Threat description



The Bofra.B worm appeared on November 9th, 2004. This worm exploits an unpatched vulnerability in Internet Explorer's IFRAME handling. Unlike regular mass-mailing worms, Bofra.B does not send itself in the emails, only an HTTP link that points to the host that sent the infected email.

As a payload Bofra.B has an IRC-controlled backdoor that allows the creator to download and execute arbitrary programs on the compromised host.


Automatic action

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You may also refer to the Knowledge Base on the F-Secure Community site for more information.

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Technical Details

The worm's body is a Windows PE executable file compressed with the MEW executable compressor. The unpacked body is around 42 KiB and was most likely hand-coded in assembly.

System Infection

When the worm's file is run, it copies itself to Windows System Folder with a random name ending in '32.exe' and creates a startup key for this file in the Registry:

[HKCU\Software\Microsoft\Windows\CurrentVersion\Run]  "Reactor5" = "%SystemDir%\32.exe"   

%SystemDir% represents the Windows System folder name, for example C:\Windows\System32 on Windows XP systems.

Email Propagation

To gather email addresses Bofra.B searches the Windows Address Book, files in Temporary Internet File and other files on the hard disk that have the following strings in their name:

"Reactor5" = "%SystemDir%\32.exe"

Using its own SMTP engine Bofra.B sends emails to the collected addresses. Sender of the mails is spoofed and the content is randomly chosen from the following components:

Email subjects:

"Reactor5" = "%SystemDir%\32.exe"

Email bodies contain an HTML-formatted text with the link:

Congratulations! PayPal has successfully charged $175 to your credit card.  Your order tracking number is A866DEC0, and your item will be shipped  within three business days  To see details please click this   DO NOT REPLY TO THIS MESSAGE VIA EMAIL! This email is being sent by  an automated message system and the reply will not be received.   


Hi! I am looking for new friends. I am from Miami, FL.  You can see my  with my last webcam photos!   


Hi! I am looking for new friends.  My name is Jane, I am from Miami, FL.  See my   with my weblog and last webcam photos!  See you!   

The email does not have any attachments. The worm only sends the link which points to the infected host. The format of the link is

h**p://<infected host ip>:port/<file_to_dowload>

Bofra.B, running on the infected host, has a stripped-down web servers listening on TCP ports starting from 1638 (0x666). The only purpose of these is to serve the potential targets with the HTML page that contains the exploit as well as the worm executable that the exploit will download.

The way this propagation technique works in explained in our weblog:

The emails sent by Bofra.B contain a fake virus scanner header (X-AntiVirus:) that might get one of the following values:

"Reactor5" = "%SystemDir%\32.exe"

The worm avoids posting to e-mail addresses that contain certain strings, among them:

"Reactor5" = "%SystemDir%\32.exe"

As a payload Bofra.B has an IRC-controlled backdoor that allows the creator to download and execute arbitrary programs on the compromised host.


Detection for Bofra.B was published on November 9th, 2004 in the following F-Secure Anti-Virus updates:

Detection Type: PC

Database: 2004-11-09_01

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