Yayih Identifying Strings

Hunt Hypothesis

This detection rule identifies specific identifying strings associated with the Yayih malware family to uncover potential initial footholds or lateral movement within the environment. Proactive hunting for these indicators in Azure Sentinel is essential because their low severity classification often causes them to be overlooked by automated alerts, requiring manual correlation to confirm early-stage compromise before escalation occurs.

YARA Rule

rule YayihStrings : Yayih Family
{
    meta:
        description = "Yayih Identifying Strings"
        author = "Seth Hardy"
        last_modified = "2014-07-11"
        
    strings:
        $ = "/bbs/info.asp"
        $ = "\\msinfo.exe"
        $ = "%s\\%srcs.pdf"
        $ = "\\aumLib.ini"

    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 5 specific false positive scenarios for the Yayih Identifying Strings detection rule in an enterprise environment, along with suggested filters or exclusions:

• Scheduled Antivirus Definition Updates via WSUS/SCCM

  • Scenario: Enterprise endpoint protection suites (e.g., Microsoft Defender, CrowdStrike) frequently push signature updates containing “Yayih” identifying strings within their payload metadata. When the WSUS server or SCCM agent distributes these updates to thousands of workstations simultaneously, the detection rule triggers on the file creation and registry modification events associated with the update package.
  • Suggested Filter: Exclude network traffic and file system events originating from known management servers (e.g., IP: 10.x.x.x or Hostname containing SCCM-Server) where the process name matches wsusagent.exe, ccmexec.exe, or MpCmdRun.exe.

• Automated Log Rotation and Archiving Jobs

  • Scenario: Centralized log management tools like Splunk Universal Forwarders, ELK Stack (Logstash), or Windows Task Scheduler jobs often compress and archive logs daily. These processes may embed “Yayih” identifying strings in the temporary file headers or compression metadata during the rotation of application logs (e.g., IIS logs or custom .NET application logs) before moving them to cold storage.
  • Suggested Filter: Create an exclusion for scheduled tasks running under the SYSTEM or specific service accounts (e.g., svc-logrotate) that execute between 02:00 and 04:00 UTC, specifically targeting file paths within standard log directories like %SystemRoot%\Logs\ or /var/log/.

• Software Deployment via Microsoft Endpoint Configuration Manager

  • Scenario: During business hours, IT administrators deploy new applications or patches using Microsoft Endpoint Configuration Manager (