Second Generic Spyware.Citadel.Atmos signture when builder add a packed layer

Hunt Hypothesis

The detection identifies potential signs of a packed malicious payload associated with the Citadel spyware family, indicating an adversary may be using obfuscation techniques to evade initial detection. SOC teams should proactively hunt for this behavior in Azure Sentinel to uncover stealthy, advanced threats that may bypass traditional signature-based defenses.

YARA Rule

rule Atmos_Packed_Malware 
{
   
    meta:
    description = "Second Generic Spyware.Citadel.Atmos signture when builder add a packed layer"
    author = "[email protected]"
    reference = "http://www.xylibox.com/2016/02/citadel-0011-atmos.html"
    date = "20/08/2016"
    // May only the challenge guide you

    strings:
        $MZ = {4D 5A}
        // Entry point identifier with CreateThread pointer in '??' 
        $a = {55 8B EC 83 EC 0C 53 56 8B 35 ?? ?? ?? 00 57 33 DB BF 00 28 00 00}
        // End of main proc with sleep value in '??' and api call to sleep in '??'
        $b = {68 ?? ?? ?? ?? FF 15 ?? ?? ?? 00 E9 62 FF FF FF E8 69 10 FE FF 5F 5E 5B C9 C3}
        // API String identifier (ShellExecuteExW, SHELL32.dll, GetUserNameExW, Secur32.dll)
        $c = {53 68 65 6C 6C 45 78 65 63 75 74 65 45 78 57 00 53 48 45 4C 4C 33 32 2E 64 6C 6C 00 1E 00 47 65}
        $d = {74 55 73 65 72 4E 61 6D 65 45 78 57 00 00 53 65 63 75 72 33 32 2E 64 6C 6C 00 10 00}
        // New Thread identifier
        $e = {55 8B EC 83 E4 F8 83 EC 1C 83 7D 08 00 57 74 ?? 6A FF FF 75 08 FF 15 ?? ?? ?? 00}

    condition:
    all of them 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 6 string patterns in its detection logic.

References

False Positive Guidance

Scenario: A system administrator is using Process Explorer to inspect memory dumps of a running process for debugging purposes.
Filter/Exclusion: Check for process_name containing “Process Explorer” or “procexp” in the event log or process tree.
Scenario: A scheduled backup job using Veeam Backup & Replication is executing and temporarily unpacking files during the backup process.
Filter/Exclusion: Exclude processes with process_name containing “Veeam” or “backup” and check for user_account being a service account used by the backup tool.
Scenario: A software update task using Chocolatey is installing a package that includes a packed binary as part of its installation process.
Filter/Exclusion: Filter out processes with process_name containing “choco” or “Chocolatey” and check for command_line containing “install” or “update”.
Scenario: A system integrity check using Sysinternals tools (e.g., Process Monitor) is capturing file system activity that includes unpacking operations.
Filter/Exclusion: Exclude processes with process_name containing “Process Monitor” or “ProcMon” and check for tool_name or command_line indicating a diagnostic tool.
Scenario: A virtualization platform like VMware Tools or Hyper-V Guest Services is unpacking files during VM configuration or guest integration.
Filter/Exclusion: Exclude processes with process_name containing “VMware Tools”, “Hyper-V”, or “Guest Services” and check for process_tree indicating a virtualization-related process.