Generic Spyware.Citadel.Atmos Signature

Hunt Hypothesis

Adversaries may use this spyware to exfiltrate sensitive data and maintain long-term persistence within the network. SOC teams should proactively hunt for this behavior in Azure Sentinel to identify and mitigate potential data breaches and stealthy malware activity.

YARA Rule

rule Atmos_Malware
{
    
    meta:
        description = "Generic Spyware.Citadel.Atmos Signature"
        author = "xylitol@temari.fr"
        reference = "http://www.xylibox.com/2016/02/citadel-0011-atmos.html"
        date = "20/08/2016"
        // May only the challenge guide you

    strings:
        // Check for the presence of MZ and kutuzov license identifier
        $MZ = {4D 5A}
        $LKEY = "533D9226E4C1CE0A9815DBEB19235AE4" wide ascii
        
        // TokenSpy identifiers
        $TS1 = "X-TS-Rule-Name: %s" wide ascii
        $TS2 = "X-TS-Rule-PatternID: %u" wide ascii
        $TS3 = "X-TS-BotID: %s" wide ascii
        $TS4 = "X-TS-Domain: %s" wide ascii
        $TS5 = "X-TS-SessionID: %s" wide ascii
        $TS6 = "X-TS-Header-Cookie: %S" wide ascii
        $TS7 = "X-TS-Header-Referer: %S" wide ascii
        $TS8 = "X-TS-Header-AcceptEncoding: %S" wide ascii
        $TS9 = "X-TS-Header-AcceptLanguage: %S" wide ascii
        $TS10 = "X-TS-Header-UserAgent: %S" wide ascii
        
        // Hidden VNC identifiers
        $VNC1 = "_hvnc_init@4" wide ascii
        $VNC2 = "_hvnc_uninit@0" wide ascii
        $VNC3 = "_hvnc_start@8" wide ascii
        $VNC4 = "_hvnc_stop@0" wide ascii
        $VNC5 = "_hvnc_wait@0" wide ascii
        $VNC6 = "_hvnc_work@0" wide ascii
        
        // Browsers identifiers
        $WB1 = "nspr4.dll" wide ascii
        $WB2 = "nss3.dll" wide ascii
        $WB3 = "chrome.dll" wide ascii
        $WB4 = "Internet Explorer" wide ascii
        $WB5 = "Firefox" wide ascii
        $WB6 = "Chrome" wide ascii

    condition:
    ($MZ at 0 and $LKEY) and ( (5 of ($TS*) and all of ($WB*)) or (3 of ($VNC*) and all of ($WB*)))
    and filesize < 300KB // Standard size (raw from builder) should be arround ~264kb
        // Remove the above line if you want to trig also on memory dumps, etc...
}

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 24 string patterns in its detection logic.

References

False Positive Guidance

Scenario: Legitimate scheduled system cleanup task using CCleaner
Filter/Exclusion: Exclude processes where the executable path contains C:\Program Files\CCleaner\ccleaner.exe or similar known legitimate paths.
Scenario: Admin performing a Windows Update or Group Policy refresh
Filter/Exclusion: Exclude processes with the command line containing wuauclt.exe or gpupdate.exe, or where the parent process is services.exe or taskhost.exe.
Scenario: Running Microsoft Endpoint Detection and Response (EDR) or Microsoft Defender signature updates
Filter/Exclusion: Exclude processes where the executable path includes C:\Windows\System32\WindowsPowerShell\v1.0\powershell.exe and the command line includes -Command "Update-MpSignature" or similar EDR-related commands.
Scenario: Executing a third-party backup tool like Veeam or Acronis
Filter/Exclusion: Exclude processes with the executable path containing C:\Program Files\Veeam\Backup\veeam.exe or similar paths for known backup tools.
Scenario: Running a custom script or job for log rotation or data archiving
Filter/Exclusion: Exclude processes where the command line includes logrotate.exe or rsync.exe and the parent process is a known system service or scheduled task.