Threat Description

Grune

Details

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

Summary



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.



Removal



Automatic Disinfection

Allow F-Secure Anti-Virus to disinfect the relevant files.

For more general information on disinfection, please see Removal Instructions.



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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