What Is a Botnet?
A botnet — short for "robot network" — is a collection of compromised computers, smartphones, routers, and other internet-connected devices that have been infected with malicious software. Once infected, each device becomes a "bot" (or zombie), responding to commands from a central controller known as the botmaster or command-and-control (C2) server. The device owner typically has no idea their machine is part of the operation.
Botnets can range from a few hundred devices to millions of machines spread across the globe. Some of the largest botnets ever discovered — like Mirai or Zeus — compromised hundreds of thousands of devices simultaneously.
How Does a Botnet Work?
The lifecycle of a botnet typically follows a few key stages:
- Infection: Devices get compromised through phishing emails, malicious downloads, unpatched software vulnerabilities, or weak passwords on routers and IoT devices.
- Recruitment: The malware silently installs itself and connects back to the attacker's command-and-control server. The infected device is now "enlisted" in the botnet.
- Activation: The botmaster sends instructions to all bots simultaneously. These instructions might tell the devices to send spam, launch attacks, steal data, or mine cryptocurrency.
- Execution: The bots carry out the assigned task, often at massive scale, because the combined power of thousands of devices is far greater than any single machine.
Modern botnets often use peer-to-peer architectures rather than a single C2 server, making them harder to shut down. If one node is removed, the rest of the network keeps functioning.
Common Uses of Botnets
Botnets are behind many of the most damaging cyberattacks on record. Here's what attackers typically use them for:
- DDoS attacks (Distributed Denial of Service): Flooding a website or server with traffic until it crashes. This is one of the most common botnet applications.
- Spam campaigns: Sending billions of phishing or advertising emails through infected machines to avoid detection.
- Credential stuffing: Using stolen username/password combinations to automatically attempt logins across thousands of websites.
- Cryptojacking: Hijacking device processing power to mine cryptocurrency for the attacker.
- Data theft: Harvesting banking credentials, personal information, and sensitive files from infected machines.
- Ad fraud: Generating fake clicks on advertisements to steal ad revenue.
Why Botnets Matter for VPN Users
VPN users aren't immune to botnets — and in some cases, VPN infrastructure can be a direct target or unintended participant.
Your device could already be a bot. A VPN encrypts your traffic, but it doesn't protect you from malware already installed on your device. If your machine is compromised, an attacker can operate through it regardless of whether a VPN is active.
Free VPNs have been used to build botnets. Some disreputable free VPN services have been caught enrolling users' devices into botnets, essentially selling their bandwidth and processing power to third parties. The infamous Hola VPN case is a well-documented example.
Botnets are used to attack VPN servers. Large-scale DDoS attacks powered by botnets can target VPN infrastructure, causing outages or forcing users onto less secure connections.
IP reputation issues: If your internet connection was previously part of a botnet, your IP address may be flagged or blacklisted by websites and services — even after the malware has been removed.
Protecting Yourself
To avoid becoming an unwitting bot, keep all software and firmware updated, use strong unique passwords, enable two-factor authentication, and run reputable antivirus software. Be cautious of free VPN services that don't clearly explain their business model. Pair a trustworthy VPN with good security hygiene to reduce your overall attack surface.
Understanding botnets is a key part of understanding modern cyber threats — because they don't just target corporations. Any connected device, including yours, is a potential recruit.