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Command and Control

The command and control tactic represents how adversaries communicate with systems under their control within a target network. There are many ways an adversary can establish command and control with various levels of covertness, depending on system configuration and network topology. Due to the wide degree of variation available to the adversary at the network level, only the most common factors were used to describe the differences in command and control. There are still a great many specific techniques within the documented methods, largely due to how easy it is to define new protocols and use existing, legitimate protocols and network services for communication.

The resulting breakdown should help convey the concept that detecting intrusion through command and control protocols without prior knowledge is a difficult proposition over the long term. Adversaries' main constraints in network-level defense avoidance are testing and deployment of tools to rapidly change their protocols, awareness of existing defensive technologies, and access to legitimate Web services that, when used appropriately, make their tools difficult to distinguish from benign traffic.
ID: TA0011


Techniques: 21
T1043Commonly Used Port

Adversaries may communicate over a commonly used port to bypass firewalls or network detection systems and to blend with normal network activity to avoid more detailed inspection. They may use commonly open ports such as

T1092Communication Through Removable Media

Adversaries can perform command and control between compromised hosts on potentially disconnected networks using removable media to transfer commands from system to system. Both systems would need to be compromised, with the likelihood that an Internet-connected system was compromised first and the second through lateral movement by Replication Through Removable Media. Commands and files would be relayed from the disconnected system to the Internet-connected system to which the adversary has direct access.

T1090Connection Proxy

A connection proxy is used to direct network traffic between systems or act as an intermediary for network communications. Many tools exist that enable traffic redirection through proxies or port redirection, including HTRAN, ZXProxy, and ZXPortMap.

T1094Custom Command and Control Protocol

Adversaries may communicate using a custom command and control protocol instead of encapsulating commands/data in an existing Standard Application Layer Protocol. Implementations include mimicking well-known protocols or developing custom protocols (including raw sockets) on top of fundamental protocols provided by TCP/IP/another standard network stack.

T1024Custom Cryptographic Protocol

Adversaries may use a custom cryptographic protocol or algorithm to hide command and control traffic. A simple scheme, such as XOR-ing the plaintext with a fixed key, will produce a very weak ciphertext.

T1132Data Encoding

Command and control (C2) information is encoded using a standard data encoding system. Use of data encoding may be to adhere to existing protocol specifications and includes use of ASCII, Unicode, Base64, MIME, UTF-8, or other binary-to-text and character encoding systems. Some data encoding systems may also result in data compression, such as gzip.

T1001Data Obfuscation

Command and control (C2) communications are hidden (but not necessarily encrypted) in an attempt to make the content more difficult to discover or decipher and to make the communication less conspicuous and hide commands from being seen. This encompasses many methods, such as adding junk data to protocol traffic, using steganography, commingling legitimate traffic with C2 communications traffic, or using a non-standard data encoding system, such as a modified Base64 encoding for the message body of an HTTP request.

T1172Domain Fronting

Domain fronting takes advantage of routing schemes in Content Delivery Networks (CDNs) and other services which host multiple domains to obfuscate the intended destination of HTTPS traffic or traffic tunneled through HTTPS. The technique involves using different domain names in the SNI field of the TLS header and the Host field of the HTTP header. If both domains are served from the same CDN, then the CDN may route to the address specified in the HTTP header after unwrapping the TLS header. A variation of the the technique, "domainless" fronting, utilizes a SNI field that is left blank; this may allow the fronting to work even when the CDN attempts to validate that the SNI and HTTP Host fields match (if the blank SNI fields are ignored).

T1008Fallback Channels

Adversaries may use fallback or alternate communication channels if the primary channel is compromised or inaccessible in order to maintain reliable command and control and to avoid data transfer thresholds.

T1188Multi-hop Proxy

To disguise the source of malicious traffic, adversaries may chain together multiple proxies. Typically, a defender will be able to identify the last proxy traffic traversed before it enters their network; the defender may or may not be able to identify any previous proxies before the last-hop proxy. This technique makes identifying the original source of the malicious traffic even more difficult by requiring the defender to trace malicious traffic through several proxies to identify its source.

T1104Multi-Stage Channels

Adversaries may create multiple stages for command and control that are employed under different conditions or for certain functions. Use of multiple stages may obfuscate the command and control channel to make detection more difficult.

T1026Multiband Communication

Some adversaries may split communications between different protocols. There could be one protocol for inbound command and control and another for outbound data, allowing it to bypass certain firewall restrictions. The split could also be random to simply avoid data threshold alerts on any one communication.

T1079Multilayer Encryption

An adversary performs C2 communications using multiple layers of encryption, typically (but not exclusively) tunneling a custom encryption scheme within a protocol encryption scheme such as HTTPS or SMTPS.

T1205Port Knocking

Port Knocking is a well-established method used by both defenders and adversaries to hide open ports from access. To enable a port, an adversary sends a series of packets with certain characteristics before the port will be opened. Usually this series of packets consists of attempted connections to a predefined sequence of closed ports, but can involve unusual flags, specific strings or other unique characteristics. After the sequence is completed, opening a port is often accomplished by the host based firewall, but could also be implemented by custom software.

T1219Remote Access Tools

An adversary may use legitimate desktop support and remote access software, such as Team Viewer, Go2Assist, LogMein, AmmyyAdmin, etc, to establish an interactive command and control channel to target systems within networks. These services are commonly used as legitimate technical support software, and may be whitelisted within a target environment. Remote access tools like VNC, Ammy, and Teamviewer are used frequently when compared with other legitimate software commonly used by adversaries.

T1105Remote File Copy

Files may be copied from one system to another to stage adversary tools or other files over the course of an operation. Files may be copied from an external adversary-controlled system through the Command and Control channel to bring tools into the victim network or through alternate protocols with another tool such as FTP. Files can also be copied over on Mac and Linux with native tools like scp, rsync, and sftp.

T1071Standard Application Layer Protocol

Adversaries may communicate using a common, standardized application layer protocol such as HTTP, HTTPS, SMTP, or DNS to avoid detection by blending in with existing traffic. Commands to the remote system, and often the results of those commands, will be embedded within the protocol traffic between the client and server.

T1032Standard Cryptographic Protocol

Adversaries may explicitly employ a known encryption algorithm to conceal command and control traffic rather than relying on any inherent protections provided by a communication protocol. Despite the use of a secure algorithm, these implementations may be vulnerable to reverse engineering if necessary secret keys are encoded and/or generated within malware samples/configuration files.

T1095Standard Non-Application Layer Protocol

Use of a standard non-application layer protocol for communication between host and C2 server or among infected hosts within a network. The list of possible protocols is extensive. Specific examples include use of network layer protocols, such as the Internet Control Message Protocol (ICMP), transport layer protocols, such as the User Datagram Protocol (UDP), session layer protocols, such as Socket Secure (SOCKS), as well as redirected/tunneled protocols, such as Serial over LAN (SOL).

T1065Uncommonly Used Port

Adversaries may conduct C2 communications over a non-standard port to bypass proxies and firewalls that have been improperly configured.

T1102Web Service

Adversaries may use an existing, legitimate external Web service as a means for relaying commands to a compromised system.