AAR

Hunt Hypothesis

This detection identifies adversary behavior characterized by specific file patterns or code structures defined by the AAR YARA signature, which may indicate early-stage reconnaissance or benign tool usage often missed by standard alerts. Proactively hunting for this signal in Azure Sentinel is essential to validate the rule’s efficacy against false positives and to uncover subtle indicators of compromise that could precede more severe attack phases.

YARA Rule

rule AAR : RAT
{
	meta:
		author = " Kevin Breen <kevin@techanarchy.net>"
		date = "2014/04"
		ref = "http://malwareconfig.com/stats/AAR"
		maltype = "Remote Access Trojan"
		filetype = "exe"

	strings:
		$a = "Hashtable"
		$b = "get_IsDisposed"
		$c = "TripleDES"
		$d = "testmemory.FRMMain.resources"
		$e = "$this.Icon" wide
		$f = "{11111-22222-20001-00001}" wide
		$g = "@@@@@"

	condition:
		all 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 7 string patterns in its detection logic.

• Source Rule

False Positive Guidance

Here are 5 specific false positive scenarios for the AAR detection rule in an enterprise environment, including suggested filters and exclusions:

• Antivirus Real-Time Scanning of Large Archives

  • Scenario: The endpoint protection agent (e.g., CrowdStrike Falcon or Microsoft Defender) performs a scheduled deep scan on large .zip or .tar.gz archives containing thousands of small files. The AAR logic may flag the rapid sequence of file creation and modification events as suspicious activity, mistaking the scanning engine’s behavior for an active attack reconnaissance phase.
  • Filter/Exclusion: Create a whitelist exclusion for the specific process names of the AV engine (e.g., MsMpEng.exe, csagent.exe) when operating on known archive extensions within designated “Data Sync” folders.

• Scheduled Software Deployment via SCCM or Intune

  • Scenario: During business hours, System Center Configuration Manager (SCCM) or Microsoft Intune pushes a mandatory application update to a batch of workstations. The installation process involves creating temporary directories, writing registry keys, and executing installer scripts that match the AAR signature for “unauthorized configuration changes.”
  • Filter/Exclusion: Exclude events originating from the ccmexec.exe (SCCM) or IntuneManagementExtension.exe processes during defined maintenance windows (e.g., 02:00–04:00 UTC), specifically targeting paths under C:\Windows\CCMCache.

• Automated Log Rotation and Archiving Scripts

  • Scenario: A nightly PowerShell script, managed by a tool like Ansible or a custom cron job on Linux servers, compresses and moves application logs to a network share. The rule triggers because the script executes file I/O operations that mimic data exfiltration patterns defined in the AAR logic.

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