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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.


Based on the settings of your F-Secure security product, it will either move the file to the quarantine where it cannot spread or cause harm, or remove it.

A False Positive is when a file is incorrectly detected as harmful, usually because its code or behavior resembles known harmful programs. A False Positive will usually be fixed in a subsequent database update without any action needed on your part. If you wish, you may also:

  • Check for the latest database updates

    First check if your F-Secure security program is using the latest updates, then try scanning the file again.

  • Submit a sample

    After checking, if you still believe the file is incorrectly detected, you can submit a sample of it for re-analysis.

    Note: If the file was moved to quarantine, you need to collect the file from quarantine before you can submit it.

  • Exclude a file from further scanning

    If you are certain that the file is safe and want to continue using it, you can exclude it from further scanning by the F-Secure security product.

    Note: You need administrative rights to change the settings.

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.