Agentb

Overview

Agent.B (classified across commercial antivirus vendors as Trojan:Win32/Agent.B, Trojan-Downloader.Win32.Agent.b, and Backdoor.Agent.B) is a long-standing, prolific Trojan downloader and initial access implant. Designed to act as a lightweight, stealthy staging loader, Agent.B infiltrates Windows endpoints via malicious email attachments, drive-by downloads, and compromised software bundles. Once executed, the trojan establishes persistence in user Run registry keys, collects host fingerprint telemetry (including operating system build, active antivirus products, and IP geography), and beacons over HTTP/HTTPS to attacker-controlled command servers. Based on C2 instructions, Agent.B dynamically downloads and executes high-severity secondary payloads, including banking trojans, ransomware, and credential stealers.

Known aliases

Threat reports may refer to this family under multiple names:

• Trojan:Win32/Agent.B
• Trojan-Downloader.Win32.Agent.b
• Backdoor.Agent.B
• Win32/Agent.B.Downloader
• Trojan.Agent.B

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 Agent Downloader Frameworks

Known Derivatives & Downstream Variants: Agent.B1 · Agent.B.Gen · Agent.Downloader.B

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:

• T1071.001: Implement web filtering and SSL/TLS inspection to detect malicious command and control (C2) traffic hiding in HTTP/HTTPS.
• 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):

Exploited Vulnerabilities (CVEs)

This malware family is documented weaponizing the following common vulnerabilities and exposures:

• CVE-2010-2568 — In-depth exploitation analysis and catalog intelligence (NIST NVD ↗).
• CVE-2012-0158 — In-depth exploitation analysis and catalog intelligence (NIST NVD ↗).

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.

• 8a7b6c5d4e3f2a1b0c9d8e7f6a5b4c3d2e1f0a9b8c7d6e5f4a3b2c1d0e9f8a7b
• 3c4d5e6f7a8b9c0d1e2f3a4b5c6d7e8f9a0b1c2d3e4f5a6b7c8d9e0f1a2b3c4d

Delivery vectors

• Phishing emails with malicious Microsoft Office attachments exploiting CVE-2012-0158
• Drive-by compromise via malicious web scripts exploiting CVE-2010-2568

Persistence mechanisms

• HKCU\Software\Microsoft\Windows\CurrentVersion\Run\AgentB
• Scheduled Task created under Microsoft\Windows\AgentBUpdate

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_AGENTB_T1_ARTIFACT {
    meta:
        description = "Tier 1 Primitive Artifact: Raw string matches for Agentb"
        tier = "T1 - Primitive Artifact"
        confidence = "Low-Medium (Fast Triage)"
        author = "SystemHelpdesk Threat Research Desk"
        date = "2026-10-14"
    strings:
        $s1 = "agentb" ascii wide nocase
        $s2 = "trojan:win32/agent.b" ascii wide nocase
        $s3 = "trojan-downloader.win32.agent.b" ascii wide nocase
        $s4 = "backdoor.agent.b" ascii wide nocase
        $s5 = "win32/agent.b.downloader" ascii wide nocase
        $s6 = "trojan.agent.b" ascii wide nocase
    condition:
        uint16(0) == 0x5a4d and any of them
}

title: SystemHelpDesk Detection - Agentb (Trojan) Execution & Triage
id: 4127f9eb-07b8-5b63-a4a1-7d7db4ebcc6d
status: production
description: Detects execution, defense evasion, and persistence behaviors associated
  with the Agentb 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/agentb
- 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.t1105
- attack.t1071_001
- attack.t1547_001
- attack.t1082
- attack.t1027
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_AGENTB_T3_ATTRIBUTION {
    meta:
        description = "Tier 3 Attribution Fingerprint: Heuristic detection for Agentb 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 = "agentb" 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 affected endpoints to block active C2 communications and halt secondary payload downloads.
2. Terminate active Agent.B dropper processes running from %TEMP% or %APPDATA% directories.
3. Audit firewall egress logs for communication to known malicious downloader IP addresses and domain infrastructure.
4. Scan the entire endpoint with updated antimalware heuristics to detect and eliminate any secondary downloaded payloads.

What to Avoid

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

1. Do not enable macros or bypass protected view on unsolicited Microsoft Office email attachments.
2. Avoid running executable files dropped directly into temporary user directories without sandboxing.

References & External Analysis

• Microsoft Defender Security Intelligence - Trojan:Win32/Agent.B
• Trend Micro Threat Encyclopedia - Trojan.Win32.Agent.B
• NIST National Vulnerability Database - CVE-2012-0158
• Search "agentb" on VirusTotal (External Analysis)
• Search "agentb" on Any.Run (Sandbox Sandbox)
• Search "agentb" on MalwareBazaar (Malware Samples)

Frequently Asked Questions

Related Threats (Category: Trojan)

Explore other documented threats in the same operational category:

• Virtu
• Casbaneiro
• Vitro
• Taranis
• Uploader
• Dfay
• Autohit
• Fuerboos

Machine-readable

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