CLS-853 Virus

Virus Name: CLS-853 Aliases: CLS V Status: Rare Discovery: December, 1992 Symptoms: .COM & .EXE growth; decrease in total system & available free memory; unexpected screen clears Origin: Unknown Eff Length: 853 Bytes Type Code: PRhAK - Parasitic Resident .COM & .EXE Infector Detection Method: IBMAV, Sweep, AVTK, F-Prot, ViruScan, NAV, NAVDX, VAlert, PCScan, ChAV, Sweep/N, NShld, Innoc, NProt, AVTK/N, NAV/N, IBMAV/N, LProt Removal Instructions: Delete infected files General Comments: The CLS-853 virus was submitted in December, 1992. CLS-853 is a memory resident infector of .COM and .EXE programs, including COMMAND.COM. When the first CLS-853 infected program is executed, the CLS-853 virus will install itself memory resident at the top of system memory but below the 640K DOS boundary. It will have hooked interrupts 08, 13, and 21. Interrupt 12's return will not be moved. Total system and available free memory, as indicated by the DOS CHKDSK program, will have decreased by 1,120 bytes. Once the CLS-853 virus is memory resident, it will infect .COM and .EXE programs when they are executed. Infected programs will have a file length increase of 853 bytes with the virus being located at the end of the file. The program's date and time in the DOS disk directory listing will not be altered. No text strings are visible within the viral code. Systems infected with the CLS-853 virus may experience the system display being cleared unexpectedly when the virus is memory resident. The virus' name, "CLS", refers to this symptom of the virus.