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