A command-and-control or C&C server is a device or machine under the control of a cybercriminal. An attacker uses a command-and-control server to maintain communications with and remotely control malware or malicious scripts within a target network.

A command-and-control server can also receive stolen data from compromised systems, including computers, mobile phones, and even Internet of Things (IoT) devices connected to an infected network. Some cyber attackers hide command-and-control servers in file-sharing services to evade detection and blocking.

A command-and-control server can be likened to the puppeteer who controls the puppets in a show. The puppets’ actions depend on the instructions issued by the command-and-control server.

Other interesting terms…

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Are All Command-and-Control Servers Malicious?

All command-and-control servers serve as threat actors’ command centers that allow them to control several devices within a target network. A command-and-control server can help hackers:

Steal Confidential Data

One of the primary goals of using a command-and-control server is to steal highly confidential data, such as financial credentials. The server acts as storage for stolen files and logs that are later on sent to a remote site still under the attackers’ control.

Shut a Network Down

Attackers can also incapacitate a target organization with the help of command-and-control servers. They do so by shutting down machines that halt the company’s entire operations.

Reboot Systems

Hackers can also choose to reboot malware-infected systems repeatedly, which can disrupt an enterprise’s operations, via a command-and-control server.

Launch a Distributed Denial-of-Service Attack

All distributed denial-of-service (DDoS) attacks use a command-and-control server. The server issues a command to all machines under its control to flood a target network with Internet traffic, overwhelming a website, and rendering it inaccessible.

What Purpose Does a Command-and-Control Server Serve in a Botnet?

A command-and-control server controls all computers infected with its malware component, turning them into bots and the entire infrastructure into a botnet. Any insufficiently protected device can be turned into a bot by:

  • Malware infection: It is pretty standard for threat actors to send email messages with malicious attachments or links. When downloaded or clicked, a malware gets automatically installed on the system, turning it into a bot or zombie.
  • Vulnerability exploitation: Another common way to take control of a computer is by exploiting a vulnerability. Plug-ins, add-ons, and unpatched software can be abused to gain entry and control a system altogether.

How Can You Block Command-and-Control Server Connections?

Command-and-control servers continuously evolve to evade detection. As such, no single way can detect them. A combination of the following strategies can, however, prove useful.

Monitor Dubious Network Activities

IT security teams should look into suspicious connection attempts, particularly outbound ones. Regularly updating service blacklists can help manage doubtful communications. For example, hundreds to thousands of users suddenly opening a blog post at the same time can be indicative of botnet activity.

Enhance Firewall and Intrusion Prevention and Detection Systems

Employing stricter network access and limiting the number of open ports can help prevent a takeover. For instance, users should only permit internal access to a Domain Name System (DNS) server. Any other attempts to connect to it can be considered red flags.

Run System Integrity Checks

To detect command-and-control server connections, teams should go beyond merely updating antivirus solutions. Administrators should run system integrity checks regularly and limit access privileges.

Is It Possible to Take Command-and-Control Servers Down?

Once you’ve identified connections to a command-and-control server within your network, the next plausible step is to sever ties. While you may wish to take the entire server down, that is no easy feat and requires law enforcement involvement. Only the authorities or Internet giants who have massive resources can take command-and-control servers down.

Law enforcement agencies work with service providers to sequester physical hosts, domain name providers to revoke service, and even security service providers to identify the components of the attack infrastructure. It is usually a global effort. A recent example of this exercise would be the Necurs botnet takedown.

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A simple malware infection can already cause tremendous damage to a machine or network. Imagine the devastation that an entire host of bots can do aided by a command-and-control server.