Casper French Espionage Malware - Win32/ProxyBot.B - Dropper http://goo.gl/VRJNLo

Hunt Hypothesis

The hypothesis detects an adversary using a known espionage malware dropper to establish a persistent backdoor and exfiltrate data through a malicious URL. SOC teams should proactively hunt for this behavior in Azure Sentinel to identify and mitigate early-stage espionage activity before data exfiltration and lateral movement occur.

YARA Rule

rule Casper_EXE_Dropper
{

    meta:
        description = "Casper French Espionage Malware - Win32/ProxyBot.B - Dropper http://goo.gl/VRJNLo"
        author = "Florian Roth"
        reference = "http://goo.gl/VRJNLo"
        date = "2015/03/05"
        hash = "e4cc35792a48123e71a2c7b6aa904006343a157a"
        score = 80

    strings:
        $s0 = "<Command>" fullword ascii
        $s1 = "</Command>" fullword ascii
        $s2 = "\" /d \"" fullword ascii
        $s4 = "'%s' %s" fullword ascii
        $s5 = "nKERNEL32.DLL" fullword wide
        $s6 = "@ReturnValue" fullword wide
        $s7 = "ID: 0x%x" fullword ascii
        $s8 = "Name: %S" fullword ascii

    condition:
        7 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 8 string patterns in its detection logic.

References

• http://goo.gl/VRJNLo
• Source Rule

False Positive Guidance

• Scenario: Legitimate Software Update via Casper (Jamf)

  • Description: A Casper (Jamf) policy is pushing a legitimate software update that includes a script or executable with a similar filename or hash to the malware.
  • Filter/Exclusion: Check the process.name for jamf or casper, and verify the file path against known Jamf update directories (e.g., /Library/Management/).

• Scenario: Scheduled Job for System Maintenance

  • Description: A scheduled task (e.g., Task Scheduler or cron) is running a legitimate system maintenance script that has a similar name or behavior to the malware.
  • Filter/Exclusion: Filter by process.parent to identify if the process is launched by a known system scheduler (e.g., svchost.exe, task scheduler, or launchd).

• Scenario: Admin Performing Remote Execution via PowerShell

  • Description: An administrator is using PowerShell to remotely execute a script or command that is being flagged due to similar network activity or file hashes.
  • Filter/Exclusion: Filter by process.name for powershell.exe and check the user.name to ensure it matches a known admin account. Also, check the destination for internal IP ranges.

• Scenario: Legitimate Data Exfiltration via Proxy Tool

  • Description: A legitimate data exfiltration tool (e.g., curl, wget, or scp) is being used to transfer data to an internal or external server, triggering the rule due to similar URL patterns.
  • Filter/Exclusion: Filter by process.name for known exfiltration tools and check the destination against internal IP ranges or approved external endpoints.

• Scenario: Malicious URL in Legitimate Email Campaign

  • Description: A phishing email contains a