Virus Name: CLS-853
V Status: Rare
Discovery: December, 1992
Symptoms: .COM & .EXE growth; decrease in total system & available free
memory; unexpected screen clears
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,
Removal Instructions: Delete infected files
The CLS-853 virus was submitted in December, 1992. CLS-853 is a
memory resident infector of .COM and .EXE programs, including
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.