F-Secure Virus Descriptions : Halfcros

O97M/Halfcros is a polymorphic VBA macro virus that infects both Microsoft Word 97 documents and Excel 97 spreadsheets.

It consist of two parts, a clear text part and an encoded part. The clear text part will take care of decoding the virus code as needed.

The virus activates when an infected document is opened in Word. It disables the built-in macro virus protection and infects the global template. When the global tempate is infected, the virus will infect all opened Word documents.

When the infected document is closed the virus launches Excel. It checks if there is a file called "Book1." in the Excel's startup directory, and if the file does not exist it disables the Excel's macro virus protection via registry. It then creates and infects the "Book1." file.

Within Excel the virus activates when an infected sheet is deactivated. It uses Word to disable Excel's built-in macro virus protection via registry, and it attempts to infects Word's global template. Then the "Book1." file will be created to the Excel's startup directory if it does not exist.

This virus has a non-destructive payload that activates if user opens an infected document in Word between 12:10 and 12:25 on any day when it displays a message box with a title:

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