McAfee > Theat Center > Virus Detail Page

Overview

This is a virus detection. Viruses are programs that self-replicate recursively, meaning that infected systems spread the virus to other systems, which then propagate the virus further. While many viruses contain a destructive payload, it's quite common for viruses to do nothing more than spread from one system to another.

Characteristics

Semtex is a memory resident, file infecting virus. It infects .COM files, including COMMAND.COM.

Upon infection, Semtex becomes memory resident at the top of system memory but below the 640K DOS boundary. Interrupt 12's return is not moved. Semtex hooks interrupts 8 and 21. Also at this time, Semtex infects COMMAND.COM.

Once Semtex is memory resident, it infects .COM files when they are executed or opened. In the case of copying files from one directory to another, both the source and target copy of the .COM file are infected.

Additional Comments:
The Semtex virus was discovered in September, 1991. It is originally from Czechoslovakia. Semtex is a memory resident infector of .COM files, including COMMAND.COM. When the first Semtex infected program is executed on a system, Semtex will install itself memory resident at the top of system memory but below the 640K DOS boundary. Total system and available free memory, as indicated by the DOS CHKDSK program, will decrease by 4,080 bytes. Interrupt 12's return will not be moved. Semtex will hook interrupts 8 and 21. Also at this time, Semtex will infect COMMAND.COM. Once Semtex is memory resident, it will infect .COM programs when they are executed or opened for any reason. In the case of copying programs from one directory to another, both the source and target copy of the .COM program will be infected. Programs infected with Semtex will increase in size by 1,000 bytes. The virus will be located at the end of infected files. There will be no visible change to the file's date and time in the DOS disk directory listing. The following text string can be found within the viral code in infected programs: "S E M T E X by Dusan Toman, CZECHOSLOVAKIA (7)213-040 or (804)213-23" This string is not displayed by the virus. After Semtex has been memory resident for 60 minutes, it will produce a screen display. Each cursor position on the screen display will be changed to contain a colored background and an ASCII character. The effect is a multi-color flashing screen of very small rectangles. The user can return the screen to normal by pressing a key on the keyboard. Known variant(s) of Semtex are:

Symptoms

The following text string is found within the viral code in infected files:

"S E M T E X by Dusan Toman, CZECHOSLOVAKIA (7)213-040 or (804)213-23"

This string is not displayed by the virus.

After Semtex has been memory resident for 60 minutes, it produces a screen display. Each cursor position on the screen display is changed to contain a colored background and an ASCII character. The effect is a multi-color flashing screen of very small rectangles. The user can return the screen to normal by pressing a key on the keyboard.

Total system and available free memory decreases by 4,080 bytes. Files infected with Semtex increase in size by 1,000 bytes. The virus is located at the end of infected files. There are visible change to the file's date and time in the DOS disk directory listing.

Method of Infection

The only way to infect a computer with a file infecting virus is to execute an infected file on the computer. The infected file may come from a multitude of sources including: floppy diskettes, downloads through an online service, network, etc. Once the infected file is executed, the virus may activate.

Removal

All Users :
Script,Batch,Macro and non memory-resident:
Use current engine and DAT files for detection and removal.

PE,Trojan,Internet Worm and memory resident :
Use specified engine and DAT files for detection. To remove, boot to MS-DOS mode or use a boot diskette and use the command line scanner:

Additional Windows ME/XP removal considerations

Users should not trust file icons, particularly when receiving files from others via P2P clients, IRC, email or other mediums where users can share files.

AVERT Recommended Updates :

* Office2000 Updates

* Malformed Word Document Could Enable Macro to Run Automatically (Information/Patch )

* scriptlet.typelib/Eyedog vulnerability patch

* Outlook as an email attachment security update

* Exchange 5.5 post SP3 Information Store Patch 5.5.2652.42 - this patch corrects detection issues with GroupShield

For a list of attachments blocked by the Outlook patch and a general FAQ, visit this link .
Additionally, Network Administrators can configure this update using an available tool - visit this link for more information .

It is very common for macro viruses to disable options within Office applications for example in Word, the macro protection warning commonly is disabled. After cleaning macro viruses, ensure that your previously set options are again enabled.

