Threat Description





A standalone malicious program which uses computer or network resources to make complete copies of itself. May include code or other malware to damage both the system and the network.


Automatic action

Once detected, the F-Secure security product will automatically disinfect the suspect file by either deleting it or renaming it.


You may wish to refer to the Support Community for further assistance. You may also refer to General Removal Instructions for a general guide on alternative disinfection actions.

Technical Details

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.

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 (E-mail)

The worm spreads by sending its infected attachment to e-mail addresses found on an infected computer. E-mail 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 e-mails to e-mail 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
  • 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 e-mail 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 e-mails 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 e-mail worms and IRC-bots.


Suspect a file or URL was wrongly detected? Submit a sample to our Labs for analysis

Submit Now

Give And Get Advice

Give advice. Get advice. Share the knowledge on our free discussion forum.

Learn More