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Text File | 1997-03-04 | 12.6 KB | 274 lines

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.