Chir

Overview

Chir (extensively documented as Virus:Win32/Chir.B@mm, W32.Chir, and Runouce) is a historic, highly aggressive mass-mailing worm and polymorphic file infector first identified in 2002. Chir is infamous for combining two destructive propagation vectors: parasitic executable file infection and automated SMTP email mass-mailing. Upon execution on a Windows workstation, Chir drops a hidden copy of itself named runouce.exe into the Windows System32 directory and secures persistence by modifying the Registry Run keys (HKLM\Software\Microsoft\Windows\CurrentVersion\Run). The worm actively monitors this registry key in memory, immediately rewriting the autostart value if deleted by administrators. Chir scans all local fixed drives (from C: to Z:) and accessible network shares, searching for executable files (.exe, .scr) to append its polymorphic viral code without changing file size attributes. Simultaneously, it scrapes email addresses from the Windows Address Book, Outlook Express caches, and local HTML files, using an integrated direct-to-MX SMTP engine to blast infected email messages across corporate contacts.

Known aliases

Threat reports may refer to this family under multiple names:

• Virus:Win32/Chir.B@mm
• W32.Chir
• Runouce
• Worm:Win32/Chir
• W32.Chir.B@mm

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: Early Mass-Mailing Pe Worms · Classic Win32 File Infectors

Known Derivatives & Downstream Variants: Chir.A · Chir.B@Mm

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:

• T1059: Restrict execution of scripting languages such as PowerShell, VBScript, or Python to authorized administrators. Enforce Script Block Logging.
• 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):

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.

• 4c5d6e7f8a9b0c1d2e3f4a5b6c7d8e9f0a1b2c3d4e5f6a7b8c9d0e1f2a3b4c5d
• 5d6e7f8a9b0c1d2e3f4a5b6c7d8e9f0a1b2c3d4e5f6a7b8c9d0e1f2a3b4c5d6e

Delivery vectors

• Mass-mailed email attachments with weaponized HTML/MIME stubs
• Infection of executable files across shared network folders

Persistence mechanisms

• HKLM\Software\Microsoft\Windows\CurrentVersion\Run\Runonce
• C:\Windows\System32\runouce.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_CHIR_T1_ARTIFACT {
    meta:
        description = "Tier 1 Primitive Artifact: Raw string matches for Chir"
        tier = "T1 - Primitive Artifact"
        confidence = "Low-Medium (Fast Triage)"
        author = "SystemHelpdesk Threat Research Desk"
        date = "2026-10-14"
    strings:
        $s1 = "chir" ascii wide nocase
        $s2 = "virus:win32/chir.b@mm" ascii wide nocase
        $s3 = "w32.chir" ascii wide nocase
        $s4 = "runouce" ascii wide nocase
        $s5 = "worm:win32/chir" ascii wide nocase
        $s6 = "w32.chir.b@mm" ascii wide nocase
    condition:
        uint16(0) == 0x5a4d and any of them
}

title: SystemHelpDesk Detection - Chir (Worm) Execution & Triage
id: 14b159bc-c65d-5bb9-85b6-ae3629b9fca0
status: production
description: Detects execution, defense evasion, and persistence behaviors associated
  with the Chir 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/chir
- 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.worm
- author.ricky-jordan
- source.systemhelpdesk-mfc
- attack.t1566_001
- attack.t1114_001
- attack.t1547_001
- attack.t1059
- attack.t1105
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_CHIR_T3_ATTRIBUTION {
    meta:
        description = "Tier 3 Attribution Fingerprint: Heuristic detection for Chir 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 = "chir" 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 infected workstation from the network to prevent mass-mailing and lateral infection of shared network drives.
2. Block outbound SMTP (TCP port 25) traffic from all endpoints at the perimeter firewall to neutralize automated email dissemination.
3. Deploy an antivirus disinfectant capable of stripping polymorphic viral code from infected .exe and .scr files rather than deleting them.
4. Purge the dropped runouce.exe binary from the Windows System32 folder and remove corresponding autostart Registry Run entries.

What to Avoid

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

1. Do not blindly delete infected executables without attempting disinfection, as Chir infects legitimate operating system and utility binaries.
2. Avoid inserting removable storage media into infected hosts, as Chir automatically searches all mapped drives to infect executable files.

References & External Analysis

• Microsoft Defender Security Intelligence - Virus:Win32/Chir.B@mm
• F-Secure Threat Description - Worm:W32/Chir
• Search "chir" on VirusTotal (External Analysis)
• Search "chir" on Any.Run (Sandbox Sandbox)
• Search "chir" on MalwareBazaar (Malware Samples)

Frequently Asked Questions

Related Threats (Category: Worm)

Explore other documented threats in the same operational category:

• Skypespam
• Drolnux
• Ngrbot
• Beebone
• Lovgate
• Brambul
• Lolbot
• Ludbaruma

Machine-readable

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