F-Secure Virus Descriptions : Infis

NAME:	Infis
ALIAS:	WinNT/Infis, WinNT.Infis.4608

Infis is a memory resident virus. It can replicate under Windows NT 4.0 with Service Packs 2, 3, 4, 5, 6 installed. It does not work on systems running Windows 95/98, Windows 2000 or other versions of Windows NT.

The virus usually arrives in an infected EXE file and being run installs itself to system. The virus copies its body to INF.SYS file in Windows NT drivers folder WinNT\System32\Drivers. Then it creates a key with three subkeys in Windows System Registry:

 \Registry\Machine\System\CurrentControlSet\Services\inf

  Type = 1                     - standard Windows NT driver
  Start = 2                    - driver start mode
  ErrorControl = 1             - continue system loading on error in driver

As a result the virus in INF.SYS file will be activated every time the operating system starts. When INF.SYS file is activated the virus first infects Windows NT memory. When this is done the virus takes control over some Windows NT internal undocumented functions. The virus traps file opening routine and if any file is opened it checks file name and file's internal format and then calls his infection routine if PE EXE file is opened.

The virus infects only PE (Portable Executable) EXE-files except CMD.EXE (Windows NT command processor). When infecting the virus increases file length by the length of its "pure code" - 4608 bytes. The virus doesn't infect files twice. It recognizes already infected files by "date and time" stamp changed to -1 (FFFFFFFFh) value upon first infection.

The Infis virus does not have any destructive payload. However, it has bugs that could result in corrupting of some files upon infection. When a corrupted file is run it the standard Windows NT application error message is shown.

[Analysis: Eugene Kaspersky, AVP Team]