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Network Intrusion

High severity Incident diary — Case #IR-2024

How a phishing email turned into a network intrusion

A single employee action opened the door. The i6 SOC identified, investigated, and contained the threat before it spread.

Threat typePhishing + RAT

Systems affected1 workstation

Lateral movementNone confirmed

OutcomeFully contained

Initial vector

Spear phishing email

Malware deployed

Remote access trojan

Detected by

i6 monitoring platform

Status

Incident closed

How the incident started

The trigger

An employee received an email appearing to come from a legitimate vendor requesting confirmation of an invoice. The message used familiar branding and language, making it difficult to recognize the threat. After opening the attachment, the user enabled macros in the document.

This single action triggered a hidden script that downloaded a remote access trojan (RAT) onto the system. Once installed, the malware allowed the attacker to remotely interact with the compromised workstation and attempt communication with external infrastructure.

The email had originated from an external domain spoofed to resemble a legitimate vendor — a carefully constructed deception designed to bypass user suspicion.

Detection by i6 security monitoring

High-priority alert triggered — i6 SOC dashboard

Shortly after the malicious file was executed, the i6 monitoring platform detected abnormal activity originating from the workstation. Several suspicious indicators triggered alerts simultaneously.

Outbound connections to an unfamiliar external domain

Execution of unusual command-line processes

New background processes running on the workstation

Attempts to access internal shared resources

Investigation by i6 security analysts

Once the alert was received, the i6 SOC team began a structured investigation to determine the cause and scope of the incident. Five investigative tracks ran in parallel.

Email and attachment analysis

i6 analysts reviewed email headers, sender infrastructure, and attachment behavior. The document was found to contain a macro-enabled script designed to download malware. The email originated from a spoofed external domain mimicking a legitimate vendor.

Endpoint activity analysis

Endpoint telemetry collected by the i6 monitoring platform confirmed that shortly after the document was opened, a malicious process executed in the background and installed a RAT. The malware attempted to establish persistence and communicate with a command-and-control server.

Network traffic investigation

Network logs and outbound traffic from the affected workstation were reviewed. Suspicious traffic was identified directed toward infrastructure associated with malicious activity — connections intended to give the attacker remote control of the system.

Lateral movement assessment

Authentication logs and file access activity across internal systems were reviewed. Although several internal resources were probed, no successful lateral movement was observed — the attacker remained confined to the initial workstation.

Scope and impact verification

The i6 SOC team conducted a broader review across the environment — reviewing endpoint alerts, scanning for similar indicators of compromise, and verifying no additional malicious processes were present across the network.

Response and remediation by i6

Once the phishing-based compromise was confirmed, the i6 incident response team immediately initiated containment actions to stop the attacker and secure the affected environment.

Containment actions

Isolating the infected workstation from the network
Blocking malicious command-and-control domains
Disabling the compromised user account
Terminating suspicious processes on the endpoint

Verification steps

Scanning endpoints across the network for similar malware
Resetting credentials associated with the affected account
Reviewing system logs for signs of unusual activity
Verifying no additional systems were infected

These actions confirmed that the incident was successfully contained and limited to a single workstation.

Security improvements implemented

Following the incident, i6 worked with the organization to strengthen its security posture and reduce the likelihood of similar attacks in the future.

Email filtering

Strengthened email filtering and phishing detection capabilities across mail infrastructure.

Macro controls

Stricter controls implemented for macro-enabled documents across the organization.

Endpoint monitoring

Enhanced endpoint monitoring and malware detection coverage across all systems.

Security awareness

Security awareness training conducted for employees to improve threat recognition.

Conclusion

Phishing attacks often rely on small user actions to create an entry point into corporate environments. What begins as a single email can quickly turn into a broader network intrusion if the activity goes unnoticed.

In this case, continuous monitoring by the i6 SOC allowed the threat to be detected and contained before the attacker could gain a stronger foothold in the network. The incident reinforces the importance of combining user awareness with proactive monitoring and rapid incident response capabilities.

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