W2K.Stream

SUMMARY

W2K.Stream virus was discovered in early September 2000. The virus was created by Benny and Ratter of the 29A virus group. This virus is a proof-of-concept virus that utilizes NTFS streams and has received much attention. It is a new subclass of the traditional companion virus and is now being referred to as the "Stream Companion" virus.

W2K.Stream virus only infects files on Windows 2000 using NTFS, and only the files in the same directory. The infected files will become 3,628 bytes in size regardless of the original file size because the original host file is replaced by the virus and stored in a different stream.

As of September 7, 2000, Symantec AntiVirus Research Center has received no reports of this virus in the wild, and it is considered a very low threat.

Protection

• Virus Definitions (LiveUpdate™ Weekly) September 5, 2000
• Virus Definitions (Intelligent Updater) September 5, 2000

Threat Assessment

Wild

• Wild Level: Low
• Number of Infections: 0 - 49
• Number of Sites: 0 - 2
• Geographical Distribution: Low
• Threat Containment: Easy
• Removal: Easy

Damage

• Damage Level: Low
• Payload Trigger: When the virus is executed on non-NTFS systems
• Payload: Displays message

Distribution

• Distribution Level: Low

TECHNICAL DETAILS

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":

• Turn off and remove unneeded services. By default, many operating systems install auxiliary services that are not critical, such as an FTP server, telnet, and a Web server. These services are avenues of attack. If they are removed, blended threats have less avenues of attack and you have fewer services to maintain through patch updates.
• If a blended 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 (for example, all Windows-based computers should have the current Service Pack installed.). Additionally, please apply any security updates that are mentioned in this writeup, in trusted Security Bulletins, or on vendor Web sites.
• 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.
• Configure your email server to block or remove email that contains file attachments that are commonly used to spread viruses, such as .vbs, .bat, .exe, .pif and .scr files.
• Isolate infected computers quickly to prevent further compromising your organization. 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.