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Win32/ConHook


Win32/Conhook is a family of Trojans that installs themselves as Browser Helper Objects (BHOs), and connects to the Internet without user consent. They also terminate specific security services, and download additional malware to the computer.


What to do now

To detect and remove this threat and other malicious software that may be installed on your computer, run a full-system scan with an appropriate, up-to-date, security solution. The following Microsoft products will detect and remove this threat:
 
 
For more information on antivirus software, see http://www.microsoft.com/windows/antivirus-partners/.

Threat behavior

Win32/Conhook is a family of Trojans that installs themselves as Browser Helper Objects (BHOs), and connects to the Internet without user consent. They also terminate specific security services, and download additional malware to the computer.
 
Installation
Win32/Conhook is installed by another executable. The installer program creates a dynamic link library (DLL) with a randomly generated file name in the Windows system folder, and also modifies the registry to load the DLL whenever a Web browser application is launched.
 
The Trojan installer may create the following registry keys (for example):
HKEY_CURRENT_USER\SOFTWARE\Microsoft\CAC
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\zxc
 
Win32/Conhook may make further modifications to the registry, as illustrated in the examples below (where specific Class IDs, keys, values and data/file names will differ among variants and specific instances):
 
HKEY_LOCAL_MACHINE\SOFTWARE\Classes\CLSID\{B0022F2A-1E0A-47D6-9B97-6EA471031820}InprocServer32\<value> = "<system folder>\<random file name>.dll"
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\Browser Helper Objects\{B0022F2A-1E0A-47D6-9B97-6EA471031820}
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\winlogon\Notify\<key>
 
These changes may be made to register the DLL as a BHO, and to register the DLL as a Winlogon notification package.
 
The trojan may also make another change where the DLL is loaded by each running process. All the DLLs that are specified in this value are loaded by each Microsoft Windows-based application that is running in the current log on session.
 
HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\CurrentVersion\Windows\
AppInit_DLLs = "<system folder>\<random file name>.dll"
 
Payload
Downloads and Executes Arbitrary Files
This Trojan injects its code into winlogon.exe and explorer.exe running processes, and creates remote threads in each. Win32/Conhook then listens for connections on UDP port 3012.
Win32/Conhook may connect to a remote Web site with IP address 83.149.105.223, using TCP port 80. This Trojan may attempt to download additional malware onto the infected computer.
 
Win32/Conhook may terminate the processes "AD-AWARE.EXE" or "GCASSERVALERT.EXE" if they are running in memory.

Symptoms

The following symptoms may be indicative of a Win32/Conhook installation:
  • Presence of the following registry subkeys
    HKEY_CLASSES_ROOT\CLSID\{64A31598-EEEC-4f1d-8D04-DACC1E2D5407}  
    HKEY_CLASSES_ROOT\CLSID\{40910BCF-0B02-417e-8C81-BC2124376133}  
    HKEY_CLASSES_ROOT\CLSID\{A5A925F3-6B88-4138-8092-16D95CD50D91}  
    HKEY_CLASSES_ROOT\CLSID\{B8FD9F6C-AA0E-4fc3-A239-1C9A0CD80D47}  
    HKEY_CLASSES_ROOT\CLSID\{DD13730A-FBA1-4f91-AB25-7FEB0563D33B}
    HKLM\Software\Classes\CLSID\{64A31598-EEEC-4f1d-8D04-DACC1E2D5407}  
    HKLM\Software\Classes\CLSID\{40910BCF-0B02-417e-8C81-BC2124376133}  
    HKLM\Software\Classes\CLSID\{A5A925F3-6B88-4138-8092-16D95CD50D91}  
    HKLM\Software\Classes\CLSID\{B8FD9F6C-AA0E-4fc3-A239-1C9A0CD80D47}  
    HKLM\Software\Classes\CLSID\{DD13730A-FBA1-4f91-AB25-7FEB0563D33B}
    HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\ShellExecuteHooks\
     {64A31598-EEEC-4f1d-8D04-DACC1E2D5407}  
     {40910BCF-0B02-417e-8C81-BC2124376133}  
     {A5A925F3-6B88-4138-8092-16D95CD50D91}  
     {B8FD9F6C-AA0E-4fc3-A239-1C9A0CD80D47}  
     {DD13730A-FBA1-4f91-AB25-7FEB0563D33B}
    HKEY_CURRENT_USER\SOFTWARE\Microsoft\CAC
    HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\zxc
    HKEY_CURRENT_USER\SOFTWARE\Microsoft\Dstr5
    HKEY_CURRENT_USER\SOFTWARE\Microsoft\Rasap2K
    HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Dstr5
    HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Rasap2K
  • Sudden termination of the process GCASSERVALERT.EXE

