Leech

Upon infection, the virus becomes memory resident at the top of system memory but below the 640K DOS boundary. Interrupt 21 is hooked by the virus. Leech also makes use of interrupt 2F.

Once Leech is memory resident, it infects .COM files over approximately 10K in size as they are executed.

It is not known what Leech does besides replicate.

Additional Comments:
The Leech virus was submitted in May, 1991. Leech is from Bulgaria. It is a memory resident infector of .COM programs, including COMMAND.COM. The first time a program infected with Leech is executed, the virus will install itself memory resident at the top of system memory but below the 640K DOS boundary. Total system and available memory, as measured by the DOS CHKDSK program, will decrease by 2,080 bytes. Interrupt 21 will be hooked by the virus. Leech also makes use of interrupt 2F. Once Leech is memory resident, it will infect .COM programs over approximately 10K in size when they are executed. Infected programs will increase in size by 1,024 bytes, but the size increase will not be visible in the disk directory if Leech is memory resident. Programs which originally had a file time of 12:00a before infection will have their time disappear from the disk directory when viewed with Leech resident. Leech is located at the beginning of infected programs. Infected files will contain the following text string: "The leech live ... April 1991 The Topler" Leech is a stealth virus. If it is memory resident, anti-viral programs which are unaware of Leech will be unable to detect its presence on the system. It is unknown if Leech does anything besides replicate. Known variant(s) of Leech are:

"The leech live ...April 1991 The Topler"

Total system and available memory decreases by 2,080 bytes. Infected files increase in size by 1,024 bytes. The file length increase is not visible in the disk directory if Leech is memory resident (Stealth techniques). Files which originally had a file time of 12:00a before infection have their time disappear from the disk directory when viewed when Leech is memory resident. Leech is located at the beginning of infected files.

The only way to infect a computer with a file infecting virus is to execute an infected file on the computer. The infected file may come from a multitude of sources including: floppy diskettes, downloads through an online service, network, etc. Once the infected file is executed, the virus may activate.

All Users :
Script,Batch,Macro and non memory-resident:
Use current engine and DAT files for detection and removal.

PE,Trojan,Internet Worm and memory resident :
Use specified engine and DAT files for detection. To remove, boot to MS-DOS mode or use a boot diskette and use the command line scanner:

Additional Windows ME/XP removal considerations

Users should not trust file icons, particularly when receiving files from others via P2P clients, IRC, email or other mediums where users can share files.

AVERT Recommended Updates :

* Office2000 Updates

* Malformed Word Document Could Enable Macro to Run Automatically (Information/Patch )

* scriptlet.typelib/Eyedog vulnerability patch

* Outlook as an email attachment security update

* Exchange 5.5 post SP3 Information Store Patch 5.5.2652.42 - this patch corrects detection issues with GroupShield

For a list of attachments blocked by the Outlook patch and a general FAQ, visit this link .
Additionally, Network Administrators can configure this update using an available tool - visit this link for more information .

It is very common for macro viruses to disable options within Office applications for example in Word, the macro protection warning commonly is disabled. After cleaning macro viruses, ensure that your previously set options are again enabled.

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