Analyzing Network Traffic for Incidents

What Is This

Analyzing Network Traffic for Incidents is the technical skill of interpreting and investigating network traffic captures and flow data to identify malicious or suspicious activity during security incidents. This process leverages tools such as Wireshark, Zeek, and NetFlow analysis to uncover evidence of adversary behaviors including command-and-control (C2) communications, lateral movement, data exfiltration, and exploitation attempts. As part of the incident response domain, this skill is critical for network forensics, PCAP (packet capture) analysis, and the validation of intrusion detection alerts.

This skill is mapped to various MITRE ATT&CK techniques, including Application Layer Protocol (T1071), Non-Application Layer Protocol (T1095), Encrypted Channel (T1573), and Protocol Tunneling (T1572), and aligns with NIST CSF response and analysis functions. It is specifically activated when tasked with analyzing network traffic, investigating packet captures, detecting C2 activity, or assessing potential data exfiltration via network evidence.

Why Use It

Network traffic analysis is essential for incident response and digital forensics. Malicious actors frequently exploit network channels for C2 communication, data theft, and lateral movement within compromised environments. Traditional endpoint forensics may miss these activities, especially if attackers use encrypted or covert channels. By analyzing raw network traffic and flow data, security professionals can:

• Confirm or dismiss SIEM and IDS/IPS alerts with packet-level evidence
• Detect covert communication channels used by adversaries
• Quantify and trace the volume and destination of potential data exfiltration
• Reconstruct attacker movement across hosts using network connections
• Satisfy compliance requirements for incident detection and response

Network traffic forensics provides a unique, often immutable source of truth about what transpired on a network during an incident.

How to Use It

The analysis process generally involves acquiring network traffic data, filtering relevant portions, and applying forensic techniques to extract meaningful indicators. The following steps outline a typical workflow:

1. Obtain Packet Captures or Flow Data

• Collect full packet captures (PCAP files) using network taps, span ports, or on-demand tools such as tcpdump or Wireshark.
• Alternatively, retrieve flow records (NetFlow, IPFIX) from routers and firewalls if full PCAP is unavailable.

Example: Capturing network traffic with tcpdump

sudo tcpdump -i eth0 -w incident_capture.pcap

2. Filter and Isolate Traffic of Interest

Use Wireshark or command-line tools to focus on relevant traffic:

Example: Extract HTTP traffic from a PCAP

tshark -r incident_capture.pcap -Y 'http' -w http_traffic.pcap

3. Analyze with Wireshark

• Open the PCAP in Wireshark.
• Filter by IP addresses, protocols, or suspicious ports.
• Inspect TCP streams for unusual patterns, encoded data, or beaconing intervals.

Example: Wireshark filter for suspected C2 traffic

ip.addr == 203.0.113.10 && tcp.port == 443

4. Leverage Zeek for Automated Analysis

Zeek (formerly Bro) can process large PCAPs and generate detailed logs for further investigation.

Example: Run Zeek on a PCAP

zeek -r incident_capture.pcap

• Review conn.log for connection summaries.
• Examine http.log, dns.log, and ssl.log for protocol-specific anomalies.

5. NetFlow and Flow Analysis

If only flow data is available, use flow analysis tools to identify large data transfers, unusual connection patterns, or communication with rare destinations.

Example: Filtering NetFlow records by destination port

nfdump -r netflow.dat 'dst port 8080'

6. Indicators of Compromise (IOCs) and Behavioral Patterns

• Search for known IOCs (malicious IPs, domains, or file hashes).
• Identify suspicious behaviors, such as repeated connection attempts, periodic beaconing, or data transfers to untrusted locations.

7. Document and Report Findings

Summarize evidence, including timelines, source/destination hosts, volumes, and protocol details, to inform incident response actions.

When to Use It

Apply this skill in scenarios such as:

• SIEM alerts that indicate anomalous network patterns and require packet-level investigation
• Suspected C2 beaconing, where confirmation and extraction of indicators are needed
• Potential data exfiltration, to validate and quantify outbound data movement
• Tracing lateral movement between systems via network connections
• Validating IDS/IPS alerts by reconstructing the triggering network traffic

Do not use this skill for host-based investigations such as process execution or file system artifact analysis; employ endpoint forensic tools for those cases.

Important Notes

• Ensure that you have legal and organizational authorization before capturing or analyzing network traffic.
• Full packet capture provides the highest fidelity but may not always be available; supplement with flow data as needed.
• Encrypted traffic can obscure contents - focus on metadata, such as connection patterns and certificate details, to infer malicious behaviors.
• Always preserve original evidence and work on copies to maintain forensic integrity.
• Be aware that sophisticated attackers may use evasion techniques such as protocol tunneling, custom encryption, or traffic obfuscation.
• Regularly update detection rules and analysis techniques to keep pace with evolving adversary tactics.

By mastering network traffic analysis, incident responders can uncover, validate, and respond to a wide range of network-based threats, improving organizational resilience against modern cyber attacks.