Modified by: Sabrina Lei Sioting


Virus:Win32/Chir.B@mm (Microsoft), Email-Worm.Win32.Runouce.b (Kaspersky), W32.Chir.B@mm (Symantec), W32/Chir.b@MM (McAfee), W32/Chir-B (Sophos), Virus:W32/Runouce.gen!A (Fsecure), Win32.chir.b (Sunbelt), Win32.Runouce.B@mm (Bitdefender), W32/Chir.B@mm (Fortinet), Email-Worm.Win32.Runouce (Ikarus), W32/Chir.B (Panda)


Windows 2000, Windows XP, Windows Server 2003


  • Threat Type: File infector

  • Destructiveness: No

  • Encrypted:

  • In the wild: Yes


Infection Channel: Dropped by other malware, Propagates via email

This file infector arrives as attachment to mass-mailed email messages. It may be dropped by other malware.

It appends its codes to target files. It infects certain file types by inserting code in the said files.

It gathers target email addresses from the Windows Address Book (WAB).


File Size: 10,748 bytes
File Type: EXE
Memory Resident: Yes
Initial Samples Received Date: 31 Jan 2011
Payload: Drops files

Arrival Details

This file infector arrives as attachment to mass-mailed email messages.

It may be dropped by the following malware:



This file infector drops the following copies of itself into the affected system:

  • %System%\runouce.exe

(Note: %System% is the Windows system folder, where it usually is C:\Windows\System32 on all Windows operating system versions.)

Autostart Technique

This file infector adds the following registry entries to enable its automatic execution at every system startup:

Runonce = "%System%\runouce.exe"

File Infection

This file infector appends its codes to target files.

It infects files with the following file extensions by inserting code in the said files:

  • .EXE
  • .SCR
  • .HTM
  • .HTML


This file infector gathers target email addresses from the Windows Address Book (WAB).

It exploits an unknown vulnerability in the following software so that the attachment automatically executes when the email message is read or previewed:

  • MS01-020 - Incorrect MIME Header Can Cause IE to Execute E-mail Attachment


Infected .EXE or .SCR files are detected as PE_CHIR.B.

To propagate, this file infector uses it own Simple Mail Transfer Protocol (SMTP) engine to mass-mail messages with a copy of itself as an attachment. The email message contains the following details:

TO: {Receipients Email Address}
FROM: imissyou@{BLOCKED}
SUBJECT: {hostname} is comming!
Attachment: pp.exe

It uses the SMTP server {BLOCKEDl}

This file infector drops the following file on the folder location of the infected .HTM or .HTML file:

  • readme.eml - detected as HTML_IFRAME.DY

It infects .HTM or .HTML by inserting a code that automatically executes the dropped README.EML when accessed. The infected .HTM or .HTML is detected as JS_NIMDA.A-1.


Minimum Scan Engine: 9.200
VSAPI OPR PATTERN File: 7.805.00
VSAPI OPR PATTERN Date: 01 Feb 2011

Step 1

Before doing any scans, Windows XP, Windows Vista, and Windows 7 users must disable System Restore to allow full scanning of their computers.

Step 2

Remove the malware/grayware file dropped/downloaded by PE_CHIR.B-O. (Note: Please skip this step if the threat(s) listed below have already been removed.)

Step 3

Restart in Safe Mode

[ Learn More ]

Step 4

Delete this registry value

[ Learn More ]

Important: Editing the Windows Registry incorrectly can lead to irreversible system malfunction. Please do this step only if you know how or you can ask assistance from your system administrator. Else, check this Microsoft article first before modifying your computer's registry.

  • In HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run
    • Runonce = "%System%\runouce.exe"

Step 5

Restart in normal mode and scan your computer with your Trend Micro product for files detected as PE_CHIR.B-O. If the detected files have already been cleaned, deleted, or quarantined by your Trend Micro product, no further step is required. You may opt to simply delete the quarantined files. Please check this Knowledge Base page for more information.

Step 6

Download and apply this security patch Refrain from using these products until the appropriate patches have been installed. Trend Micro advises users to download critical patches upon release by vendors.

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