W32.Hezhi

SUMMARY

W32.Hezhi is a polymorphic virus that infects Portable Executable (PE) files. It replicates to network shares and mapped network drives.

Behavior

Symptoms

Transmission

Protection

• Virus Definitions (LiveUpdate™ Weekly) April 22, 2002
• Virus Definitions (Intelligent Updater) April 22, 2002

Threat Assessment

Wild

• Wild Level: Low
• Number of Infections: 0 - 49
• Number of Sites: 0 - 2
• Geographical Distribution: Low
• Threat Containment: Easy
• Removal: Easy

Damage

• Damage Level: Low

Distribution

• Distribution Level: Low

TECHNICAL DETAILS

When W32.Hezhi is run, it does the following:

1. Decrypts itself and resolves the API functions that it needs for replication.
   
2. Spawns a new sub-process to run its host in parallel with the virus, at which time the main process starts infecting new files.
   
3. Lowers the priority of the viral process in an attempt to go unnoticed.
   
4. Infects the files on all the available network shares, as well as on all the fixed and remote drives from C through Z. The files that W32.Hezhi infects must meet the following criteria to be considered for infection:
   • Must not be located in a folder whose name begins with "system."
   • Must have a .exe extension.
   • Must be longer than 8 KB.
   • Must not start with any of the following prefixes:
     • aler
     • amon
     • <character 0x7f>avp
     • avp3
     • avpm
     • f-pr
     • navw
     • scan
     • smss
     • ddhe
     • dpla
     • mpla
     • user
     • mpre
     • krnl
     • gdi.
     • vshw
     • rundll32
     • runonce
     • rav
     • lsass
     • services
     • winlogon
     • spoolsv
     • mstask
     • rpcss

W32.Hezhi infects files by replacing the code at the entry point with a decryptor routine. The encrypted body of the virus is placed in the last section. The virus uses five encryption layers.

There is one chance out of 64 that the virus will append an additional section named, ".cao," to the infected PE files.

On November 20th, the virus will corrupt the files instead of infecting them. Additionally the virus sometimes corrupts files during infection.

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

The following instructions pertain to 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 the files detected as W32.Hezhi.

For specific details on each of these steps, read the following instructions.

1. Updating the virus definitions
Symantec Security Response fully tests all the virus definitions for quality assurance before they are posted to our servers. There are two ways to obtain the most recent virus definitions:

• Running LiveUpdate, which is the easiest way to obtain virus definitions: These virus definitions are posted to the LiveUpdate servers once each week (usually on Wednesdays), unless there is a major virus outbreak. To determine whether definitions for this threat are available by LiveUpdate, refer to the Virus Definitions (LiveUpdate).
• Downloading the definitions using the Intelligent Updater: The Intelligent Updater virus definitions are posted on U.S. business days (Monday through Friday). You should download the definitions from the Symantec Security Response Web site and manually install them. To determine whether definitions for this threat are available by the Intelligent Updater, refer to the Virus Definitions (Intelligent Updater).
  
  The 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.

2. Scanning for and deleting the infected files

1. Start your Symantec antivirus program and make sure that it is configured to scan all the files.
   • For Norton AntiVirus consumer products: Read the document, "How to configure Norton AntiVirus to scan all files."
   • For Symantec AntiVirus Enterprise products: Read the document, "How to verify that 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 W32.Hezhi, click Delete.
4. Replace the deleted files from a clean backup or re-install them.