Semtex Virus

Virus Name: Semtex Aliases: V Status: Rare Discovered: September, 1991 Symptoms: .COM file growth; decrease in total system and available free memory; screen display Origin: Czechoslovakia Eff Length: 1,000 Bytes Type Code: PRhCK - Parasitic Resident .COM Infector Detection Method: Sweep, AVTK, F-Prot, NAVDX, VAlert, IBMAV, NAV, ViruScan, PCScan, ChAV, LProt, Sweep/N, NShld, Innoc, NProt, AVTK/N, NAV/N, IBMAV/N Removal Instructions: Delete infected files General 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: Semtex-B: A 1,000 byte variant of the Semtex virus, this variant's size in memory is 4,080 bytes, hooking interrupt 21. It infects .COM programs when they are executed or opened for any reason. Infected programs will have a file length increase of 1,000 bytes with the virus being located at the end of the file. The file length increase is not hidden when the virus is memory resident. The program's date and time in the DOS disk directory listing will not be altered. The following text string is visible within the viral code in all Semtex-B infected programs: "S E M T E X by Dusan Toman, CZECHOSLOVAKIA *** Have a nice day ***" System hangs may occur when infected programs are executed. Origin: Czechoslovakia December, 1992. Semtex-C: A 619 byte variant of the Semtex virus, this variant's size in memory is 4,080 bytes, hooking interrupts 08 and 21. It infects .COM programs when they are executed or opened for any reason. Infected programs will have a file length increase of 619 bytes with the virus being located at the end of the file. The file length increase is not hidden when the virus is memory resident. The program's date and time in the DOS disk directory listing will not be altered. The following text string is visible within the viral code in all Semtex-C infected programs: "S E M T E X by Dusan Toman, CZECHOSLOVAKIA (7)213-040 or (804)213-23" System hangs may occur when infected programs are executed. Origin: Czechoslovakia December, 1992. Semtex-D: A 1,000 byte variant of the Semtex virus, this variant's size in memory is 4,080 bytes, hooking interrupts 08 and 21. It infects .COM programs when they are executed or opened for any reason. Infected programs will have a file length increase of 1,000 bytes with the virus being located at the end of the file. The file length increase is not hidden when the virus is memory resident. The program's date and time in the DOS disk directory listing will not be altered. The following text string is visible within the viral code in all Semtex-D infected programs: "S E M T E X by Dusan Toman, CZECHOSLOVAKIA (7)213-040 or (804)213-23" System hangs may occur when infected programs are executed. Origin: Czechoslovakia December, 1992.