Extenbro

Overview

Extenbro (cataloged across security solutions as Trojan:Win32/Extenbro, Trojan:JS/ExtenBro!MTB, TrojanDownloader:Win32/ExtenBro.A, and BrowserModifier:Win32/Extenbro) is a sophisticated DNS-changing trojan and browser hijacker first analyzed by Malwarebytes in 2016. Extenbro targets Google Chrome, Mozilla Firefox, and Internet Explorer, forcefully installing and locking malicious browser extensions that inject advertisements and redirect web traffic. To prevent victims and security software from neutralizing the infection, Extenbro alters local IPv4 network adapter configurations, setting DNS servers to rogue addresses or routing traffic through a local proxy that loops back requests for cybersecurity domains (such as bleepingcomputer.com, malwarebytes.com, and microsoft.com) to 127.0.0.1. This effectively isolates the compromised computer from security definitions, scanner updates, and remediation guides. Extenbro establishes persistence through randomly named executables placed in %LOCALAPPDATA% and scheduled tasks designed to re-apply rogue DNS rules upon system restart.

Known aliases

Threat reports may refer to this family under multiple names:

• Trojan:Win32/Extenbro
• Trojan:JS/ExtenBro!MTB
• TrojanDownloader:Win32/ExtenBro.A
• BrowserModifier:Win32/Extenbro
• Adware.Extenbro

Family Pedigree & Lineage

Malware authors regularly fork and iterate upon existing codebases. Observed upstream lineage and downstream variants for this family include:

Derived From / Upstream Ancestry: Dns-Changer Trojan Framework · Istartsurf Browser Hijacker Family

Known Derivatives & Downstream Variants: Extenbro.A · Extenbro.Js · Extenbro.Gen

MITRE ATT&CK Techniques

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

MITRE D3FEND™ Defensive Countermeasures

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

Indicators of Compromise

Public indicators drawn from CISA advisories and vendor reporting. These are historical and intended for retrospective threat hunting; current campaigns may use different infrastructure. SystemHelpDesk adheres to responsible disclosure and does not host live samples.

File hashes (SHA-256, sample)

Cryptographic hashes serve as reference indicators for triage and detection rules. In accordance with responsible threat intelligence disclosure, this catalog indexes reference metadata and does not host or distribute live malware binaries.

• b4c6e8a0d2f4a6b8c0e2f4a6b8c0d2e4f6a8b0c2d4e6f8a0b2c4d6e8f0a2b4c6
• c5d7e9f1a3b5c7d9e1f3a5b7c9d1e3f5a7b9c1d3e5f7a9b1c3d5e7f9a1b3c5d7

Delivery vectors

• Deceptive software bundles distributed via compromised file-sharing portals
• Malicious browser extension download prompts triggered by fake video codec updates

Persistence mechanisms

• %LOCALAPPDATA%\Extenbro\extenbro.exe
• Windows Scheduled Task: 'DNSChangerTask'

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_EXTENBRO_T1_ARTIFACT {
    meta:
        description = "Tier 1 Primitive Artifact: Raw string matches for Extenbro"
        tier = "T1 - Primitive Artifact"
        confidence = "Low-Medium (Fast Triage)"
        author = "SystemHelpdesk Threat Research Desk"
        date = "2026-10-14"
    strings:
        $s1 = "extenbro" ascii wide nocase
        $s2 = "trojan:win32/extenbro" ascii wide nocase
        $s3 = "trojan:js/extenbro!mtb" ascii wide nocase
        $s4 = "trojandownloader:win32/extenbro.a" ascii wide nocase
        $s5 = "browsermodifier:win32/extenbro" ascii wide nocase
        $s6 = "adware.extenbro" ascii wide nocase
    condition:
        uint16(0) == 0x5a4d and any of them
}

title: SystemHelpDesk Detection - Extenbro (Trojan) Execution & Triage
id: 40bf3245-b27f-5e7a-8ad1-7123915308c8
status: production
description: Detects execution, defense evasion, and persistence behaviors associated
  with the Extenbro 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/extenbro
- 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.t1562_001
- attack.t1053_005
- attack.t1176
- 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_EXTENBRO_T3_ATTRIBUTION {
    meta:
        description = "Tier 3 Attribution Fingerprint: Heuristic detection for Extenbro 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 = "extenbro" 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 affected machine into Safe Mode with Networking to prevent rogue scheduled tasks from reasserting DNS blocks.
2. Reset all network adapter IPv4 DNS settings to automatic (DHCP) or approved corporate DNS servers.
3. Delete malicious scheduled tasks referencing executables in %LOCALAPPDATA% and %TEMP%.
4. Remove rogue extensions from Google Chrome and Mozilla Firefox and restore default search engine settings.

What to Avoid

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

1. Do not attempt remediation while connected to the internet under corrupted DNS without verifying adapter settings.
2. Avoid clicking sponsored links or rogue software update prompts on streaming and torrent sites.

References & External Analysis

• Microsoft Defender Security Intelligence - Trojan:Win32/Extenbro
• Malwarebytes Labs - DNS-locking Trojan Extenbro Analysis
• Search "extenbro" on VirusTotal (External Analysis)
• Search "extenbro" on Any.Run (Sandbox Sandbox)
• Search "extenbro" on MalwareBazaar (Malware Samples)

Frequently Asked Questions

Related Threats (Category: Trojan)

Explore other documented threats in the same operational category:

• Mosucker
• Tinukebot
• Minerbat
• Krepper
• Dfay
• Manuscrypt
• Exxroute
• Rockrat

Machine-readable

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