Kuluoz

Overview

Kuluoz (identified across antimalware research as Trojan:Win32/Kuluoz, TrojanDownloader:Win32/Kuluoz, and closely associated with the historic Asprox botnet infrastructure) is a modular Windows trojan downloader and spam botnet implant. Active in widespread phishing and malicious spam operations disguised as postal delivery notifications (FedEx, UPS, DHL, USPS) or court summonses, Kuluoz executes an encrypted loader that checks for sandbox environments and debuggers. Upon infection, it configures persistence through startup registry keys (HKCU\Software\Microsoft\Windows\CurrentVersion\Run) and injects malicious code into legitimate processes such as svchost.exe. Kuluoz establishes encrypted C2 communications over HTTP/HTTPS, receiving instructions to send high-volume spam emails, harvest stored credentials, scan for SQL injection vulnerabilities across external websites, and download secondary malware families such as ransomware and banking trojans.

Known aliases

Threat reports may refer to this family under multiple names:

• Trojan:Win32/Kuluoz
• TrojanDownloader:Win32/Kuluoz
• Trojan:Win32/Kuluoz.A
• Trojan:Win32/Kuluoz.B
• Worm:Win32/Asprox

MITRE ATT&CK Techniques

Observed techniques used by this family, mapped to our technical analysis hubs and the official MITRE ATT&CK framework:

Tactical Mitigations

Based on the techniques used by this family, consider the following defensive strategies:

• T1059.003: Restrict execution of Windows Command Shell (cmd.exe) and block unauthorized batch scripts.
• T1071.001: Implement web filtering and SSL/TLS inspection to detect malicious command and control (C2) traffic hiding in HTTP/HTTPS.
• T1566.001: Scan email attachments for malicious macros, scripts, or suspicious archive files.

MITRE D3FEND™ Defensive Countermeasures

Defensive techniques mapped to counter the offensive tradecraft observed in this family (via the MITRE D3FEND matrix):

Tiered Detection Rules (Cisco CAIRN Model)

Modern threat detection requires multi-layered rule engineering. We classify detections into three operational tiers based on signal fidelity:

rule MALWARE_WIN_KULUOZ_T1_ARTIFACT {
    meta:
        description = "Tier 1 Primitive Artifact: Raw string matches for Kuluoz"
        tier = "T1 - Primitive Artifact"
        confidence = "Low-Medium (Fast Triage)"
        author = "SystemHelpdesk Threat Research Desk"
        date = "2026-10-14"
    strings:
        $s1 = "kuluoz" ascii wide nocase
        $s2 = "trojan:win32/kuluoz" ascii wide nocase
        $s3 = "trojandownloader:win32/kuluoz" ascii wide nocase
        $s4 = "trojan:win32/kuluoz.a" ascii wide nocase
        $s5 = "trojan:win32/kuluoz.b" ascii wide nocase
        $s6 = "worm:win32/asprox" ascii wide nocase
    condition:
        uint16(0) == 0x5a4d and any of them
}

title: SystemHelpDesk Detection - Kuluoz (Trojan) Execution & Triage
id: a87913eb-a333-5d17-8275-18665aff4fa5
status: production
description: Detects execution, defense evasion, and persistence behaviors associated
  with the Kuluoz malware family. Detects suspicious execution from temporary user
  directories with hidden window flags and background transfer utilities.
references:
- https://malware-families-catalog.systemhelpdesk.com/families/kuluoz
- https://malware-families-catalog.systemhelpdesk.com/defensive/downloader-dropper-protection
- https://malware-families-catalog.systemhelpdesk.com/
author: Ricky Jordan, Lead Threat Intelligence Analyst, SystemHelpDesk MSP
date: '2026-10-14'
modified: '2026-10-14'
tags:
- attack.trojan
- author.ricky-jordan
- source.systemhelpdesk-mfc
- attack.t1566_001
- attack.t1059_003
- attack.t1055
- attack.t1547_001
- attack.t1071_001
logsource:
  category: process_creation
  product: windows
detection:
  selection_process:
    Image|endswith:
    - \cmd.exe
    - \powershell.exe
    - \cscript.exe
    - \wscript.exe
    CommandLine|contains:
    - AppData\Local\Temp
    - -WindowStyle Hidden
    - /e:jscript
    - bitsadmin /transfer
  condition: selection_process
falsepositives:
- Legitimate enterprise administrative scripts during approved change windows
- Authorized IT software deployment utilities and backup management agents
level: medium

rule MALWARE_WIN_KULUOZ_T3_ATTRIBUTION {
    meta:
        description = "Tier 3 Attribution Fingerprint: Heuristic detection for Kuluoz tradecraft"
        tier = "T3 - Attribution Fingerprint"
        confidence = "High (Confirmed Family Signature)"
        author = "SystemHelpdesk Threat Research Desk"
        date = "2026-10-14"
    strings:
        // Core family identifier & heuristic payload signatures
        $attrib_str = "kuluoz" ascii wide nocase
        $pe_magic = "MZ" ascii
        $net_marker = "http" ascii nocase
    condition:
        uint16(0) == 0x5a4d and ($attrib_str and $net_marker) and filesize < 15MB
}

Containment & Response Steps

Ordered checklist for responders. Adapt to your environment and engage professional support for active incidents.

1. Isolate the infected host from the enterprise network immediately to stop spam distribution and payload downloads.
2. Kill injected system processes and delete staged executable droppers in %TEMP% and %APPDATA%.
3. Remove Run registry values under HKCU\Software\Microsoft\Windows\CurrentVersion\Run.
4. Block identified C2 IP addresses, domains, and outbound SMTP traffic from unauthorized endpoints.
5. Conduct full antimalware scanning to detect and eliminate secondary payloads dropped by Kuluoz.

What to Avoid

Common mistakes during response to this family that can destroy evidence, spread the infection, or worsen recovery.

1. Do not allow infected systems to send outbound SMTP or HTTP traffic.
2. Do not assume removing the primary loader cleans secondary injected modules.

References & External Analysis

• Microsoft Defender Threat Encyclopedia - Trojan:Win32/Kuluoz
• US-CERT / CISA Alert on Asprox/Kuluoz Botnet Activity
• Malwarebytes Threat Intelligence - Kuluoz Malware Analysis
• Search "kuluoz" on VirusTotal (External Analysis)
• Search "kuluoz" on Any.Run (Sandbox Sandbox)
• Search "kuluoz" on MalwareBazaar (Malware Samples)

Frequently Asked Questions

Related Threats (Category: Trojan)

Explore other documented threats in the same operational category:

• Triusor
• Premium
• Multidl
• Corebot
• Scarcruft
• Redirected
• Collector
• Favadd

Machine-readable

Get this profile as JSON: https://malware-families-catalog.systemhelpdesk.com/api/kuluoz.json

Ecosystem & Interactive Environments

This profile is part of the Malware Families Catalog, a public dataset of 2,899 malware families. The catalog is also published across our ecosystem: Hugging Face, Kaggle, Zenodo, Replit, StackBlitz, CodeSandbox, and CodePen.