Disclosed hacktool set (old stuff) - file fscan.exe

Hunt Hypothesis

This detection identifies adversaries utilizing legacy FScan tools to conduct network reconnaissance and asset discovery, which often indicates early-stage information gathering or lateral movement preparation. Proactively hunting for this behavior in Azure Sentinel is critical because outdated scanning utilities may lack modern security controls, creating potential entry points for attackers to map the environment before launching more complex attacks.

YARA Rule

rule sig_238_fscan {
	meta:
		description = "Disclosed hacktool set (old stuff) - file fscan.exe"
		author = "Florian Roth"
		date = "23.11.14"
		score = 60
		hash = "d5646e86b5257f9c83ea23eca3d86de336224e55"
	strings:
		$s0 = "FScan v1.12 - Command line port scanner." fullword ascii
		$s2 = " -n    - no port scanning - only pinging (unless you use -q)" fullword ascii
		$s5 = "Example: fscan -bp 80,100-200,443 10.0.0.1-10.0.1.200" fullword ascii
		$s6 = " -z    - maximum simultaneous threads to use for scanning" fullword ascii
		$s12 = "Failed to open the IP list file \"%s\"" fullword ascii
		$s13 = "http://www.foundstone.com" fullword ascii
		$s16 = " -p    - TCP port(s) to scan (a comma separated list of ports/ranges) " fullword ascii
		$s18 = "Bind port number out of range. Using system default." fullword ascii
		$s19 = "fscan.exe" fullword wide
	condition:
		4 of them
}

Deployment Notes

This YARA rule can be deployed in the following contexts:

• Microsoft Defender for Endpoint — Custom indicators / advanced hunting
• Email Gateway — Attachment scanning
• File Share Monitoring — Periodic scanning of shared drives
• YARA CLI — Manual threat hunting on endpoints

This rule contains 9 string patterns in its detection logic.

• Source Rule

False Positive Guidance

• Legitimate Network Discovery by IT Operations

  • Scenario: The internal Security Operations team or Network Engineering group regularly executes fscan.exe (a popular Chinese network scanning and asset discovery tool) from a dedicated management workstation to audit active hosts, open ports, and patch levels across the corporate LAN. This often occurs during scheduled maintenance windows.
  • Suggested Filter/Exclusion: Create an exclusion rule that ignores alerts where the Source Host is part of the “IT-Management” security group or the Process Name matches fscan.exe running under a specific service account (e.g., DOMAIN\svc_network_audit).

• Scheduled Vulnerability Assessment Job

  • Scenario: A nightly scheduled task runs on a centralized vulnerability scanner server to perform deep asset inventory and configuration checks. The job triggers fscan.exe to scan the “Development” subnet for legacy endpoints that lack modern agents, generating a high volume of file access events that mimic tool behavior.
  • Suggested Filter/Exclusion: Configure an exclusion based on the Scheduled Task Name (e.g., Nightly_FScan_Audit) and restrict the alert to only trigger if the execution time falls outside the defined maintenance window (e.g., 02:00–04:00 UTC).

• Endpoint Remediation by Helpdesk Technicians

  • Scenario: During a major incident response or routine remediation, helpdesk technicians remotely launch fscan.exe on user workstations to quickly identify missing patches, unauthorized software, and network connectivity issues. This is often done via RDP sessions where the tool runs interactively rather than as a background service.
  • Suggested Filter/Exclusion: Implement a filter that suppresses alerts when the User Context belongs to the “Helpdesk-Support” group and the `Process