What is a Botnet? How are they used? What harm can be caused, and how can you prevent a computer from becoming part of a botnet? These and other questions answered.

What is a Botnet?

A botnet is simply a collection of computers and other Internet-connected devices that are controlled by a threat actor. Usually that control is achieved via a malware installation, with the malware communicating with the threat actor’s command and control server.

Once malware has been installed on one device, potentially it can propagate to other devices on the same network, creating a mini-army of slave devices under the threat actor’s control. Any computer with the malware installed is part of the botnet and can be used on its own or collectively with other compromised devices for malicious purposes.

What are Botnets Used For?

Botnets are often used to conduct Distributed Denial of Service (DDoS) attacks, with the devices in the botnet used to access a particular service simultaneously and flooding it with traffic making that service temporarily unavailable. The Mirai botnet, which mostly consists of vulnerable IoT devices, was used to take down large sections of the Internet, including some of the most popular websites such as Twitter and Netflix. DDoS attacks are now being conducted that exceed 1 terabits per second, largely due to sheer number of devices that are part of the botnet.

One of the biggest botnets ever assembled was made possible with Zeus malware, a banking Trojan that was particularly difficult to detect. In the United States, an estimated 3.6 million computers had been infected with the malware, making Zeus one of the biggest botnets ever created.

In addition to DDoS attacks, botnets are also used to send huge quantities of spam and phishing emails. The Necurs botnet is the world’s largest spamming botnet, delivering 60% of all spam emails. The Gamut spam botnet delivers around 37% of spam botnet traffic. These two spamming botnets are primarily used to send malicious messages containing email attachments with malicious macros that download malware such as the Dridex banking Trojan, and the ransomware variants Locky, Globelmposter, and Scarab.

Recently, the rise in the value of cryptocurrencies has made it highly profitable to use the processing power of botnets to mine cryptocurrency. When processing power is used for cryptocurrency mining, the performance of the computers will reduce significantly.

How Are Botnets Created?

Botnets can be created through several different methods. In the case of IoT devices, attackers often take advantage of weak passwords and default credentials that have not been changed. Since IoT devices are less likely to be updated automatically with the latest software and firmware, it is easier to exploit flaws to gain access to the devices. IoT Devices also rarely have antivirus controls, making infection easier and detection of malware much harder.

Computers are most commonly recruited into botnets through malware sent via spam email campaigns – such as those sent out by the spamming botnets. Malware is delivered via infected email attachments or links to malicious websites where malicious code is hosted. Messages can be sent via social media networks and chat apps, which also direct users to malicious websites where malware is downloaded.

Drive-by downloads are also common – Malware is downloaded by exploiting vulnerabilities in browsers, add-ons or browser plug-ins, often through exploit kits loaded on compromised websites.

Prevent a Computer from Becoming Part of a Botnet

It is much easier to prevent a computer from becoming part of a botnet than identifying a malware infection and eradicating it once it has been installed. To prevent a computer from becoming part of a botnet, it is necessary to use technological controls and adopt security best practices.

Businesses need to ensure all staff are trained to be more security aware and are told about the risks of opening email attachments or clicking links in emails from unknown senders. They should also be told not to automatically trust messages from contacts as their email accounts could have been compromised. Employees should be taught security best practices and risky behavior, such as connecting to public WiFi networks without using a VPN, should be eradicated.

All software must be kept up to date with patches applied promptly. This will reduce the risk of vulnerabilities being exploited to deliver malware. Antivirus software should be installed and configured to update automatically, and regular AV scans should be performed.

Firewalls should be used to implemented to prevent unauthorized network access and allow security teams to monitor internet traffic.

Spam filtering solutions should be implemented to block the majority of malicious messages from being delivered to end users’ inboxes. The more messages that are blocked, the less chance there is of an employee responding to a phishing email and inadvertently installing malware.

One way to prevent a computer from becoming part of a botnet that is often forgotten, is the use of a web filtering solution. A web filter, such as WebTitan, will prevent malware and ransomware downloads and block access to malicious websites sent via email or through web browsing.

Implement these controls and it will make it much harder for your organization’s computers to be infected with malware and added to a botnet.