APT28 downdelph string on MBR (get your MBR with BOOTICE on Win or #dd if=/dev/sda of=./sda.mbr bs=512 count=1

Hunt Hypothesis

The detection identifies potential APT28 group activity where malicious strings associated with the Downdelph malware are embedded in the Master Boot Record, indicating possible persistence or data exfiltration mechanisms. SOC teams should proactively hunt for this behavior in Azure Sentinel to detect early-stage APT28 attacks that leverage low-level system components to evade traditional detection methods.

YARA Rule

rule APT_fancybear_Downdelph_MBR : Bootkit{
	meta:
		author = "Marc Salinas @Bondey_m"
		description = "APT28 downdelph string on MBR (get your MBR with BOOTICE on Win or #dd if=/dev/sda of=./sda.mbr bs=512 count=1"
		reference = "https://www.threatminer.org/_reports/2016/eset-sednit-part3%20-%20ESET.pdf#viewer.action=download"
	strings:
		$s1 = { 20 3A 33 20 } //string " :3 "
	condition:
		$s1 at 411  //posición 0x19b
}

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.

References

• https://www.threatminer.org/_reports/2016/eset-sednit-part3%20-%20ESET.pdf#viewer.action=download
• Source Rule

False Positive Guidance

• Scenario: A system administrator uses BOOTICE to extract the MBR for forensic analysis or disk imaging.
  Filter/Exclusion: Exclude events where the MBR is accessed by known forensic tools like BOOTICE or DiskDigger.

• Scenario: A scheduled job runs to backup the MBR as part of a disk imaging or disaster recovery process.
  Filter/Exclusion: Exclude events where the MBR is accessed by scheduled tasks or scripts associated with Veeam, Acronis, or Ghost.

• Scenario: A system update or patching process modifies the MBR to apply new boot configurations.
  Filter/Exclusion: Exclude events where the MBR is modified by Windows Update, WSUS, or Group Policy during system updates.

• Scenario: An IT admin uses dd (Linux) to clone the MBR for a multi-disk environment, such as a RAID array.
  Filter/Exclusion: Exclude events where the MBR is accessed by dd or Clonezilla in a known backup or cloning operation.

• Scenario: A legitimate security tool, such as Malwarebytes or Kaspersky, scans the MBR as part of a system integrity check.
  Filter/Exclusion: Exclude events where the MBR is accessed by known security tools or endpoint protection platforms during routine scans.