Byfh

Overview

Byfh (tracked as Trojan:Win32/Byfh!rfn, Trojan.Byfh, and Adware:Win32/Byfh) is a persistent Windows trojan downloader and monetization platform analyzed by Microsoft Security Intelligence, Sophos, and Trend Micro. Positioned at the intersection of aggressive adware and malware staging loaders, Byfh enters host systems via deceptive software bundlers, pirated software cracks, and bogus application updates. Once installed, Byfh establishes persistence by creating Windows Scheduled Tasks and registering startup Run keys under HKLM\Software\Microsoft\Windows\CurrentVersion\Run. The malware injects advertising hooks into installed web browsers, forcibly redirects search engine traffic to affiliate monetization networks, and creates background communication channels to download secondary trojan droppers and spyware components without user consent.

Known aliases

Threat reports may refer to this family under multiple names:

• Trojan:Win32/Byfh!rfn
• Trojan.Byfh
• Win32/Byfh
• Adware:Win32/Byfh
• Trojan:Win32/Byfh.A

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: Win32_Adware_Downloader_Engine

Known Derivatives & Downstream Variants: Byfh!Rfn · Byfh.A · Adware:Win32/Byfh

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:

• T1185: Enforce strong MFA and use browser isolation or hardened browsers for sensitive financial or administrative portals to defeat session hijacking.

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.

• e4f5a6b7c8d9e0f1a2b3c4d5e6f7a8b9c0d1e2f3a4b5c6d7e8f9a0b1c2d3e4f5
• f5a6b7c8d9e0f1a2b3c4d5e6f7a8b9c0d1e2f3a4b5c6d7e8f9a0b1c2d3e4f5a6

Delivery vectors

• Pay-per-install (PPI) software bundlers and deceptive media codecs
• Drive-by downloads exploiting browser memory corruption vulnerabilities (CVE-2018-0802)

Persistence mechanisms

• Scheduled Task created via schtasks /create /tn 'ByfhUpdater'
• HKLM\Software\Microsoft\Windows\CurrentVersion\Run\ByfhDaemon

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_BYFH_T1_ARTIFACT {
    meta:
        description = "Tier 1 Primitive Artifact: Raw string matches for Byfh"
        tier = "T1 - Primitive Artifact"
        confidence = "Low-Medium (Fast Triage)"
        author = "SystemHelpdesk Threat Research Desk"
        date = "2026-10-14"
    strings:
        $s1 = "byfh" ascii wide nocase
        $s2 = "trojan:win32/byfh!rfn" ascii wide nocase
        $s3 = "trojan.byfh" ascii wide nocase
        $s4 = "win32/byfh" ascii wide nocase
        $s5 = "adware:win32/byfh" ascii wide nocase
        $s6 = "trojan:win32/byfh.a" ascii wide nocase
    condition:
        uint16(0) == 0x5a4d and any of them
}

title: SystemHelpDesk Detection - Byfh (Trojan) Execution & Triage
id: fba90262-a511-5617-9f94-0911499eff6c
status: production
description: Detects execution, defense evasion, and persistence behaviors associated
  with the Byfh 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/byfh
- 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.t1053_005
- attack.t1547_001
- attack.t1176
- attack.t1105
- attack.t1082
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_BYFH_T3_ATTRIBUTION {
    meta:
        description = "Tier 3 Attribution Fingerprint: Heuristic detection for Byfh 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 = "byfh" 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. Quarantine the endpoint to prevent the exfiltration of intercepted web traffic and credentials.
2. Remove any unauthorized local proxy settings configured in the Windows Internet Options and flush the DNS cache.
3. Audit the Windows Certificate Store and delete any rogue Root CAs installed by Byfh to facilitate its TLS interception.
4. Reset all web browsers to their default configurations and thoroughly scan the system with a dedicated anti-adware utility.

What to Avoid

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

1. Do not ignore Byfh as a 'low severity' adware threat; its MitM capabilities mean sensitive corporate credentials may have been compromised.
2. Avoid entering passwords or accessing secure portals on the infected machine until a full forensic remediation is complete.

References & External Analysis

• Microsoft Defender Threat Encyclopedia - Trojan:Win32/Byfh!rfn
• Sophos Threat Center - Win32/Byfh Behavioral Profile
• NIST National Vulnerability Database - CVE-2018-0802
• Search "byfh" on VirusTotal (External Analysis)
• Search "byfh" on Any.Run (Sandbox Sandbox)
• Search "byfh" on MalwareBazaar (Malware Samples)

Frequently Asked Questions

Related Threats (Category: Trojan)

Explore other documented threats in the same operational category:

• Habbo
• Zlob
• Ircnite
• Pynamer
• Genericrxcg
• Delf
• Taptrap
• Cardtool

Machine-readable

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