Sober.I

Threat description

Details

CATEGORYMalware
TYPEWorm

Summary

F-secure has raised alert level of this virus to Radar LEVEL 2:

Email Worm Sober.I is spreading, mostly in Europe. It sends highly variable German and English emails with an attachment. The virus drops several files to infected systems, including spool32dir.exe.

Removal

Automatic action

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

More scanning & removal options

More information on the scanning and removal options available in your F-Secure product can be found in the Help Center.

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

Contact Support

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Technical Details

The worm is written in Visual Basic. The worm's file is a UPX packed PE executable with length of 56 KiBytes.

Installation to system

When the worm's file is started it shows the following messagebox:

Then the worm installs itself to system. It makes two copies itself to Windows System folder with a semi-randomly generated name and EXE extension. The following text strings are used to generate the file names of the worm's executable:

sys
host
dir
expoler
win
run
log
32
disc
crypt
data
diag
spool
service
smss32
 

After that the worm creates startup keys for its files in Windows Registry. The key names are also semi-randomly generated from the above given list. The following keys are created:

[HKCU\Software\Microsoft\Windows\CurrentVersion\Run]  "" = "%WinSysDir%\.exe"  
[HKCU\Software\Microsoft\Windows\CurrentVersion\Run]  "" = "%WinSysDir%\.exe %srun%"  
[HKLM\Software\Microsoft\Windows\CurrentVersion\Run]  "" = "%WinSysDir%\.exe"  
[HKLM\Software\Microsoft\Windows\CurrentVersion\Run]  "" = "%WinSysDir%\.exe %srun%"   

During its installation cycle the worm creates the following files in Windows System folder:

[HKCU\Software\Microsoft\Windows\CurrentVersion\Run]
"" = "%WinSysDir%\.exe"

[HKCU\Software\Microsoft\Windows\CurrentVersion\Run]
"" = "%WinSysDir%\.exe %srun%"

[HKLM\Software\Microsoft\Windows\CurrentVersion\Run]
"" = "%WinSysDir%\.exe"

[HKLM\Software\Microsoft\Windows\CurrentVersion\Run]
"" = "%WinSysDir%\.exe %srun%"
 

These files have zero length and they are used to disable previous variants of Sober if they are installed on an infected computer.

Additionally the worm creates the following files:

[HKCU\Software\Microsoft\Windows\CurrentVersion\Run]
"" = "%WinSysDir%\.exe"

[HKCU\Software\Microsoft\Windows\CurrentVersion\Run]
"" = "%WinSysDir%\.exe %srun%"

[HKLM\Software\Microsoft\Windows\CurrentVersion\Run]
"" = "%WinSysDir%\.exe"

[HKLM\Software\Microsoft\Windows\CurrentVersion\Run]
"" = "%WinSysDir%\.exe %srun%"
 

These files are MIME-encoded copies of the worm's executable file and a ZIP archive with the worm's file. These files will be used for spreading of the worm in e-mail messages.

Spreading in e-mails

The worm sends e-mail messages with English and German texts and its file attached. The attached file can be an executable or a ZIP archive containing the worm's executable. The worm composes several different types of messages and the content of these messages is variable. Here's an example of a English message sent by the worm:

Before spreading the worm scans files with certain extensions on all hard disks to harvest e-mail addresses. Files with the following extensions are scanned:

[HKCU\Software\Microsoft\Windows\CurrentVersion\Run]
"" = "%WinSysDir%\.exe"

[HKCU\Software\Microsoft\Windows\CurrentVersion\Run]
"" = "%WinSysDir%\.exe %srun%"

[HKLM\Software\Microsoft\Windows\CurrentVersion\Run]
"" = "%WinSysDir%\.exe"

[HKLM\Software\Microsoft\Windows\CurrentVersion\Run]
"" = "%WinSysDir%\.exe %srun%"
 

The found e-mail addresses and user names are saved in these 2 files that the worm created in Windows System folder:

[HKCU\Software\Microsoft\Windows\CurrentVersion\Run]
"" = "%WinSysDir%\.exe"

[HKCU\Software\Microsoft\Windows\CurrentVersion\Run]
"" = "%WinSysDir%\.exe %srun%"

[HKLM\Software\Microsoft\Windows\CurrentVersion\Run]
"" = "%WinSysDir%\.exe"

[HKLM\Software\Microsoft\Windows\CurrentVersion\Run]
"" = "%WinSysDir%\.exe %srun%"
 

When the worm is active in memory it blocks access to these files as well as to its MIME-encoded files and its executable file.

