malware) designed to infect devices. This malware can be delivered through phishing emails, malicious downloads, infected websites, or vulnerabilities in software. Once the malware is installed, the device becomes part of the botnet without the owner’s knowledge.
Connection to the command-and-control (C&C) server. After infection, the compromised device, now a "bot" or "zombie," establishes communication with a central command-and-control (C&C) server, which is operated by the attacker. The C&C server acts as the brain of the botnet, sending instructions to the infected devices and receiving data from them.
Stealth and expansion. Botnets are designed to avoid detection. The malware may disable antivirus software or operate in the background with minimal disruption to the infected device’s performance. During this time, the attacker can continue to infect more devices, growing the botnet.
Execution of commands. Once a botnet is large enough, the attacker can issue commands to all the infected devices simultaneously. These commands can be used to perform a variety of malicious actions, such as launching DDoS attacks, sending spam, mining cryptocurrencies, or stealing data.
Maintenance and evasion. Botnets are typically maintained over time, with infected devices regularly communicating with the C&C server. Some advanced botnets use decentralized or peer-to-peer structures to make it harder to take down the network. They may also use evasion techniques to avoid detection and removal, such as changing the location of the C&C server or using encryption to hide their communication.
Exploitation and attack. The attacker uses the botnet for the intended malicious purposes, such as overwhelming websites with traffic in a DDoS attack, spreading malware, or generating fraudulent clicks in advertising schemes. These attacks can cause significant harm, often on a global scale, while the individual owners of the compromised devices may remain unaware that their systems are being used for such activities.

How Are Botnets Controlled?

Botnets are controlled through a central mechanism that allows cybercriminals to communicate with and manage the infected devices, coordinating their activities for malicious purposes. There are two primary methods for controlling botnets: centralized control and decentralized control.

Centralized Control

In a centralized control model, a botnet relies on a single command-and-control (C&C) server to communicate with the infected devices. Here’s how it works:

Command-and-control (C&C) server. The attacker operates a central server, which sends instructions to the bots (infected devices) and receives data from them. The botnet malware installed on each compromised device includes the location of the C&C server, allowing it to connect and await commands.
Communication protocols. Communication between the bots and the C&C server typically occurs using common internet protocols such as HTTP, IRC (internet relay chat), or custom protocols. This allows botnets to blend in with normal network traffic, making them harder to detect.
Vulnerabilities. The centralized control model has a single point of failure — if the C&C server is discovered and shut down by law enforcement or cybersecurity experts, the botnet is rendered ineffective. To mitigate this risk, attackers often move the C&C server frequently or use multiple servers.

Decentralized Control (Peer-to-Peer Botnets)

In a decentralized or peer-to-peer (P2P) control model, there is no central C&C server. Instead, bots communicate with one another, forming a network where each device can relay commands:

Peer-to-peer (P2P) networks. In P2P botnets, each infected device acts as both a client and a server, passing along commands to other bots. This makes the botnet more resilient, as there’s no single point of failure. If one or more bots are taken down, the rest of the network remains operational.
Distributed architecture. In this model, botnet control is spread across the network, with each bot potentially receiving instructions from several other bots. Attackers can issue commands from any device within the network, making it harder to locate the source of control.
Resilience and anonymity. P2P botnets are much harder to take down since there is no centralized server to target. This distributed approach also provides greater anonymity for the attacker, as their control is masked within the network.

Advanced Control Techniques

In addition to traditional centralized and decentralized models of control, botnets have evolved to incorporate more sophisticated methods of manipulation. These advanced control techniques provide attackers with increased flexibility, resilience, and stealth, making botnet operations more difficult to detect and dismantle. The following methods not only enhance the coordination of botnet activities but also help evade modern cybersecurity defenses, contributing to the persistence and effectiveness of large-scale, malicious networks.

