Command-and-control (C2) traffic is communication an adversary uses to direct or receive information from compromised systems. It can travel through familiar protocols such as HTTP/S or DNS, but a familiar protocol name—or encryption—does not make a connection benign. To assess possible C2, compare what the traffic does with the protocol’s expected behavior and your network’s normal baseline.
What command-and-control traffic means
C2 describes the purpose of a communication: an adversary is communicating with systems it has compromised to control them or support its activity. It is not a synonym for all suspicious outbound traffic. Unusual traffic can warrant investigation, but the label C2 should be reserved for communications serving that adversarial control function.
Adversaries may try to make those communications resemble expected network activity. MITRE ATT&CK’s protocol-tunneling material, hosted by CISA, notes: “Adversaries commonly attempt to mimic normal, expected traffic to avoid detection.” MITRE ATT&CK: Protocol Tunneling (T1572)
How C2 can hide in ordinary protocols
A protocol is a set of rules for communicating. A legitimate protocol can carry malicious communication, so identifying HTTP, HTTPS, DNS, or another application protocol is only one part of the analysis. Concealment methods differ: the application protocol carrying the traffic, any protocol encapsulation, the route it takes, and the port used are separate properties.
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Using an application protocol as the carrier
C2 can use application-layer protocols such as web or file-transfer protocols. In its account of APT40 tradecraft, CISA lists web protocols, file-transfer protocols, proxies, encrypted channels, domain fronting, and protocol tunneling among observed technique categories. These are examples from that advisory, not a claim that all adversaries use them. CISA: APT40 Advisory (AA20-200A)
Tunneling one protocol inside another
Protocol tunneling explicitly encapsulates one protocol within another. The outer protocol carries the inner communication, which can make the traffic resemble use of the outer protocol. MITRE ATT&CK gives SSH tunneling—forwarding arbitrary data over an encrypted SSH tunnel—as one example. It also describes DNS over HTTPS (DoH), in which DNS queries are carried inside encrypted HTTPS packets and can be used to resolve C2 infrastructure.
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DoH illustrates why encryption and protocol identification are not verdicts: the DNS activity is inside HTTPS, and encryption limits what a network observer can see in the payload. The presence of HTTPS alone does not establish whether a connection is ordinary or malicious. MITRE ATT&CK: Protocol Tunneling (T1572)
Routing through a proxy or relay
A proxy or relay changes how traffic reaches its destination; it is distinct from the protocol used to carry the communication. CISA includes proxies among technique categories observed in its APT40 advisory and discusses proxy use in guidance on malicious communications infrastructure. A proxy can be part of legitimate network operations as well, so its presence alone does not prove C2. CISA: APT40 Advisory (AA20-200A) CISA: Detecting and Mitigating Malicious Communications Infrastructure
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Using an unexpected port
A protocol and a port are different facts. CISA’s mapping guide uses “HTTP-based Command and Control (C2) traffic over port 8088” as an example: HTTP-based traffic does not necessarily use port 80. Analysts should record and assess both the protocol and the port rather than infer one from the other. CISA: Mapping MITRE ATT&CK and CISA Cybersecurity Advisories
How to investigate traffic that may be C2
Look for departures from expected protocol standards and traffic flows, then interpret them in context. CISA recommends monitoring protocol traffic and packet inspection for issues such as extraneous packets outside established flows, gratuitous or anomalous traffic patterns, and anomalous syntax or structure. CISA: Technical Approaches to Uncovering and Remediating Malicious Activity
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- Establish what is normal. Build a baseline of ordinary network behavior for the environment. CISA’s communications-infrastructure guidance recommends baselining normal behavior and configuring security appliances to alert on abnormal behavior. CISA: Detecting and Mitigating Malicious Communications Infrastructure
- Check the protocol flow and structure. Compare observed packets, syntax, and flow with the protocol’s expected standards. Look for extraneous packets, unexpected patterns, or malformed or unusual structure.
- Assess the connection in local context. Ask whether the source host, destination, timing, volume, protocol, and port fit the system’s role and established baseline. These are useful investigative dimensions, not a universal official checklist or standalone proof of C2.
- Correlate with other evidence. Interpret network observations alongside endpoint and incident context. A single anomaly—such as encryption, an unusual port, or proxy use—does not by itself identify command-and-control activity.
Why a single signal is not enough
Encryption can restrict visibility into payload content, while legitimate software may use unusual services or produce traffic patterns that stand out from a general baseline. The cited guidance supports checking protocol behavior and deviations from normal activity, but it does not provide a universal threshold for declaring traffic malicious. Treat an anomaly as a lead for investigation, not a verdict; thresholds need to reflect the organization’s environment.
The sources describe several concealment techniques but do not rank them by prevalence or effectiveness. No one method—HTTP/S, DNS, tunneling, proxies, or non-standard ports—should be assumed to be the default explanation for suspicious traffic.
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