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Discovered: December 01, 2002
Updated: February 13, 2007 11:41:25 AM
Type: Virus
Systems Affected: Windows

W32.Cervan.6256 is a virus that infects Windows Portable Executable (PE) files. The virus is polymorphic and entry-point obscuring (EPO).

What are Portable Executable (PE) files?
PE files are files that are portable across all Microsoft 32-bit operating systems. The same PE-format executable can be executed on any version of Windows 95, 98, Me, NT, 2000, and XP. All PE files are executable, but not all executable files are portable.

A common example of a Portable Executable file is a screen saver (.scr) file.

Antivirus Protection Dates

  • Initial Rapid Release version December 02, 2002
  • Latest Rapid Release version September 28, 2010 revision 054
  • Initial Daily Certified version December 02, 2002
  • Latest Daily Certified version September 28, 2010 revision 036
  • Initial Weekly Certified release date December 04, 2002

Click here for a more detailed description of Rapid Release and Daily Certified virus definitions.

Writeup By: Neal Hindocha

Discovered: December 01, 2002
Updated: February 13, 2007 11:41:25 AM
Type: Virus
Systems Affected: Windows

W32.Cervan.6256 is a fairly complex virus that infects Windows PE files. However, the virus contains several bugs that may cause the it to not work as intended.

W32.Cervan.6256 is entry-point obscuring (EPO). This means that the virus does not run directly when an infected file is executed. Instead, it modifies the import table so that it runs whenever an imported function is called. The virus creates its own import table to do two imports: LoadLibrary and GetProcAddress.

W32.Cervan.6265 is polymorphic. This means that it encrypts itself in a different way each time that it infects a file. The decryption routine is fairly large and contains many instructions that do nothing; therefore, it takes from 1,000,000 to 3,000,000 instructions to decrypt the viral body.

After the virus has decrypted itself, it does the following:

First, the virus manually imports the addresses of several functions in five separate .dll files. It stores the addresses in a table for later use.

Next the virus attempts to allocate about 6 KB of memory. However, as memory on Windows 32-bit platforms is allocated in 4 KB pages, 8 KB of memory will be allocated. After the memory has been allocated, the virus moves its entire body to the newly allocated memory block. From this point, the virus will continue execution in the newly allocated memory block.

W32.Cervan.6256 is multithreaded. Therefore, at this point the virus creates an object to control the secondary thread. However, due to bugs in the viral code, this control mechanism may not work as intended. In some cases, the secondary thread is halted in a wait state instead of executing as intended by the author of the virus.

After the virus creates the object to control the secondary thread, the virus creates and runs the secondary thread.

The main thread manually imports all of the APIs that the original program uses. Because the virus is called when the original host tries to call an API, the virus must call this API itself, or the original host would not work. Therefore, after the virus has completed all of the imports, it finds the API that the original host tried to call and calls it. After this API gives control back to the virus, the virus returns and lets the original host execute.

It is the secondary thread that the virus launched which infects files. As previously mentioned, this code in the virus contains bugs. Therefore, in some cases the secondary thread does nothing. However, if the secondary thread does run, which is generally the case, it does the following:

The virus infects files as they are opened by the original host program. For example, if Notepad.exe were infected, the virus would infect all PE files that are opened in Notepad. However, if Telnet.exe were infected, the virus would not infect any additional files because Telnet.exe does not open files.

The virus infects files by modifying the import table in a manner that causes all imports to point to the virus decryptor through a jumptable. As stated above, the virus does the imports manually to allow the original host to execute normally.


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: Neal Hindocha

Discovered: December 01, 2002
Updated: February 13, 2007 11:41:25 AM
Type: Virus
Systems Affected: Windows

NOTE: These instructions are for all current and recent Symantec antivirus products, including the Symantec AntiVirus and Norton AntiVirus product lines.

  1. Update the virus definitions.
  2. Run a full system scan, and delete all files that are detected as W32.Cervan.6526.

For details on how to do this, read the following instructions.

To update the virus definitions:
All virus definitions receive full quality assurance testing by Symantec Security Response before being posted to our servers. There are two ways to obtain the most recent virus definitions:
  • Run LiveUpdate, which is the easiest way to obtain virus definitions. These virus definitions are posted to the LiveUpdate servers once each week (usually Wednesdays) unless there is a major virus outbreak. To determine whether definitions for this threat are available by LiveUpdate, look at the Virus Definitions (LiveUpdate) line at the top of this write-up.
  • Download the definitions using the Intelligent Updater. Intelligent Updater virus definitions are posted on U.S. business days (Monday through Friday). They must be downloaded from the Symantec Security Response Web site and installed manually. To determine whether definitions for this threat are available by the Intelligent Updater, look at the Virus Definitions (Intelligent Updater) line at the top of this write-up.

    Intelligent Updater virus definitions are available here. For detailed instructions on how to download and install the Intelligent Updater virus definitions from the Symantec Security Response Web site, click here.

To scan for and delete the infected files:
  1. Start your Symantec antivirus program, and make sure that it is configured to scan all files.
  2. Run a full system scan.
  3. If any files are detected as infected with W32.Cervan.6526, click Delete. Replace these files from a clean backup, or reinstall them.

Writeup By: Neal Hindocha