Strings for Surtr

Hunt Hypothesis

This detection rule identifies adversary behavior characterized by specific string patterns associated with the “Surtr” activity, which may indicate early-stage reconnaissance or data exfiltration attempts within Azure workloads. Proactively hunting for these strings in Azure Sentinel is critical to validate low-severity signals before they escalate, ensuring that subtle indicators of compromise are not overlooked amidst routine noise.

YARA Rule

rule SurtrStrings : Surtr Family {	
	meta: 
		author = "Katie Kleemola"
		description = "Strings for Surtr"
		last_updated = "2014-07-16"

	strings:
		$ = "\x00soul\x00"
		$ = "\x00InstallDll.dll\x00"
		$ = "\x00_One.dll\x00"
		$ = "_Fra.dll"
		$ = "CrtRunTime.log"
		$ = "Prod.t"
		$ = "Proe.t"
		$ = "Burn\\"
		$ = "LiveUpdata_Mem\\"

	condition:
		any 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
• Source Rule

False Positive Guidance

Here are 4 specific false positive scenarios for the “Strings for Surtr” detection rule in an enterprise environment, along with recommended filters or exclusions:

• Scheduled Backup Verification Jobs

  • Scenario: Enterprise backup solutions like Veeam Backup & Replication or Commvault frequently run nightly integrity checks. These jobs often parse large configuration files (XML/JSON) containing the specific “Surtr” string signature to validate data consistency before archiving, triggering the rule during the maintenance window (e.g., 02:00 – 04:00 UTC).
  • Filter/Exclusion: Create an exclusion for process names VeeamAgent.exe and SimService.exe running on hosts within the Backup-Servers AD security group, specifically during the defined maintenance window.

• Compliance Scanning by Qualys or Tenable

  • Scenario: Vulnerability management tools such as Qualys Cloud Agent or Tenable Nessus perform deep content scans on application directories. When scanning legacy applications or custom scripts that reference “Surtr” in their manifest files, the scanner’s file parsing engine generates events matching the detection logic while running under the qualysguard service account.
  • Filter/Exclusion: Exclude events where the User Account is a known service account (e.g., svc-qualys-scanner) and the Source IP belongs to the internal vulnerability scanning subnet (e.g., 10.20.50.0/24).

• CI/CD Pipeline Artifact Analysis

  • Scenario: In a DevOps environment using Jenkins or Azure DevOps, automated build pipelines often execute static code analysis on source repositories. If the “Surtr” string is part of a standard library name or a hardcoded configuration key in