The CIH virus was first located in Taiwan in early June 1998. After that, it has been confirmed to be in the wild worldwide. It has been among the ten most common viruses for several months. CIH has been spreading very quickly as it has been distributed through pirated software.
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It seems that at least four underground pirate software groups got infected with the CIH virus during summer 1998. They inadvertently spread the virus globally in new pirated softwares they released through their own channels. These releases include some new games which will spread world-wide very quickly. There's also a persistent rumor about a 'PWA-cracked copy' of Windows 98 which would be infected by the CIH virus but F-Secure has been unable to confirm this.
Later on, CIH was distributed by accident from several commercial sources, such as:
The CIH virus infects Windows 95 and 98 executable files (EXE files). After an infected EXE is executed, the virus will stay in memory and will infect other programs as they are accessed. It does not infect Word or Excel documents. CIH works under both Windows 95 and Windows 98, but it does not work under Windows NT.
CIH uses a peculiar way of infecting executables. As a result, the size of the infected files does not grow at all. The actual size of the virus code is around 1 kB. The virus also employees advanced tricks in jumping from processor ring 3 to ring 0 in order to hook file system calls.
What makes the CIH case really serious is that the virus activates destructively. When it happens the virus overwrites most of the data on the computers hard drive. This can be recovered with recent backups.
However, the virus has another, unique activation routine: It will try to overwrite the Flash BIOS chip of the machine. If this succeeds, the machine will be unable to boot at all unless the chip is reprogammed. The Flash routine will work on many types of Pentium machines - for example, on machines based on the Intel 430TX chipset. On most machines, the Flash BIOS can be protected with a jumper. By default, protection is usually off.
There are four known closely-related variants:
Activates on June 26th. Contains this text:
Activates on 26th of every month. It is in the wild, but not particularily common. It contains this text:
In December 2002, over four years after the original CIH virus was found, a modified variant known as CIH.1106 was found. However, this minor version is not widespread. The only way CIH.1106 might spread in any significant numbers happens when it infects programs that are sent via email by other, more modern mass mailer viruses - such as Klez. In such scenario a user would receive an infected email where the infected executable attachment would contain two viruses. However, CIH.1106 (like other CIH versions) works only under Windows 95 and 98.