These advanced control techniques include:

Fast flux. This is a technique used to hide the location of the C&C server by rapidly changing the IP addresses of infected bots that act as proxies for the server. This helps the botnet avoid detection and makes it difficult to locate and shut down the C&C infrastructure.
Domain generation algorithms (DGAs). Some botnets use DGAs to generate a list of domain names that bots can use to contact the C&C server. The server’s domain name changes periodically, and only the attacker knows which domain will be active at a given time, making it harder for defenders to block communications.
Encryption and steganography. To avoid detection, botnets may encrypt their communication with the C&C server or use techniques like steganography, where commands are hidden within seemingly harmless data (e.g., images), making it difficult for security systems to identify botnet traffic.

Common Botnet Attacks

Botnets are used to execute various cyberattacks that leverage compromised devices' collective power. Below are some of the most common types of botnet attacks, explained in detail:

Distributed Denial-of-Service (DDoS) Attacks

A DDoS attack is one of the most well-known uses of a botnet. In this type of attack, the attacker uses a network of infected devices (bots) to flood a target server, website, or network with an overwhelming amount of traffic. This causes the target to slow down, become unresponsive, or crash, making it inaccessible to legitimate users. Botnets are particularly effective at DDoS attacks because they can generate traffic from multiple locations simultaneously, making it harder for defenses to mitigate the attack by blocking specific IP addresses.

Spam Campaigns

Botnets are frequently used to distribute massive amounts of spam emails. These emails often contain phishing links, malicious attachments, or advertisements for scams and counterfeit products. Since each bot in the network can send emails from its own IP address, botnets help spammers bypass spam filters and flood inboxes with unwanted messages. Botnet-driven spam campaigns may also spread malware, infecting more devices and expanding the botnet.

Credential Stuffing

Credential stuffing is an automated attack where a botnet attempts to log into multiple accounts using stolen username and password combinations. Often, attackers rely on credentials leaked from previous data breaches, trying them across multiple platforms in the hope that users have reused passwords. The botnet can make thousands of login attempts simultaneously across different services, making it easier for attackers to gain unauthorized access to accounts.

Brute-Force Attacks

In brute-force attacks, attackers use botnets to try numerous password combinations in rapid succession to break into systems or accounts. By distributing the attack across many devices in the botnet, the attacker can avoid detection or rate-limiting defenses that might block repetitive login attempts from a single IP address. These attacks target weak passwords or poorly secured systems.

Ad Fraud

Ad fraud occurs when a botnet is used to generate fake clicks or views on online advertisements. In this attack, infected devices are programmed to simulate human behavior, clicking on ads or visiting websites to artificially inflate traffic and ad revenue. This type of botnet attack is highly profitable for attackers but causes financial losses for advertisers and damages the integrity of online advertising networks.

Cryptojacking

Botnets are increasingly used in cryptojacking attacks, where infected devices are forced to mine cryptocurrencies. The attacker profits from the computational resources of the bots, while the device owners experience slower performance, higher energy costs, and potential hardware damage from prolonged mining activity.

Proxy Networks

Botnets are sometimes used to create vast networks of proxy servers, where infected devices act as intermediaries to anonymize the attacker’s traffic. These proxy networks can be sold or rented to other criminals who want to hide their real IP addresses while conducting illegal activities.

Data Theft

Botnets can be used to steal sensitive information from compromised devices, including login credentials, financial data, personal information, or intellectual property. This type of botnet attack may involve keylogging, which captures everything the user types, or extracting stored data like browser cookies or saved passwords. Stolen data is then sold on the dark web or used for further attacks, such as identity theft or financial fraud.

Ransomware Distribution

Botnets are frequently employed to distribute ransomware, malicious software that encrypts a victim’s files and demands a ransom payment in exchange for the decryption key. The botnet allows attackers to simultaneously infect thousands of devices with ransomware, maximizing their chances of receiving multiple ransom payments. Ransomware botnets can also move laterally across a network, increasing the scope of the attack.

Man-in-the-Middle (MitM) Attacks

In a MitM attack, a botnet intercepts and manipulates communication between two parties without their knowledge. This allows attackers to eavesdrop on sensitive communications, steal credentials, or inject malicious code into data transfers. Botnets can perform MitM attacks by compromising network infrastructure or using infected devices as relays to spy on or alter network traffic.

