Virus.DOS.Frodo

Parent class: VirWare

Viruses and worms are malicious programs that self-replicate on computers or via computer networks without the user being aware; each subsequent copy of such malicious programs is also able to self-replicate. Malicious programs which spread via networks or infect remote machines when commanded to do so by the “owner” (e.g. Backdoors) or programs that create multiple copies that are unable to self-replicate are not part of the Viruses and Worms subclass. The main characteristic used to determine whether or not a program is classified as a separate behaviour within the Viruses and Worms subclass is how the program propagates (i.e. how the malicious program spreads copies of itself via local or network resources.) Most known worms are spread as files sent as email attachments, via a link to a web or FTP resource, via a link sent in an ICQ or IRC message, via P2P file sharing networks etc. Some worms spread as network packets; these directly penetrate the computer memory, and the worm code is then activated. Worms use the following techniques to penetrate remote computers and launch copies of themselves: social engineering (for example, an email message suggesting the user opens an attached file), exploiting network configuration errors (such as copying to a fully accessible disk), and exploiting loopholes in operating system and application security. Viruses can be divided in accordance with the method used to infect a computer:

• file viruses
• boot sector viruses
• macro viruses
• script viruses

Any program within this subclass can have additional Trojan functions. It should also be noted that many worms use more than one method in order to spread copies via networks.

Class: Virus

Viruses replicate on the resources of the local machine. Unlike worms, viruses do not use network services to propagate or penetrate other computers. A copy of a virus will reach remote computers only if the infected object is, for some reason unrelated to the virus function, activated on another computer. For example: when infecting accessible disks, a virus penetrates a file located on a network resource a virus copies itself to a removable storage device or infects a file on a removable device a user sends an email with an infected attachment.

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Platform: DOS

No platform description

Description

This is a memory-resident stealth virus, 4096 (1000h) bytes long. It infects files upon execution or closing. Contamination of data-files is also possible. The virus completes its copy in such a way that the size of an infected file will grow exactly by 4096 bytes (see "Eddie.2000"). In infected files, the virus makes the time of the last modification increase by 100 years. In COM-files, it alters the first 6 bytes, and in EXE-files it alters the header.

Upon entering the files, "Frodo" uses the true values of interrupt vectors 13h and 21h, which it receives using the "Yankee":algorithm. In addition to this, "Frodo" modifies the first 5 bytes of the INT 21h handler.

When creating its TSR-copy, the virus occupies the top addresses that results in infecting the COMMAND.COM file. "Frodo" sets the owner address in its MCB, coinciding with the address of the first MCB owner in the system, masking it in such a way as DOS. Later, the copy of the virus might move through the memory in the direction of lower addresses, allocating new memory areas and clearing old ones.

A genuine stealth virus: intercepts INT 21h, handles 20 (!) functions of it (FindFirst, FindNext, Read, Write, Lseek, Open, Create, Close, Exec etc.) and effectively masks itself. When DOS tries to access an infected file, the virus substitutes its original length and the last modification time. Upon reading or loading a file into the memory, it modifies the information read from the disk in such a way that the file appears as though it is uninfected. Upon opening an infected file for writing, the virus cures it (because writing to the file might delete part of the virus), and reinfects it upon closing.

The virus runs itself from September 22nd until December 31st every year. It is not known how the virus runs itself, because the corresponding area of the virus code happens to be deleted. It may be assumed that the virus deletes the Boot-sector of a floppy-disk and the MBR-sector of the hard disk, writing its own code there. Upon rebooting from such a disk, the screen displays (using pseudo-graphic symbols) the message "FRODO LIVES!" in large letters.

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