Virus.Win9x.Yobe

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: Win9x

No platform description

Description

Technical Details

This is a dangerous memory resident parasitic Windows virus. It uses system calls that are valid under Win95/98 only, and can't spread under NT. The virus also has bugs and often halts the system when run. Despite this, the virus has very unusual way of spreading, and it is interesting enough from a technical point of view.

The virus can be found only in two files: "SETUP.EXE" on floppy disks and "SETUP .EXE" in the root of the C: drive (there is one space between the file name and ".EXE" extension).

On floppy disks, the virus uses a trick to hide its copy. It writes its complete code to the last disk sectors and modifies the SETUP.EXE file to read and execute this code.

The infected SETUP.EXE file looks just as a 512-byte DOS EXE program, but it is not. While infecting this file, the virus uses a DirII virus method: by direct disk sectors read/write calls, the virus gets access to disk directory sectors, modifies the "first file cluster" field and makes necessary changes in disk FAT tables. As a result, the original SETUP.EXE code is not modified, but the directory enters points to the virus code instead of the original file clusters.

When the infected SETUP.EXE is run from the infected floppy disk, this DOS component of the virus takes control, reads the complete virus body from the last sectors on the floppy disk, then creates the "C:SETUP .EXE" file, writes these data (complete virus code) to there and executes. The virus installation routine takes control then, installs the virus into the system and disinfects the SETUP.EXE file on the floppy drive.

While installing itself into the system, the virus creates a new key in the system registry to activate itself upon each Windows restart:

 HKLMSoftwareMicrosoftWindowsCurrentVersionRun 
  YOBE=""C:SETUP .EXE" YOBE"

The virus then switches to the Windows kernel level (Ring0), allocates a block of system memory, copies itself to there and hooks disk-file access Windows functions (IFS API). This hook intercepts file opening calls, and upon opening the SETUP.EXE file on the A: drive, the virus infects it.

The virus has additional routines. First, one of them looks for "AVP Monitor" and "Amon Antivirus Monitor" windows and closes them; the second one, depending on the random counter, displays a line with the words "YOBE" to the left side of the screen.

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