http://flatassembler.net

Hunt Hypothesis

This rule identifies outbound HTTP traffic to the known suspicious domain flatassembler.net, which may indicate an adversary establishing a Command and Control channel or exfiltrating data via a low-fidelity web request. Proactively hunting for this activity allows the SOC to detect early-stage reconnaissance or beaconing that might otherwise be overlooked due to the low severity classification and lack of specific MITRE technique mapping.

YARA Rule

rule FASM : flat assembler {
//abit weak, needs more targets & testing
	meta:
		author = "_pusher_"
		date = "2016-01"
		description = "http://flatassembler.net"
	//strings:
		//$c0 = { 55 89 E5 83 EC 1C 8D 45 E4 6A 1C 50 FF 75 08 FF 15 ?? ?? ?? ?? 8B 45 E8 C9 C2 04 00 }
	condition:
		(
		//linker 1.60..1.79
		(pe.linker_version.major == 1) and ((pe.linker_version.minor >= 60) and (pe.linker_version.minor < 80))
		) 
		//and $c0
}

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 1 string patterns in its detection logic.

• Source Rule

False Positive Guidance

• Legacy Web Application Testing or Staging Environments: Developers or QA engineers may use flatassembler.net as a mock endpoint, placeholder resource, or test fixture within staging environments for legacy web applications or internal tools that have not yet been migrated to modern infrastructure.
  • Filter/Exclusion: Exclude traffic from known staging subdomains (e.g., *.staging.internal) or specific developer workstations (OU: Dev-Team) where the destination IP or hostname matches the known CIDR range or FQDN of the staging cluster.
• Automated Health Check or Uptime Monitoring Scripts: Internal monitoring agents (e.g., Nagios, Zabbix, or custom PowerShell/Bash scripts) might use http://flatassembler.net as a lightweight HTTP endpoint to verify outbound internet connectivity or DNS resolution for specific network segments, particularly if the main health check endpoint is down or being tested.
  • Filter/Exclusion: Exclude events where the source user account is a service account (e.g., svc-monitoring, svc-healthcheck) or where the process name is a known monitoring agent (e.g., nagios.exe, zabbix_agent2.exe, python.exe running a specific script path).
• Browser Cache or Historical Artifact Access: End users may inadvertently trigger this rule by accessing a bookmarked URL, restoring a browser session, or clicking a link in an old email or document that still references http://flatassembler.net as a resource, even if the site is no longer actively used for production.
  • Filter/Exclusion: Exclude events where the initiating process is a web browser (e.g., chrome.exe, edge.exe, firefox.exe) and the user agent string contains standard browser identifiers, or where the event type is “HTTP Request” rather than “HTTP Response” to focus on active data exfiltr