1. Symantec/
  2. Security Response/
  3. W32.Funlove.4099


Risk Level 2: Low

November 8, 1999
February 13, 2007 11:34:17 AM
Also Known As:
Win32.FunLove.4070 [KAV], W32/FunLove.gen [McAfee], PE_FUNLOVE.4099 [Trend], W32/Flcss [Sophos], Win32.Funlove.4099 [CA]
Systems Affected:

How FunLove works
Files infected with W32.FunLove.4099 insert the Flcss.exe file into the \Windows\System (Windows 95/98/Me) or \Winnt\System32 (Windows NT) folder. Whenever the 4,608-byte Flcss.exe file can be created, the virus attempts to execute it as a service on computers running Windows NT. If for any reason the service can not be executed, the virus creates a thread inside the infected program. This thread infects local and network drives by searching for Portable Executable (PE) files with .exe, .scr, or .ocx extensions. The thread then executes inside the infected process and the main thread of the program takes control. In most cases, this does not cause any noticeable delays. When the virus can execute itself as a service process under the "FLC" name, other infected programs will try to insert the Flcss.exe file, but will not create a new infection thread. W32.FunLove.4099 is the second virus that runs as a service on Windows NT.

The WNT.RemEx.A (W32.RemoteExplore) virus is very similar in its functions to W32.FunLove.4099, but W32.FunLove.4099 can run on both Windows 95/98 and Windows NT. It is, therefore, considered more successful than WNT.RemEx.A. When the virus runs as a service, it can spread on the local drives, even if no one is logged on. Because of this, the virus can infect files that are normally not accessible after the logon. For example, the virus can infect Explorer.exe on a Windows NT system.

On Windows 95/98 computers, infected programs place the Flcss.exe file in the \System folder and try to execute it as a regular process. If the process cannot be executed, the virus tries to execute the infection thread inside the infected host program.

This virus also attacks the Windows NT file security system. For the virus to attempt the attack, it needs administrative rights in Windows NT Server or Windows NT Workstation during the initial infiltration. Once the Administrator or someone with the equivalent rights logs on, W32.FunLove.4099 has the opportunity to modify the Ntoskrnl.exe file, the Windows NT kernel located in the \Winnt\System32 folder. The virus modifies only two bytes in a security API named SeAccessCheck. W32.FunLove.4099 is then able to give full access to all files to all users, regardless of its original protection, whenever the computer is booted with the modified kernel. This means that a Guest--who has the lowest possible rights on the system--can read and modify all files, including files that are normally accessible only by the Administrator. This is a potential problem, because the virus can spread everywhere, regardless of the actual access restrictions on the particular computer. Furthermore, after the attack, no data can be considered protected from modification by any user.

Unfortunately, the consistency of Ntoskrnl.exe is checked only once during the startup process. The loader, Ntldr, checks Ntoskrnl.exe when it loads into physical memory during startup. If the kernel becomes corrupted, Ntldr is supposed to stop loading Ntoskrnl.exe and display an error message, even before a "blue screen" appears. To avoid this, W32.FunLove.4099 patches Ntldr so that no error messages are displayed, and Windows NT will boot successfully, even if its checksum does not match the original. Since no code checks the consistency of Ntldr itself, the patched kernel will be loaded without notifying the user. Because Ntldr is a hidden, system, and read-only file, W32.FunLove.4099 changes the attributes of it to "archive" before it attempts to patch it. The virus does not change the attribute of Ntldr back to its original value after the patch. FunLove can also infect local and network drives. It enumerates the mapped network drives and infects PE files on those computers. In addition, the Ntoskrnl.exe and Ntldr patch is performed on the network drives. Whenever a computer with sufficient rights maps the System drive of a computer running Windows NT, the virus modifies the kernel and the loader components over the network.

The Ntoskrnl.exe and Ntldr patches are executed by a routine picked up from the Bolzano virus. In fact, more than 50 percent of the virus code shows similarities to the Bolzano virus. It is very likely that the author of these two viruses is the same person.

How FunLove locates the mapped drives on a system
FunLove uses the Windows function call WNetEnumResourceA. Details on this function can be found in the Microsoft Developer Network documentation.

Can Ntoskrnl.exe be infected across the network, without Flcss.exe actually being copied to the system?
The worm infects every network drive that it finds through the call to WNetEnumResourceA. As long as the drive is writeable, FunLove will modify Ntoskrnl.exe over the network, even without dropping Flcss.exe onto the system. FunLove does not actually infect Ntoskrnl.exe, but it changes the file's security function. Once the affected computer is restarted, the modified Ntoskrnl.exe and Ntldr are loaded, and security is compromised.

Files not infected
The virus does not infect files that begin with the following characters in their names:

These are partial file names of antivirus programs, as well as a few other programs.


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
Summary| Technical Details| Removal

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