Fourthrem

Overview

Trojan:Win32/Fourthrem is a persistent, stealthy trojan designed to breach endpoint defenses and securely deliver secondary, high-severity payloads like ransomware, banking trojans, and botnet agents, or act as a persistent backdoor.

Understanding Fourthrem
To an end-user, a Fourthrem infection is typically invisible until the devastating secondary payload executes. For threat intelligence analysts, Fourthrem is a critical staging mechanism. It functions as an 'Initial Access Broker' tool. Its primary objective is to securely bypass endpoint defenses, profile the infected machine to ensure it is not a sandbox, and reach out to a Command-and-Control (C2) server to pull down the final, destructive payload.

Execution and Evasion Strategies
Fourthrem is commonly distributed via massive malspam campaigns containing weaponized attachments (like ZIP files or Office macros) or through exploit kits. Upon execution, it utilizes heavy packing and dynamic API resolution to evade static antivirus signatures. It drops a randomized executable into the `%Temp%` or `%AppData%` directory. It establishes persistence by creating a hidden scheduled task or modifying the Registry Run keys. Fourthrem frequently injects its downloading routine into legitimate system processes (like `explorer.exe` or `svchost.exe`) to mask its outbound network traffic.

Indicators of Compromise (IoCs)
Threat hunters should investigate EDR alerts related to 'Suspicious Process Injection' or 'Anomalous Child Process Spawning'. Network logs will often reveal Fourthrem reaching out to compromised domains using encrypted HTTPS traffic. The presence of unexpected, hidden scheduled tasks designed to execute randomly named binaries in the user's profile is a strong IoC. Memory analysis is necessary to extract the injected downloader modules and determine what payloads were requested.

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.
• 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_FOURTHREM_T1_ARTIFACT {
    meta:
        description = "Tier 1 Primitive Artifact: Raw string matches for Fourthrem"
        tier = "T1 - Primitive Artifact"
        confidence = "Low-Medium (Fast Triage)"
        author = "SystemHelpdesk Threat Research Desk"
        date = "2026-10-14"
    strings:
        $s1 = "fourthrem" ascii wide nocase
    condition:
        uint16(0) == 0x5a4d and any of them
}

title: SystemHelpDesk Detection - Fourthrem (Ransomware) Execution & Triage
id: e2deadff-76c3-5fc9-8dcb-8a355f325f4a
status: production
description: Detects execution, defense evasion, and persistence behaviors associated
  with the Fourthrem malware family. Detects volume shadow copy deletion, boot status
  tampering, and recovery inhibition characteristic of modern ransomware operations.
references:
- https://malware-families-catalog.systemhelpdesk.com/families/fourthrem
- https://malware-families-catalog.systemhelpdesk.com/defensive/ransomware-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.ransomware
- author.ricky-jordan
- source.systemhelpdesk-mfc
- attack.t1055
- attack.t1027
- attack.t1105
- attack.t1566_001
- attack.t1547_001
logsource:
  category: process_creation
  product: windows
detection:
  selection_process:
    Image|endswith:
    - \vssadmin.exe
    - \wmic.exe
    - \wbadmin.exe
    - \bcdedit.exe
    CommandLine|contains:
    - delete shadows
    - shadowcopy delete
    - resize shadowstorage
    - recoveryenabled no
    - ignoreallfailures
  condition: selection_process
falsepositives:
- Legitimate enterprise administrative scripts during approved change windows
- Authorized IT software deployment utilities and backup management agents
level: critical

rule MALWARE_WIN_FOURTHREM_T3_ATTRIBUTION {
    meta:
        description = "Tier 3 Attribution Fingerprint: Heuristic detection for Fourthrem 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 = "fourthrem" 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 endpoint immediately to prevent Fourthrem from downloading and executing its secondary payloads (e.g., ransomware).
2. Audit the Windows Task Scheduler and Registry Run keys to identify and remove the Fourthrem persistence mechanisms.
3. Review firewall and proxy logs to identify the C2 domains Fourthrem attempted to contact, and block them enterprise-wide.
4. Capture a live memory image (RAM dump) to extract the injected Fourthrem modules and identify the secondary payloads.

What to Avoid

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

1. Do not close an incident simply because the initial Fourthrem dropper was quarantined; always verify if secondary payloads were downloaded.
2. Avoid relying solely on manual file deletion, as the injected processes will likely just recreate the dropped binaries.

References & External Analysis

• Search "fourthrem" on VirusTotal (External Analysis)
• Search "fourthrem" on Any.Run (Sandbox Sandbox)
• Search "fourthrem" on MalwareBazaar (Malware Samples)

Frequently Asked Questions

Related Threats (Category: Ransomware)

Explore other documented threats in the same operational category:

• Dnguard
• Zonobindi
• Fdoh
• Cxkh
• Genericrxdf
• Adject
• Serveradmin
• Cmhfbw9R

Machine-readable

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