Variants

Variants

• Semtex-B
• Semtex-C
• Semtex-D

All Information

Overview -

This is a virus detection. Viruses are programs that self-replicate recursively, meaning that infected systems spread the virus to other systems, which then propagate the virus further. While many viruses contain a destructive payload, it's quite common for viruses to do nothing more than spread from one system to another.

Characteristics

Characteristics -

Semtex is a memory resident, file infecting virus. It infects .COM files, including COMMAND.COM.

Upon infection, Semtex becomes memory resident at the top of system memory but below the 640K DOS boundary. Interrupt 12's return is not moved. Semtex hooks interrupts 8 and 21. Also at this time, Semtex infects COMMAND.COM.

Once Semtex is memory resident, it infects .COM files when they are executed or opened. In the case of copying files from one directory to another, both the source and target copy of the .COM file are infected.

Additional Comments:
The Semtex virus was discovered in September, 1991. It is originally from Czechoslovakia. Semtex is a memory resident infector of .COM files, including COMMAND.COM. When the first Semtex infected program is executed on a system, Semtex will install itself memory resident at the top of system memory but below the 640K DOS boundary. Total system and available free memory, as indicated by the DOS CHKDSK program, will decrease by 4,080 bytes. Interrupt 12's return will not be moved. Semtex will hook interrupts 8 and 21. Also at this time, Semtex will infect COMMAND.COM. Once Semtex is memory resident, it will infect .COM programs when they are executed or opened for any reason. In the case of copying programs from one directory to another, both the source and target copy of the .COM program will be infected. Programs infected with Semtex will increase in size by 1,000 bytes. The virus will be located at the end of infected files. There will be no visible change to the file's date and time in the DOS disk directory listing. The following text string can be found within the viral code in infected programs: "S E M T E X by Dusan Toman, CZECHOSLOVAKIA (7)213-040 or (804)213-23" This string is not displayed by the virus. After Semtex has been memory resident for 60 minutes, it will produce a screen display. Each cursor position on the screen display will be changed to contain a colored background and an ASCII character. The effect is a multi-color flashing screen of very small rectangles. The user can return the screen to normal by pressing a key on the keyboard. Known variant(s) of Semtex are:

Symptoms

Symptoms -

The following text string is found within the viral code in infected files:

"S E M T E X by Dusan Toman, CZECHOSLOVAKIA (7)213-040 or (804)213-23"

This string is not displayed by the virus.

After Semtex has been memory resident for 60 minutes, it produces a screen display. Each cursor position on the screen display is changed to contain a colored background and an ASCII character. The effect is a multi-color flashing screen of very small rectangles. The user can return the screen to normal by pressing a key on the keyboard.

Total system and available free memory decreases by 4,080 bytes. Files infected with Semtex increase in size by 1,000 bytes. The virus is located at the end of infected files. There are visible change to the file's date and time in the DOS disk directory listing.

Method of Infection

Method of Infection -

The only way to infect a computer with a file infecting virus is to execute an infected file on the computer. The infected file may come from a multitude of sources including: floppy diskettes, downloads through an online service, network, etc. Once the infected file is executed, the virus may activate.

Removal -

Removal -

All Users :
Script,Batch,Macro and non memory-resident:
Use current engine and DAT files for detection and removal.

PE,Trojan,Internet Worm and memory resident :
Use specified engine and DAT files for detection. To remove, boot to MS-DOS mode or use a boot diskette and use the command line scanner:

Additional Windows ME/XP removal considerations

Users should not trust file icons, particularly when receiving files from others via P2P clients, IRC, email or other mediums where users can share files.

AVERT Recommended Updates :

* Office2000 Updates

* Malformed Word Document Could Enable Macro to Run Automatically (Information/Patch )

* scriptlet.typelib/Eyedog vulnerability patch

* Outlook as an email attachment security update

* Exchange 5.5 post SP3 Information Store Patch 5.5.2652.42 - this patch corrects detection issues with GroupShield

For a list of attachments blocked by the Outlook patch and a general FAQ, visit this link .
Additionally, Network Administrators can configure this update using an available tool - visit this link for more information .

It is very common for macro viruses to disable options within Office applications for example in Word, the macro protection warning commonly is disabled. After cleaning macro viruses, ensure that your previously set options are again enabled.

Variants

Variants -

• Semtex-B
• Semtex-C
• Semtex-D