Threat Description



Category: Malware
Type: Virus
Platform: W32
Aliases: Grune


This destructive virus uses INT 21h/AX=4BCCh as an "Are-you-there" call. If this function return FCFEh, virus concludes that it is already resident. The virus installs itself by manipulating the Memory Control Blocks. DOS calls are used to install the resident INT 21h handler.


Automatic action

Once detected, the F-Secure security product will automatically disinfect the suspect file by either deleting it or renaming it.

More scanning & removal options

More information on scanning or removal options is available in the documentation for your F-Secure security product on the Downloads section of our Home - Global site.

You may also refer to the Knowledge Base on the F-Secure Community site for more information.

Contact Support

For further assistance, F-Secure customers can request support online via the Request support or the Chat forms on our Home - Global site.

Technical Details

The INT 21h handler defines the residence test and intercepts DOS function 4B00h (load/exec) to infect files. Files with an EXE header are excluded by testing for 'MZ' as the first word of the file. Files larger than 62.5 KB are also ignored. If a victim file start with a jump to a location of 1241 bytes before the end of the file, virus concludes that the file is already installed. File attribute is cleared and restored afterwards. File date/time are preserved. Infection is done by appending the virus.

Virus checks the system data. If the date is November 1991 or later and it is the 19th of an even-numbered month, a text message is decrypted (with XOR 0B5h). The message reads:

 Arbeiten Sie jetzt wirklich umweltfreundlich ?           Sie haben nun viel Zeit dar ber nachzudenken !           Es grsst Sie die "Grne Partei der Schweiz" !  

The damage routine reads the disk system parameters (INT 13h/AH=8h) and the partition and boot sectors to get the disk parameters. All partitions on all hard disks have their system area overwritten with garbage from memory location 1000:0000h. Then the message is printed on the screen. When the message is displayed for the first time, data on disks is lost already.

Next each hard disk is completely overwritten (all sectors from beginning to end of physically formatted area) with the same garbage, and the message is printed once more for each disk. A maximum of 4 attempts are made at each disk operation.


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