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Definition

command-and-control server (C&C server)

Contributor(s): Matthew Haughn

A command and control server (C&C server) is a computer that issues directives to digital devices that have been infected with rootkits or other types of malware, such as ransomware. C&C servers can be used to create powerful networks of infected devices capable of carrying out distributed denial-of-service (DDoS) attacks, stealing data, deleting data or encrypting data in order to carry out an extortion scheme. In the past, a C&C server was often under an attacker's physical control and could remain active for several years. Today, C&C servers generally have a short shelf life; they often reside in legitimate cloud services and use automated domain generation algorithms (DGAs) to make it more difficult for law enforcement and white hat malware hunters to locate them. 

A malicious network under a C&C server's control is called a botnet and the network nodes that belong to the botnet are sometimes referred to as zombies. In a traditional botnet, the bots are infected with a Trojan horse and use Internet Relay Chat (IRC) to communicate with a central C&C server. These botnets were often used to distribute spam or malware and gather misappropriated information, such as credit card numbers. 

Popular botnet topologies include:

  • Star topology - the bots are organized around a central server.
  • Multi-server topology - there are multiple C&C servers for redundancy.
  • Hierarchical topology - multiple C&C servers are organized into tiered groups.
  • Random topology - coopted computers communicate as a peer-to-peer botnet (P2P botnet).

Since IRC communication was typically used to command botnets, it is often guarded against. This has motivated the drive for more covert ways for C&C servers to issue commands. Alternative channels used for botnet command include JPG images, Microsoft Word files and posts from LinkedIn or Twitter dummy accounts.

Learn more about botnet command and control in this video:

This was last updated in January 2017

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Dig Deeper on Network security

Botnet research suggests progress in cybercrime war The recent arrests of those suspected of being connected to the Mariposa botnet and the legal action by Microsoft to take down the command and control of the Waledac botnet may be a sign that those defending the Internet are gaining ground against botnets and the cybercriminals behind them. But a complex web of legal and jurisdictional issues remain, said botnet expert Joe Stewart, director of malware research at SecureWorks Inc. In a wide ranging interview at the 2010 RSA Conference, Stewart explained whether the tools and the resources are available to win the botnet war. He said more work needs to be done before victory can be declared. As soon as a botnet is taken down, cybercriminals have shown they can quickly rebound, deploying new bots to spread malware and harvest data, Stewart said.

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