Click Fraud

Click fraud is another common botnet attack, where bots are used to click on pay-per-click (PPC) advertisements illegitimately. These fake clicks are designed to drain advertisers' budgets or inflate the revenue of a malicious advertiser. Since botnets distribute clicks across many devices, it becomes challenging to distinguish fraudulent activity from legitimate user interactions, allowing attackers to siphon ad revenue.

Social Media Manipulation

Botnets can be used to manipulate social media platforms by creating fake accounts or hijacking existing ones to spread misinformation, inflate follower counts, or influence public opinion. These fake accounts may be used to post comments, like content, or participate in polls, giving the appearance of widespread support or engagement. Social media botnets are often used in political campaigns, marketing schemes, or to damage competitors’ reputations.

Manipulating Online Polls and Reviews

Botnets can be used to manipulate online content, such as inflating poll results, online ratings, or reviews on websites. In this use case, a botnet might submit fake votes, reviews, or comments to skew public perception, promote a product or service, or cause harm to a competitor.

Search Engine Poisoning

In search engine poisoning attacks, botnets manipulate search engine rankings by generating fake traffic or placing malicious links at the top of search results. This promotes fraudulent websites, distributes malware, or directs users to phishing sites. By boosting the visibility of malicious sites, attackers can increase the likelihood that unsuspecting users will visit and fall victim to these scams.

Targeted Attacks and Espionage

Advanced botnets can be used in targeted attacks, such as industrial espionage or state-sponsored cyber operations. These botnets may be employed to gather intelligence, monitor specific individuals or organizations, or sabotage critical infrastructure. The ability to control numerous devices makes botnets a powerful tool for large-scale, covert surveillance or data exfiltration operations.

Network Reconnaissance

Botnets can be used to perform network reconnaissance by scanning and probing target systems to identify vulnerabilities. The bots in the network are programmed to search for open ports, weak services, or unpatched software that could be exploited in future attacks. This information is then relayed back to the attacker, who can plan a more targeted and effective attack based on the findings.

Botnet Protection Best Practices

Here are some best practices for protecting against botnets:

Educate users. Provide cybersecurity training, teaching users to recognize phishing scams, use strong passwords, and follow safe browsing practices. Informed users are less likely to fall victim to botnet infections.
Use antivirus and anti-malware software. Regularly install and update reputable antivirus and anti-malware software to detect and remove botnet-related infections. This software helps identify malware before it can turn your device into a bot.
Apply software patches and updates. Ensure that operating systems, applications, and firmware are always up to date. Regularly applying patches closes security vulnerabilities that botnets may exploit.
Implement strong passwords and multi-factor authentication (MFA). Use complex, unique passwords for all accounts and enable multi-factor authentication to reduce the risk of credential-based botnet attacks like credential stuffing and brute-force attacks.
Use a firewall. A firewall helps block unauthorized incoming and outgoing network traffic, reducing the risk of botnet infections. Configuring firewalls to block suspicious communication also prevents botnet control from external servers.
Monitor network traffic. Regularly monitor network traffic for unusual spikes or behavior, which could indicate a botnet infection. Network monitoring tools can detect abnormal patterns that are common in botnet activities like DDoS attacks or data exfiltration.
Disable unnecessary services and ports. Close unnecessary services and ports on devices to reduce potential attack vectors. Botnets often exploit open ports or unused services to gain access to devices.
Avoid clicking suspicious links or attachments. Be cautious when opening emails, attachments, or links from unknown or untrusted sources. Phishing emails and malicious downloads are common methods for spreading botnet malware.
Segment your network. Use network segmentation to limit the spread of infections. If a botnet compromises one part of the network, segmentation ensures it doesn’t easily infect the entire system.
Use DNS filtering. DNS filtering can block access to malicious domains and prevent devices from communicating with botnet command-and-control (C&C) servers, cutting off the control chain.