The worm ignores e-mail addresses that contain any of the following substrings:

[HKCU\Software\Microsoft\Windows\CurrentVersion\Run]
"" = "%WinSysDir%\.exe"

[HKCU\Software\Microsoft\Windows\CurrentVersion\Run]
"" = "%WinSysDir%\.exe %srun%"

[HKLM\Software\Microsoft\Windows\CurrentVersion\Run]
"" = "%WinSysDir%\.exe"

[HKLM\Software\Microsoft\Windows\CurrentVersion\Run]
"" = "%WinSysDir%\.exe %srun%"
 

If the worm sends infected messages to domains with suffixes '.de', '.ch', '.at', '.li' or to 'gmx.' domain, it composes messages in German, otherwise English messages are composed.

The worm can compose English messages from the following text string arrays:

Subject (one of the following):

[HKCU\Software\Microsoft\Windows\CurrentVersion\Run]
"" = "%WinSysDir%\.exe"

[HKCU\Software\Microsoft\Windows\CurrentVersion\Run]
"" = "%WinSysDir%\.exe %srun%"

[HKLM\Software\Microsoft\Windows\CurrentVersion\Run]
"" = "%WinSysDir%\.exe"

[HKLM\Software\Microsoft\Windows\CurrentVersion\Run]
"" = "%WinSysDir%\.exe %srun%"
 

Message body (one of the following):

[HKCU\Software\Microsoft\Windows\CurrentVersion\Run]
"" = "%WinSysDir%\.exe"

[HKCU\Software\Microsoft\Windows\CurrentVersion\Run]
"" = "%WinSysDir%\.exe %srun%"

[HKLM\Software\Microsoft\Windows\CurrentVersion\Run]
"" = "%WinSysDir%\.exe"

[HKLM\Software\Microsoft\Windows\CurrentVersion\Run]
"" = "%WinSysDir%\.exe %srun%"
 

Attachment name (one of the following):

[HKCU\Software\Microsoft\Windows\CurrentVersion\Run]
"" = "%WinSysDir%\.exe"

[HKCU\Software\Microsoft\Windows\CurrentVersion\Run]
"" = "%WinSysDir%\.exe %srun%"

[HKLM\Software\Microsoft\Windows\CurrentVersion\Run]
"" = "%WinSysDir%\.exe"

[HKLM\Software\Microsoft\Windows\CurrentVersion\Run]
"" = "%WinSysDir%\.exe %srun%"
 

Sober's attachment name can contain a random number and can have one of the following extensions:

[HKCU\Software\Microsoft\Windows\CurrentVersion\Run]
"" = "%WinSysDir%\.exe"

[HKCU\Software\Microsoft\Windows\CurrentVersion\Run]
"" = "%WinSysDir%\.exe %srun%"

[HKLM\Software\Microsoft\Windows\CurrentVersion\Run]
"" = "%WinSysDir%\.exe"

[HKLM\Software\Microsoft\Windows\CurrentVersion\Run]
"" = "%WinSysDir%\.exe %srun%"
 

Also the attachment can be sent in a ZIP archive. In this case the worm's file inside the archive will have double extension. The first extension is selected from the following list:

[HKCU\Software\Microsoft\Windows\CurrentVersion\Run]
"" = "%WinSysDir%\.exe"

[HKCU\Software\Microsoft\Windows\CurrentVersion\Run]
"" = "%WinSysDir%\.exe %srun%"

[HKLM\Software\Microsoft\Windows\CurrentVersion\Run]
"" = "%WinSysDir%\.exe"

[HKLM\Software\Microsoft\Windows\CurrentVersion\Run]
"" = "%WinSysDir%\.exe %srun%"
 

The subject of an infected e-mail can have 'FwD:' or 'Re:' string.

Sober.I can place a fake anti-virus scanner report in the message body trying to persuade a recipient that the message was scanned by an on-line scanner and no infection was found.

Detection

This Sober variant is detected with FSAV updates:

Detection Type: PC

Database: 2004-11-19_01

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