Zlob

Overview

Zlob (historically one of the most prolific malware families in cybersecurity history, classified across antivirus engines as Trojan:Win32/Zlob, Trojan.Zlob, and Trojan-Downloader.Win32.Zlob) was a massive global trojan ecosystem active from 2005 onwards that infected tens of millions of computers worldwide. Disguised as a required video codec (such as ActiveX Video Codec or VideoLAN plug-in) needed to view online media, Zlob tricks victims into manual installation. Upon execution, Zlob modifies Windows DNS server settings under HKLM\System\CurrentControlSet\Services\Tcpip\Parameters\Interfaces, redirecting all DNS queries to rogue attacker-controlled DNS servers (DNSChanger). This allows threat actors to hijack web traffic, display fake security alerts, redirect searches to rogue antivirus scareware landing pages, and inject pay-per-click advertisements into browsing sessions.

Known aliases

Threat reports may refer to this family under multiple names:

• Trojan:Win32/Zlob
• Trojan.Zlob
• Win32/Zlob.Trojan
• DNSChanger.Zlob
• Trojan-Downloader.Win32.Zlob
• Zlob Video Codec

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:

• T1105: Implement network intrusion detection systems (NIDS) and host-based firewalls to block unauthorized inbound or outbound file transfers.

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_ZLOB_T1_ARTIFACT {
    meta:
        description = "Tier 1 Primitive Artifact: Raw string matches for Zlob"
        tier = "T1 - Primitive Artifact"
        confidence = "Low-Medium (Fast Triage)"
        author = "SystemHelpdesk Threat Research Desk"
        date = "2026-10-14"
    strings:
        $s1 = "zlob" ascii wide nocase
        $s2 = "trojan:win32/zlob" ascii wide nocase
        $s3 = "trojan.zlob" ascii wide nocase
        $s4 = "win32/zlob.trojan" ascii wide nocase
        $s5 = "dnschanger.zlob" ascii wide nocase
        $s6 = "trojan-downloader.win32.zlob" ascii wide nocase
        $s7 = "zlob video codec" ascii wide nocase
    condition:
        uint16(0) == 0x5a4d and any of them
}

title: SystemHelpDesk Detection - Zlob (Trojan) Execution & Triage
id: 493e7e4a-6d3e-5e76-bf7e-0a2994b3dd41
status: production
description: Detects execution, defense evasion, and persistence behaviors associated
  with the Zlob 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/zlob
- 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.t1112
- attack.t1547_001
- attack.t1071_004
- attack.t1491
- attack.t1189
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_ZLOB_T3_ATTRIBUTION {
    meta:
        description = "Tier 3 Attribution Fingerprint: Heuristic detection for Zlob 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 = "zlob" 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. Boot the infected machine into 'Safe Mode with Networking' to bypass the malware's autostart mechanisms and fake AV blockers.
2. Check and reset the system's DNS settings (IPv4 properties) to ensure traffic is not being routed through malicious servers.
3. Utilize a standalone, reputable malware removal tool to scan the system, specifically targeting the `System32` directory and BHO registry keys.
4. Advise the end-user NOT to enter their credit card information into any of the aggressive pop-up warnings.

What to Avoid

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

1. Do not trust the 'Antivirus' software demanding payment; it is the secondary payload dropped by Zlob.
2. Avoid attempting manual removal of Zlob DLLs while the OS is running normally, as they are often injected into `explorer.exe` and cannot be deleted.

References & External Analysis

• Microsoft Defender Threat Encyclopedia - Trojan:Win32/Zlob
• Symantec Security Response - Trojan.Zlob Threat Assessment
• MITRE ATT&CK - Modify Registry (T1112)
• Search "zlob" on VirusTotal (External Analysis)
• Search "zlob" on Any.Run (Sandbox Sandbox)
• Search "zlob" on MalwareBazaar (Malware Samples)

Frequently Asked Questions

Related Threats (Category: Trojan)

Explore other documented threats in the same operational category:

• Zamg
• Krepper
• Skeeyah
• Qhosts
• Miragefox
• Dsbot
• Scarcruft
• Y3K_Rat

Machine-readable

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