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Category: Malware

Type: Worm

Aliases: Worm:W32/Mytob.A


Worm:W32/Mytob.A is a worm that has functionality similar to the MyDoom worm family functionality. This worm includes code to spread over a network by exploiting the known LSASS vulnerability.


Automatic action

Based on the settings of your F-Secure security product, it will either automatically delete, quarantine or rename the detected program or file, or ask you for a desired action.

Knowledge Base

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About the product

See the manual for your F-Secure product on the Help Center.

Contact Support

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Submit a sample

Submit a file or URL for further analysis.

Technical Details

In addition to propagating, Mytob.A is also able to function as an IRC bot.

The worm is a PE executable file 42512 bytes long, packed with FSG file compressor.


When run, the worm copies under %SYSTEM% directory using the name 'msnmsgr.exe' and creates a mutex named 'D66'.

It will then alters the registry entries to ensure that it gets started when a user logs on or the system is restarted:

  • [HKCU\Software\Microsoft\Windows\CurrentVersion\Run]
  • [HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Run]
  • [HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\RunServices]
  • [HKCU\SYSTEM\CurrentControlSet\Control\Lsa] "MSN" = "msnmsgr.exe"

The worm tries to connect to an IRC channel at a predefined address using TCP port 6667. An attacker who knows channel password can instruct the created bot to execute the following actions:

  • Request worm uptime
  • Request worm version
  • Shutdown worm
  • Download and execute files
  • Delete files
  • Update worm
Propagation (email)

The worm spreads by sending its infected attachment to email addresses found on an infected computer. email addresses are harvested from Windows Address Book (WAB) and from files with the following extensions:

  • htm
  • sht
  • php
  • asp
  • dbx
  • tbb
  • adb
  • wab
  • pl

The worm avoids sending emails to email addresses that contain any of the following substrings:

  • accoun
  • acketst
  • admin
  • anyone
  • arin.
  • be_loyal
  • berkeley
  • borlan
  • bugs
  • certific
  • contact
  • .edu
  • example
  • feste
  • fido
  • foo.
  • fsf.
  • gold-certs
  • google
  • .gov
  • gov.
  • help
  • hotmail
  • iana
  • ibm.com
  • icrosof
  • icrosoft
  • ietf
  • info
  • inpris
  • isc.o
  • isi.e
  • kernel
  • linux
  • listserv
  • math
  • .mil
  • mit.e
  • mozilla
  • msn.
  • mydomai
  • nobody
  • nodomai
  • noone
  • nothing
  • ntivi
  • page
  • panda
  • postmaster
  • privacy
  • rating
  • rfc-ed
  • ripe.
  • root
  • ruslis
  • samples
  • secur
  • sendmail
  • service
  • site
  • soft
  • somebody
  • someone
  • sopho
  • submit
  • support
  • syma
  • tanford.e
  • the.bat
  • unix
  • usenet
  • utgers.ed
  • webmaster
  • your

The email message is composed from randomly chosen subject line, body text and additional parts. The worm has a selection of attachment names that it uses for its attachment. The subject of infected emails is selected from the following variants:

  • Error
  • Status
  • Server Report
  • Mail Transaction Failed
  • Mail Delivery System
  • hello
  • hi

The attachment name is composed using the following predefined keywords:

  • body
  • message
  • test
  • data
  • file
  • text
  • doc

The extension for the filename can be one of the following:

  • bat
  • cmd
  • exe
  • scr
  • pif

For example:

  • body.scr
Propagation (Exploit)

The worm spreads to remote computers using LSASS vulnerability. It contacts remote computers on TCP port 445, exploits the vulnerability and copies its file to a remote system.


Mytob.B is a minor variant of Mytob.A that includes functionality from the MyDoom family of email worms and IRC-bots.