W95.Spaces

The virus has two known variants, which append either 1,633 or 1,245 bytes to the last section of the Portable Executable (PE) files. The PE header's entry point will point to the start of the virus in the last section. The last section characteristic is changed to writeable and the PE header's Reserved1 field will contain two spaces (0x2020). Hence the name of the virus.

When the virus is executed, it checks for an active copy of itself in memory by calling VxDcallIFSMgr_Get_Version and passing 0x2020 (Spaces) in the AX register. On return, the AX is 0xDEAD when the virus is active in memory. In such a case, the virus checks the time, calls its payload routine on June 1 of each year, and corrupts the AT hard disk.

When the virus does not detect itself in memory, the virus allocates memory for itself and hooks the file system to itself. This way it can infect all files that are accessed with the .exe file name extension. Since VxD calls are patched on the fly by Windows 9x systems, the virus fixes its copy for those places before it writes itself to the file. Such a mechanism was first used in the W95.Anxiety virus.

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.

Summary