Prevention

Take the following steps to help prevent infection on your computer:
  • Enable a firewall on your computer.
  • Get the latest computer updates for all your installed software.
  • Use up-to-date antivirus software.
  • Limit user privileges on the computer.
  • Use caution when opening attachments and accepting file transfers.
  • Use caution when clicking on links to webpages.
  • Avoid downloading pirated software.
  • Protect yourself against social engineering attacks.
  • Use strong passwords.
Enable a firewall on your computer
Use a third-party firewall product or turn on the Microsoft Windows Internet Connection Firewall.
Get the latest computer updates
Updates help protect your computer from viruses, worms, and other threats as they are discovered. It is important to install updates for all the software that is installed in your computer. These are usually available from vendor websites.
 
You can use the Automatic Updates feature in Windows to automatically download future Microsoft security updates while your computer is on and connected to the Internet.
Use up-to-date antivirus software
Most antivirus software can detect and prevent infection by known malicious software. To help protect you from infection, you should always run antivirus software, such as Microsoft Security Essentials, that is updated with the latest signature files. For more information, see http://www.microsoft.com/windows/antivirus-partners/.
Limit user privileges on the computer
Starting with Windows Vista and Windows 7, Microsoft introduced User Account Control (UAC), which, when enabled, allowed users to run with least user privileges. This scenario limits the possibility of attacks by malware and other threats that require administrative privileges to run.
 
You can configure UAC in your computer to meet your preferences:
Use caution when opening attachments and accepting file transfers
Exercise caution with email and attachments received from unknown sources, or received unexpectedly from known sources. Use extreme caution when accepting file transfers from known or unknown sources.
Use caution when clicking on links to webpages
Exercise caution with links to webpages that you receive from unknown sources, especially if the links are to a webpage that you are not familiar with, unsure of the destination of, or suspicious of. Malicious software may be installed in your computer simply by visiting a webpage with harmful content.
Avoid downloading pirated software
Threats may also be bundled with software and files that are available for download on various torrent sites. Downloading "cracked" or "pirated" software from these sites carries not only the risk of being infected with malware, but is also illegal. For more information, see 'The risks of obtaining and using pirated software'.
Protect yourself from social engineering attacks
While attackers may attempt to exploit vulnerabilities in hardware or software to compromise a computer, they also attempt to exploit vulnerabilities in human behavior to do the same. When an attacker attempts to take advantage of human behavior to persuade the affected user to perform an action of the attacker's choice, it is known as 'social engineering'. Essentially, social engineering is an attack against the human interface of the targeted computer. For more information, see 'What is social engineering?'.
Use strong passwords
Attackers may try to gain access to your Windows account by guessing your password. It is therefore important that you use a strong password – one that cannot be easily guessed by an attacker. A strong password is one that has at least eight characters, and combines letters, numbers, and symbols. For more information, see http://www.microsoft.com/protect/yourself/password/create.mspx.

Alert level: High
This entry was first published on: Nov 05, 2007
This entry was updated on: Apr 17, 2011

This threat is also detected as:
  • Win32/Darksma!generic (CA)
  • Trojan-Downloader.Win32.ConHook (Kaspersky)
  • Downloader-AWX (McAfee)
  • W32/ConHook (Norman)
  • Troj/ConHook (Sophos)
  • Downloader (Symantec)
  • TROJ_CONHOOK (Trend Micro)