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Backdoors! +--------+ Qark/VLAD The information in this article concerns the backdoors in MS-DOS and BIOS that can be used and abused by a virus for it's own ends! Most of them concern the Int21h DOS services interrupt and Int13h Disk services. Int40h - The floppy disk handler gets relocated to Int40h by the Hard disk BIOS and is called by Int 13h. Int40h can be hooked and used to infect floppy disks. Be cautious when using this because if no harddisk is present it isn't set. Int30h - This is not infact an interrupt at all. It is a far jump to the original interrupt 21h handler that is stored at the address of int30h. It was originally used for the CP/M emulation in earlier versions of DOS and remains there today. If you look at the PSP information, at PSP: is a 'call to the DOS function dispatcher'. It actually calls the int30h we are talking about (but due to a microsoft stuff up it misses by two bytes). There are two ways of using this backdoor, the first way could be for a really hard method for infection or a good destructive payload. You can use it directly by calling it in an unusual fashion. This function could be messed up by some programs but I have yet to see it not work anywhere. Thanx go out to John Switzer for supplying me for this information although I'm sure he wouldn't appreciate it's use! :) Method One: You can only use DOS functions AH=0 to 24h with this and any functions that require AL can't be used. Int21h Proc Near ;Call this from your code with the same parameters ; as the real DOS int 21h function. ;Truly weird I'm sure you'll agree! mov cl,ah ;It uses CL. mov ax,offset return_addr ;Stack is backwards push ax push cs pushf ;Flags are last!! db 0eah ;JMP FAR PTR dw 0c0h ;30h * 4 dw 0 ;Interrupt table. return_addr: ret ;Back to user. Int21h EndP Method Two: This is different in that it uses the segment:offset address of the Int30h to get the original 'proper' Int21h that we are all used to. This method is used by the writers of the MG virus (who also wrote creeping death, very talented and good researchers!) Anyway you can work that out yourself, thats why it's called research! Int2fh - When DOS gets loaded it hooks int13h and saves the original ah=13h addresses for its own use. When this function is called it returns two addresses where one is slightly closer to the original int13h than the other, but I'm not too sure which is the closer of the two (they are often equal). If you play with this yourself look it up in Ralf Brown's, you can probably point the DOS calls to your virus if you do it right. To grab the original int13h without messing up DOS: mov ah,13h int 2fh ;Get the int13h's push es ;Save them push ds push dx push bx int 2fh ;Put them back to what they were. pop bx ;Now we've got our handlers. pop dx pop ds pop es ;From here you can either choose to use ES:BX or DS:DX ;as your int13h. Seg70h - Segment 70h is used by DOS. All DOS disk access passes through it at sometime. All you have to do is scan through it for the bytes of the different calls. This method was first used by the Creeping Death virus and is used in the 1984 (listed as 'ignorant' by CARO) and Daemon viruses. I'd suggest running through this with a debugger and having a look to work out what's going on. DOS has been using 70:B4 to store the original Int 13h since DOS 3.3. mov ax,70h mov ds,ax mov si,2 first_backdoor: or si,si jz wherever dec si ;SI-1 lodsw ;DS:[SI] to AX SI+2 cmp ax,1effh ;FF1E = CALL FAR PTR [xxxx] jnz first_backdoor cmp word ptr [si],0b4h ;This is just there :) jnz first_backdoor jmp set_fake_int13 ;We've found it! set_fake_int13h: mov si,[si] ;SI=Where the address is ;stored. ;save the int13h into the virus mov cs:orig_store,word ptr [si] mov cs:orig_store+2,word ptr [si+2] ;point it to our virus mov word ptr [si],offset our_int13 mov word ptr [si+2],cs ;ret or whatever... Int2fh - Have a look at this interrupt in Ralf Browns (a must for every virus programmer) it can do ALL the interrupt 21h functions! The only problem is working out the DOS stacks and so on. It is handy for bypassing AV monitors, but it is much too huge to go into in any detail. BIOS - Within BIOS lurk a number of stationary entry points to entry interrupts. There are a few problems with these, as alot points of BIOSes are incompatible and QEMM won't work with them but they can be useful because there isn't ANYTHING that can be done to stop it. Here are a list of addresses that are guaranteed not to work half the time but have a look anyway. F000:EC59 Floppy disk int 13h F000:F859 Int 15h, sometimes useful Int2ah - This is called by Int 21h on every file related function. By ah=82h modifying the stack or certain registers you can change the function that was called to whatever you want. DOS stores the function multiplied by two in BL (eg Int 21h AH=40h will be BL=80h when the int 2ah is called.). If you change this BL to another function it should fool most AV monitors. This may only work for some versions of DOS. Int21h - If you call this service you can do any DOS function. Have a ax=5d00h look! All you have to do is set your registers up in a table. It should be easy to write a basic simulated int21h using this. Anymore ? Not that I can think of! If you know any... tell me!!