Win32/Canahom is a family of trojans that can be used to allow unauthorized access and control of an affected machine. Canahom consists of two components; a dropper and a DLL component that may be used to download and execute arbitrary files and to perform Distributed Denial of Service (DDoS) attacks.
Installation
The dropper component may be detected as either TrojanDropper:Win32/Canahom.A or TrojanDropper:Win32/Canahom.B. When the dropper component is executed, it initially checks for previous or 'outdated' installations of itself by querying the value of the following registry key:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon\Notify\sysfldr\ver
Should it find a previous installation, it moves the existing installed file from "<system folder>\sysfldr.dll" to "<system folder>\fldrsys.dll". Otherwise it creates a DLL, <system folder>\sysfldr.dll, with the SYSTEM attribute set. It also sets the timestamp of this file to that of 'kernel32.dll'
Note - <system folder> refers to a variable location that is determined by the malware by querying the Operating System. The default installation location for the System folder for Windows 2000 and NT is C:\Winnt\System32; and for XP and Vista is C:\Windows\System32.
It creates a winlogon notification package for the dropped DLL by making the following modification to the registry:
Adds value: DLLName
With data: "sysfldr.dll"
To subkey: HKEY_LOCAL_MACHINE\Software\Microsoft\WindowsNT\CurrentVersion\Winlogon\Notify\sysfldr
Note: A Winlogon notification package is a DLL that exports functions that handle Winlogon events. For example, when a user logs onto the system, Winlogon calls each notification package's logon event handler function to provide information about the event.
If this fails, the dropper creates a scheduled task to launch the payload (contained in the DLL).
The dropper also creates a new 'svchost.exe' process. It then injects code into the newly created process, which loads the DLL 'sysfldr.dll', and then deletes the dropper.
Payload
Downloads and Executes Arbitrary Files/Performs DDoS Attacks
The DLL component performs the trojan's payload and may be detected as Trojan:Win32/Canahom.gen!A, B, or C. This component attempts to contact a control server in order to retrieve instructions. These instructions may include the following:
- Download and execute arbitrary file
- Download and update itself
- Perform denial of service attack using different types of flood such as ICMP, SYN, TCP, UDP and HTTP
- Delete "<system folder>\fldrsys.dll"
- Create a thread which attempts to continually set the registry keys associated with the winlogon notification package.
Those variants detected as Trojan:Win32/Canahom.gen!B, and Trojan:Win32/Canahom.gen!C may also perform the following actions:
- Attempt to stop the services 'SharedAccess' and 'wscsvc'.
- Disable the 'SharedAccess', 'wscsvc', and 'wuauserv' services via registry modifications.
Analysis by Scott Molenkamp
Take the following steps to help prevent infection on your system:
Enable a firewall on your computer.
Get the latest computer updates for all your installed software.
Use up-to-date antivirus software.
Use caution when opening attachments and accepting file transfers.
Use caution when clicking on links to web pages.
Enable a firewall on your computer
Use a third-party firewall product or turn on the Microsoft Windows Internet Connection Firewall.
To turn on the Windows Firewall in Windows Vista
Click Start, and click Control Panel.
Click Security.
Click Turn Windows Firewall on or off.
Select On.
Click OK.
To turn on the Internet Connection Firewall in Windows XP
Click Start, and click Control Panel.
Click Network and Internet Connections. If you do not see Network and Internet Connections, click Switch to Category View.
Click Change Windows Firewall Settings.
Select On.
Click OK.
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.
To turn on Automatic Updates in Windows Vista
Click Start, and click Control Panel.
Click System and Maintainance.
Click Windows Updates.
Select a setting. Microsoft recommends selecting Install updates automatically and choose a time that is convenient for you. If you do not choose Automatic, but you choose to be notified when updates are ready, a notification balloon appears when new downloads are available to install. Click the notification balloon to review and install the updates.
To turn on Automatic Updates in Windows XP
Click Start, and click Control Panel.
Click System.
Click Automatic Updates.
Select a setting. Microsoft recommends selecting Automatic. If you do not choose Automatic, but you choose to be notified when updates are ready, a notification balloon appears when new downloads are available to install. Click the notification balloon to review and install the updates.
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 that is updated with the latest signature files. Antivirus software is available from several sources. For more information,
see http://www.microsoft.com/protect/computer/viruses/vista.mspx.Use caution when opening attachments and accepting file transfers
Exercise caution with e-mail 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 web pages
Exercise caution with links to web pages that you receive from unknown sources, especially if the links are to a web page that you are not familiar with or are suspicious of. Malicious software may be installed in your system simply by visiting a web page with harmful content.
