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W2K.Stream

Risk Level 1: Very Low

Discovered:
September 5, 2000
Updated:
February 13, 2007 11:32:40 AM
Also Known As:
PE_STREAM.A [Trend], W2K/Stream [McAfee, Sophos], Win2K.Stream.a [Kaspersky], Win2K.Stream [Computer Associa
Type:
Virus
Systems Affected:
Windows 2000

W2K.Stream is a virus that only replicates on Windows 2000 systems that use an NTFS partition. W2K.Stream utilizes an NTFS feature that exists on both Windows NT and Windows 2000. The virus writers believed that this particular feature did not exist on Windows NT and therefore reduced the virus to be Windows 2000 specific by having the virus check the OS version (similar to the W2K.Installer virus).

NTFS streams are virtually hidden from users. This is because NT commands or standard Windows 2000 applications do not display them. A given file on an NTFS partition is basically an unnamed stream of a file. Any file can have associated named streams. These streams can be accessed during standard file operations. Most Windows NT/2000 applications do not use named streams.

W2K.Stream virus is 3628 bytes. The virus is compressed with a popular Portable Executable (PE) file compressor called Petite. The actual virus code inside is very short. First the virus checks the Windows version of the current system. If it is not Windows 2000, the virus displays the following message:



The virus is basically a new subclass of companion viruses, a "stream companion" virus. When the virus infects a file it replaces the host application with itself. Basically the virus implements the simplest possible virus infection by overwriting the host program with its own code. In other words each infected file will be 3628 bytes long. The trick of the virus is that it saves the original host application as a named stream of the host program.

For instance when Notepad.exe gets infected, the size of the file changes to 3628 bytes. At the same time the virus creates "NOTEPAD.EXE:STR" stream that has the copy of Notepad.exe content. This way the virus can execute the host program as long as the infected file remains on a NTFS partition. When someone copies an infected file to a diskette, the host program will be lost since the diskette uses FAT instead of NTFS storage format. However, the virus and the host are copied over a network from an NTFS to an NTFS partition.

Since the virus displays a message box, it is very unlikely that someone would not notice the infection.

The virus uses the file compression flag as an infection marker. This way the used disk space of the virus is not obvious. The virus infects all .exe files in the current directory. It does not pay attention to the actual file type.

Whenever the "STR" stream is not available, the virus will also display its message. The virus does not pay attention to the read-only attribute. During infection operations the virus uses temporary files to copy the data streams.

Recommendations

Symantec Security Response encourages all users and administrators to adhere to the following basic security "best practices":

  • Use a firewall to block all incoming connections from the Internet to services that should not be publicly available. By default, you should deny all incoming connections and only allow services you explicitly want to offer to the outside world.
  • Enforce a password policy. Complex passwords make it difficult to crack password files on compromised computers. This helps to prevent or limit damage when a computer is compromised.
  • Ensure that programs and users of the computer use the lowest level of privileges necessary to complete a task. When prompted for a root or UAC password, ensure that the program asking for administration-level access is a legitimate application.
  • Disable AutoPlay to prevent the automatic launching of executable files on network and removable drives, and disconnect the drives when not required. If write access is not required, enable read-only mode if the option is available.
  • Turn off file sharing if not needed. If file sharing is required, use ACLs and password protection to limit access. Disable anonymous access to shared folders. Grant access only to user accounts with strong passwords to folders that must be shared.
  • Turn off and remove unnecessary services. By default, many operating systems install auxiliary services that are not critical. These services are avenues of attack. If they are removed, threats have less avenues of attack.
  • If a threat exploits one or more network services, disable, or block access to, those services until a patch is applied.
  • Always keep your patch levels up-to-date, especially on computers that host public services and are accessible through the firewall, such as HTTP, FTP, mail, and DNS services.
  • Configure your email server to block or remove email that contains file attachments that are commonly used to spread threats, such as .vbs, .bat, .exe, .pif and .scr files.
  • Isolate compromised computers quickly to prevent threats from spreading further. Perform a forensic analysis and restore the computers using trusted media.
  • Train employees not to open attachments unless they are expecting them. Also, do not execute software that is downloaded from the Internet unless it has been scanned for viruses. Simply visiting a compromised Web site can cause infection if certain browser vulnerabilities are not patched.
  • If Bluetooth is not required for mobile devices, it should be turned off. If you require its use, ensure that the device's visibility is set to "Hidden" so that it cannot be scanned by other Bluetooth devices. If device pairing must be used, ensure that all devices are set to "Unauthorized", requiring authorization for each connection request. Do not accept applications that are unsigned or sent from unknown sources.
  • For further information on the terms used in this document, please refer to the Security Response glossary.
Writeup By: Peter Szor
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