W95.Paddi

SUMMARY

W95.Paddi is a direct infecting polymorphic virus that infects under Windows 95/98/Me. When executed, it will infect all Portable Executable (PE) files on the drive that the virus is executed from. The virus also inserts a text file on the system. This text file contains the source code of the virus.

Behavior

Symptoms

Transmission

Protection

• Virus Definitions (LiveUpdate™ Weekly) November 4, 2002
• Virus Definitions (Intelligent Updater) November 4, 2002

TECHNICAL DETAILS

W95.Paddi is a polymorphic virus. This means that the virus is encrypted, and each time that it infects a new file, it will re-encrypt itself using a new key. The encryption method will always remain the same.

When the virus is executed, it will decrypt itself in memory. Once the viral body is decrypted, it starts its malicious actions.

First, the virus will find a certain byte in Kernel32.dll in memory. The memory address of Kernel32.dll is hard coded in the virus. The address is correct for all Windows 95/98/Me systems, but not for Windows NT/2000/XP. This is one of the reasons it will not work under Windows NT/2000/XP.

The byte that the virus is searching for is located close to the GetProcAddress function in Kernel32.dll. Once the virus find this byte, it will have the address of GetProcAddress.

GetProcAddress is a Windows function that enables importing of other functions. As the virus now has the address for GetProcAddress, as well as the address of Kernel32.dll in memory, it can now import functions from Kernel32.dll.

The virus imports several functions from Kernel32.dll and saves the addresses to these functions so that it can use them later.

The virus next attempts to insert the file Paddingx.txt in the C:WindowsSystem folder. If it fails to insert this file on the system, the virus will call the infection routine. This file contains the source code of the virus itself. The virus also contains a small routine that displays a message on the screen and then enters an infinite loop. However, this routine has to be manually cut out of the text file before it can be executed.

Next, the virus calls the infection routine. The infection routine is fairly simple. It will find PE files in all folders on the drive from which it is executed and infect them. It will hook the entry point of the file. This means that when an infected file is executed, it will start by running the viral code.

Before the virus infects a file, it re-encrypts itself using a random key. It also updates the decryption routine, so that the virus can be decrypted when it is executed. The encryption will always be four bytes long, changing the xor key. However, the virus also inserts several random junk instructions in the decryption loop. The junk instructions do not actually do anything, and they appear to be an attempt to make it more difficult to analyze the virus.

When the virus has finished infecting files, it returns to the original entry point of the program. This will cause the original file to execute as normal.

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.

REMOVAL

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, delete all files that are detected as W95.Paddi and restore them from a clean backup copy if necessary.

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.
   • Norton AntiVirus consumer products: Read the document How to configure Norton AntiVirus to scan all files.
   • Symantec enterprise antivirus products: Read the document How to verify a Symantec Corporate antivirus product is set to scan All Files.
2. Run a full system scan.
3. If any files are detected as infected with W95.Paddi, click Delete.
4. Replace all deleted files with clean backup copies if necessary.