Virus.DOS.Beast

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 dangerous stealth virus that affects COM files, writing itself at the file beginning. A file is infected as it is executed or closed. The beginning of the file is saved at the first unused sector of the last cluster of the file.

? <----------- File --------------------------> ?



+-------------------------  --------------------------+



? cluster   ? cluster   ? ... ? cluster   ? cluster   ?



+-------------------------  --------------------------+



^                                               ^  ^



+- Virus beginning  Saved beginning of file  ---+  ?



Unused sector            ------+

The length of an infected file does not change. The time of the file last modification is set to 62 sec. On infecting, the interrupt vectors table (0000:0200 - 0000:03FF) is used by infector as a work area.

On its activation the virus enters one of the system buffers. Upon infecting files the virus actively uses non documented DOS area - System File Table. During a read of the infected file beginning it substitutes the true beginning. By these actions the "Beast" virus successfully masks its presence in the system.

The virus is very dangerous. This virus affects files with a .CO? extension. As an infected file is copied it may be lost (the last file cluster is not copied in full). The file will be lost if the disk has one sector on a cluster. The virus alters INT 21h, some of them contain the string "666".

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