Strings used by iexpl0re

Hunt Hypothesis

The detection identifies potential adversary use of the iexpl0re tool through suspicious string patterns, which may indicate reconnaissance or initial compromise activities. SOC teams should proactively hunt for this behavior in Azure Sentinel to identify early-stage threats and prevent lateral movement or data exfiltration.

YARA Rule

rule iexpl0reStrings : iexpl0re Family
{
    meta:
        description = "Strings used by iexpl0re"
        author = "Seth Hardy"
        last_modified = "2014-07-21"
        
    strings:
        $ = "%USERPROFILE%\\IEXPL0RE.EXE"
        $ = "\"<770j (("
        $ = "\\Users\\%s\\AppData\\Roaming\\Microsoft\\Windows\\Start Menu\\Programs\\Startup\\IEXPL0RE.LNK"
        $ = "\\Documents and Settings\\%s\\Application Data\\Microsoft\\Internet Explorer\\IEXPL0RE.EXE"
        $ = "LoaderV5.dll"
        // stage 2
        $ = "POST /index%0.9d.asp HTTP/1.1"
        $ = "GET /search?n=%0.9d&"
        $ = "DUDE_AM_I_SHARP-3.14159265358979x6.626176"
        $ = "WHO_A_R_E_YOU?2.99792458x1.25663706143592"
        $ = "BASTARD_&&_BITCHES_%0.8x"
        $ = "c:\\bbb\\eee.txt"
        
    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

• Scenario: Legitimate use of Internet Explorer for internal web applications
  Filter/Exclusion: Check for known internal URLs (e.g., intranet.example.com) in the request or response data, and exclude traffic to internal domains.

• Scenario: Scheduled job running Internet Explorer to generate reports
  Filter/Exclusion: Exclude processes associated with scheduled tasks (e.g., schtasks.exe) or specific job names (e.g., GenerateMonthlyReport).

• Scenario: Admin using Internet Explorer to access Microsoft Endpoint Manager (Intune) portal
  Filter/Exclusion: Exclude traffic to portal.microsoft.com or admin.microsoft.com and filter by user roles (e.g., Admin).

• Scenario: Legacy application requiring Internet Explorer for compatibility
  Filter/Exclusion: Exclude processes running under specific service accounts (e.g., LegacyAppSvc) or applications known to require IE (e.g., LegacyApp.exe).

• Scenario: Malicious payload disguised as a legitimate IE tool
  Filter/Exclusion: Exclude processes that match known legitimate IE tools (e.g., iexplore.exe, ieframe.dll) and filter by process parent or command-line arguments.