Msilperseus

Overview

MSIL/Perseus (identified in endpoint telemetry as Trojan:MSIL/Perseus, Trojan.MSIL.Perseus, and MSIL/Perseus.A) is a modular .NET-based information stealer and Remote Access Trojan documented by Microsoft Defender, Trend Micro, and ESET. Written in Microsoft Intermediate Language (MSIL), Perseus targets sensitive user credentials, browser artifacts, and cryptocurrency wallet databases on infected Windows hosts. The malware executes through multi-stage obfuscation, frequently leveraging process hollowing techniques to inject unmanaged shellcode or .NET assemblies into trusted legitimate binaries such as RegSvcs.exe, MSBuild.exe, or svchost.exe. Once active in memory, Perseus extracts stored passwords, autofill data, and session cookies from Chromium- and Gecko-based browsers (including Google Chrome, Microsoft Edge, and Mozilla Firefox), enumerates local file systems for cryptocurrency wallet files, and transmits harvested intelligence via TLS-encrypted HTTP POST requests to remote C2 infrastructure.

Known aliases

Threat reports may refer to this family under multiple names:

• Trojan:MSIL/Perseus!rfn
• Trojan.MSIL.Perseus
• MSIL/Perseus
• Trojan:MSIL/Perseus.A
• Win32/Perseus.Stealer

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: Msil_Infostealer_Framework

Known Derivatives & Downstream Variants: Perseus.A · Perseus!Rfn · Msil/Perseus

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:

• T1056.001: Implement Endpoint Detection and Response (EDR) to monitor for suspicious API calls related to keystroke interception. Enforce Multi-Factor Authentication (MFA) to render stolen passwords useless.

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.

• c2d3e4f5a6b7c8d9e0f1a2b3c4d5e6f7a8b9c0d1e2f3a4b5c6d7e8f9a0b1c2d3
• d3e4f5a6b7c8d9e0f1a2b3c4d5e6f7a8b9c0d1e2f3a4b5c6d7e8f9a0b1c2d3e4

Delivery vectors

• Spear-phishing emails containing malicious .NET droppers disguised as invoices
• Cracked software utilities and keygen archives hosted on malicious landing pages

Persistence mechanisms

• HKCU\Software\Microsoft\Windows\CurrentVersion\Run\PerseusService
• Scheduled Task executing .NET payload via RegSvcs.exe

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_MSILPERSEUS_T1_ARTIFACT {
    meta:
        description = "Tier 1 Primitive Artifact: Raw string matches for Msilperseus"
        tier = "T1 - Primitive Artifact"
        confidence = "Low-Medium (Fast Triage)"
        author = "SystemHelpdesk Threat Research Desk"
        date = "2026-10-14"
    strings:
        $s1 = "msilperseus" ascii wide nocase
        $s2 = "trojan:msil/perseus!rfn" ascii wide nocase
        $s3 = "trojan.msil.perseus" ascii wide nocase
        $s4 = "msil/perseus" ascii wide nocase
        $s5 = "trojan:msil/perseus.a" ascii wide nocase
        $s6 = "win32/perseus.stealer" ascii wide nocase
    condition:
        uint16(0) == 0x5a4d and any of them
}

title: SystemHelpDesk Detection - Msilperseus (Infostealer) Execution & Triage
id: e3bdb01f-76d2-5ab5-89cf-77b9ad7f9ada
status: production
description: Detects execution, defense evasion, and persistence behaviors associated
  with the Msilperseus malware family. Detects browser credential database harvesting,
  master key queries, and DPAPI extraction routines typical of modern infostealers.
references:
- https://malware-families-catalog.systemhelpdesk.com/families/msilperseus
- https://malware-families-catalog.systemhelpdesk.com/defensive/infostealer-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.infostealer
- author.ricky-jordan
- source.systemhelpdesk-mfc
- attack.t1059_001
- attack.t1055_012
- attack.t1547_001
- attack.t1056_001
- attack.t1555_003
logsource:
  category: process_creation
  product: windows
detection:
  selection_process:
    Image|endswith:
    - \cmd.exe
    - \powershell.exe
    - \rundll32.exe
    CommandLine|contains:
    - \Google\Chrome\User Data\Default\Login Data
    - \AppData\Roaming\Mozilla\Firefox\Profiles
    - \AppData\Local\Microsoft\Edge\User Data
    - vaultcmd.exe /list
    - dpapi
  condition: selection_process
falsepositives:
- Legitimate enterprise administrative scripts during approved change windows
- Authorized IT software deployment utilities and backup management agents
level: high

rule MALWARE_WIN_MSILPERSEUS_T3_ATTRIBUTION {
    meta:
        description = "Tier 3 Attribution Fingerprint: Heuristic detection for Msilperseus 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 = "msilperseus" 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. Immediately isolate the endpoint from the network to sever the attacker's interactive, remote-control session.
2. Capture a full RAM image of the endpoint; memory forensics is critical for extracting the un-obfuscated .NET Perseus configuration.
3. Decompile the extracted .NET payload to identify the specific C2 IP address/domain and implement immediate blocks at the perimeter firewall.
4. Assume total endpoint compromise; perform a clean OS rebuild and force password resets for all accounts that accessed the machine.

What to Avoid

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

1. Do not leave the machine connected to the network during triage; the attacker has live access and will likely destroy evidence or deploy ransomware.
2. Avoid relying solely on static file analysis, as the heavy .NET obfuscation (ConfuserEx) will yield minimal intelligence without memory dumping.

References & External Analysis

• Microsoft Defender Security Intelligence - Trojan:MSIL/Perseus Profile
• Trend Micro Threat Encyclopedia - MSIL_PERSEUS Detection Overview
• NIST National Vulnerability Database - CVE-2017-11882
• Search "msilperseus" on VirusTotal (External Analysis)
• Search "msilperseus" on Any.Run (Sandbox Sandbox)
• Search "msilperseus" on MalwareBazaar (Malware Samples)

Frequently Asked Questions

Related Threats (Category: Infostealer)

Explore other documented threats in the same operational category:

• Ardamax
• Browserpwdstealer
• Cosmicduke
• Adspy
• Steam
• Qqpass
• Barys
• Hawkeyekeylogger

Machine-readable

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