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CIH, also known as Chernobyl or Spacefiller, is a Microsoft Windows 9x computer virus which first emerged in 1998. Its payload is highly destructive to vulnerable systems, overwriting critical information on infected system drives, and in some cases destroying the system BIOS. The virus was created by Chen Ing-hau (陳盈豪, pinyin: Chén Yíngháo) who was a student at Tatung University in Taiwan. Now he is the chief executive officer, and founder of 8tory. 60 million computers were believed to be infected by the virus internationally, resulting in an estimated $1 billion US dollars in commercial damages.
Chen claimed to have written the virus as a challenge against bold claims of antiviral efficiency by antivirus software developers. Chen stated that after the virus was spread across Tatung University by classmates, he apologized to the school and made an antivirus program available for public download; the antivirus program was co-authored with Weng Shi-hao (翁世豪), a student at Tamkang University. Prosecutors in Taiwan could not charge Chen at the time because no victims came forward with a lawsuit. These events led to new computer crime legislation in Taiwan.
The name "Chernobyl Virus" was coined some time after the virus was already well known as CIH, and refers to the complete coincidence of the payload trigger date in some variants of the virus (actually the virus creation date in 1998, to trigger exactly a year later) and the Chernobyl accident, which happened in the Soviet Union on April 26, 1986.
The name "Spacefiller" was introduced because most viruses write their code to the end of the infected file - however, CIH looks for gaps in the existing program code, where it then writes its own code. This does not increase the file size and in that way helps the virus avoid detection.