Win95.CIH

This virus is also known as: Chernobyl, PE_CIH, W32.Spacefiller, WIN95/CIH, CIH, and W32.CIH

This is a Windows95 specific parasitic PE files (Portable Executable) infector about 1Kbyte of length. This virus was found "in-the-wild"

in Taiwan in June 1998 - it was posted by the virus author to a local Internet conference as a some utility. 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 and the list keeps growing).

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

motherboard and chipset allow to write 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 for switch position and this protection has no effect at all. Some other motherboard designs provide write

protection that can be disabled/overriden by software.

During tests in our lab the virus did not overwrite the Flash BIOS and just halted the computer. We do however have reports from other

sources telling that the virus really is able to mess it up.

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 virus versions known, which are very closely related and only differ in 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 CCIH 1.2 TTIT on April 26th YES

1010 CCIH 1.3 TTIT on April 26th NO

1019 CCIH 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

and 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 a not used block not less than 184 bytes of 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 file header - out of loadable file data. Despite this, infected programs are run with no problems - Windows does not

pay attention for such "strange" files, loads the file header into the memory, then file sections, and then passes control to the virus

startup routine in 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 allocated block of memory. The virus then hooks 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 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).

Detection and Cleaning instructions

We have made a special edition of AntiViral Toolkit Pro Lite (AVPLite) to detect and remove Win95.CIH and variants easily from your computer.

Step 1.

Download this special edition of AntiViral Toolkit Pro Lite from Central Command at:

<a href="ftp://ftp.avp.com/pub/nocih/nocih.exe">ftp://ftp.avp.com/pub/nocih/nocih.exe </A>

This is a special utility to create a diskette version of AVPLite. To use this simply download the file nocih.exe to a folder on your computer.

The Win95.CIH virus will not infect this archive.

Step 2.

Insert a blank formatted diskette into your floppy diskette drive.

Step 3.

Locate the file nocih.exe that you downloaded from Central Command and run the program using the example below:

nocih.exe [Drive letter of your floppy drive]

Example:

nocih.exe a: <press enter>

This will create a diskette with AVPLite for detection and removal of Win95.CIH

Step 4.

Make the diskette bootable by typing:

SYS [Drive letter of your floppy drive]

Example:

Sys a: <press enter>

Step 5.

You need to copy one file to the diskette named HIMEM.SYS This file is located in your c:\windows or c:\windows\system\ folder

Copy himem.sys [Drive letter of your floppy drive]

Example:

copy himem.sys a:

Step 6.

Now remove the diskette, write protect it and shut down and shut off your computer.

Re-insert the diskette and turn your computer on again.

This will start AVPLite and detect and remove all Win95.CIH infections. When it's complete simply remove the diskette and restart your computer.

PLEASE NOTE! This virus may corrupt Winzip archives and leave them in a state the in NOT repairable! You will need to replace these files after

disinfection.