Nygus is a resident file virus which infects both COM and EXE files.


Automatic action

Based on the settings of your F-Secure security product, it will either move the file to the quarantine where it cannot spread or cause harm, or remove it.

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Technical Details

When infected file is executed for the first time, virus will install itself in memory and reserves 1024 bytes for itself. At this time the virus will also hook interrupt 21h.

The int21 handler intercepts calls to DOS function 4B00h (loadexec). The read-only attribute is cleared if set, but not restored afterwards. Infection is only attempted if at least 1 cluster is available on the default drive. Files matching COMM*.*, MKS_*.*, COS_*.* or MKMM*.* are not infected. These files include some antivirus programs and DOS' command interpreter COMMAND.COM.

Virus infects all EXE files which are not already infected and are between 768 bytes and 192 kilobytes. Virus infects all uninfected COM files which are longer than 640 bytes. Virus contains an encrypted text message saying '(c)Nygus v2.0'. The text is not used anywhere by the virus.

There is one known 752 byte variant of this virus.

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