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Detect Date 11/20/2002
Class Virus
Platform Win9x

Technical Details

This is a Windows95/98 specific parasitic virus infecting Windows PE files

(Portable Executable), which is about 1Kbyte in length. Also known as Chernobyl. This virus was found

“in-the-wild” in Taiwan in June 1998, being released by its virus author who was studying at a local university at the time. The virus (accidentally?) was posted at a local Internet conference that released the virus out of Taiwan. Within a week, the virus was found in Austria,

Australia, Israel, United Kingdom, and was also reported from several other countries (Switzerland, Sweden, USA, Russia, Chile, etc.).

In about a month, the infected files were accidentally put on several Web sites in the USA (game software distribution sites) that caused a global virus epidemic. In about a year after the virus’ appearance on March 26th 1999, the “time-bomb” in virus code caused a computer catastrophe when about half of a million computers were damaged because of virus infection: all of them lost data on the hard drive, and many of them also had the motherboard BIOS destroyed (plus hard-drive data damaged). This incident was significant as there had been no such global and terrible computer incidents known to date at this time.

Because the virus “bomb” day falls on the day of the Chernobyl catastrophe that shocked the world on 26th April 1986, the virus, already known as “CIH” got

its second name – “Chernobyl”.

Despite this, the virus author did not link his “bomb” with Chernobyl

(maybe he had never even heard of Chernobyl). It seems the “bomb” day was selected for another reason. The first virus version (that fortunately hasn’t left Taiwan) was released on April 26 1998, so the virus celebrated its “birthday” on April 26 1999.

How the virus works

The virus installs itself into the Windows memory, hooks file access calls and infects EXE files that are opened. Depending on the system date (see below), the virus runs its trigger routine. The virus has bugs and in some cases, halts the computer when an infected application is run.

The virus’ trigger routine operates with Flash BIOS ports and tries to overwrite Flash memory with “garbage”. This is possible only if the motherboard and chipset allow for the writing to Flash memory. Usually writing to Flash memory can be disabled by a DIP switch, however, this depends on the motherboard design. Unfortunately, there are modern motherboards that cannot be protected by a DIP switch – also, some of them do not pay attention to the switch position and this protection has no effect at all. Some other motherboard designs provide written protection that can be disabled/overridden by software.

The trigger routine then overwrites data on all installed hard drives. The virus uses direct disk write calls to achieve this and bypasses standard BIOS virus protection while overwriting the MBR and boot sectors.

There are three “original” virus versions known, which are very closely related and only differ in a few parts of their code. They have different lengths, texts inside the virus code and trigger date:

Length Text Trigger date Found In-The-Wild

1003 CIH 1.2 TTIT on April 26th YES

1010 CIH 1.3 TTIT on April 26th NO

1019 CIH 1.4 TATUNG on 26th of any month YES – many reports

Technical details

While infecting a file, the virus looks for “caves” in the file body. These caves are a result of the PE file structure: all file sections are aligned by a value that is defined in the PE file header, and there are unused blocks of file data between the end of the previous section and the next one. The virus looks for these caves and writes its code into them. The virus then increases the size of sections by the necessary values. As a result, the file length is not increased while infecting.

If there is a cave of enough size, the virus saves its code in one section.

Otherwise, it splits its code into several parts and saves them to the end of several sections. As a result, the virus code may be found as set of pieces, not as a single block in infected files.

The virus also looks for a cave in the PE header. If there is an unused block not less than 184 bytes in length, the virus writes its startup routine to there. The virus then patches the entry address in the PE header with a value that points to the startup routine placed in the header. This is the same trick that was used in the “Win95.Murkry” virus: address of program entry points not to some file section, but to the file header – out of a loadable file data. Despite this, infected programs are run with no problems – Windows does not pay any attention to such “strange” files, loads the file header into the memory, then file sections, and then passes control to the virus startup routine in the PE header.

When the virus startup routine takes control, it allocates a block of memory by using the PageAllocate VMM call, copies itself to there, locates other blocks of virus code and also copies them to the allocated block of memory. The virus then hooks the system IFS API and returns control to the host program.

The most interesting thing in this part of the virus code is that the virus uses quite complex tricks to jump from Ring3 to Ring0: when the virus jumps to newly allocated memory, its code is then executed as Ring0 routine, and the virus is able to hook the file system calls (it is not possible in

Ring3, where all users applications are run).

The IFS API virus handler intercepts only one function – file opening. When

PE .EXE files are opened, the virus infects them, provided there are caves of enough size. After infection, the virus checks the file date and calls the trigger routine (see above).

While running its trigger routine, the virus uses direct access to Flash

BIOS ports and VxD direct disk access calls (IOS_SendCommand).

Other known virus versions

The original virus author released not only virus code in affected EXE files to the wild, but a virus source (assembler) code as well. These source code were patched, recompiled, and new virus versions were found because of this. Most of these versions are buggy and not able to replicate, but others can do. All of them are very closed to original viruses, but there are a few differences. The main difference is that the “bomb” date has changed, and new variants of the virus either erase data and Flash BIOS on other days, or this routine is never called.

There are also “original” versions of the virus patched so that they have other “bomb” days. The reason for this is actually humorous: the virus checks the trigger date by comparing the current day and month number with two constants (two bytes). By patching these constants, it is possible to select any day the virus will destroy computers.

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