Threat behavior
When Trojan:Win32/Yidvar.A runs, it first tries to create a Mutex named "CMD". If a Mutex with this name already exists, then Trojan:Win32/Yidvar.A terminates. Trojan:Win32/Yidvar.A also tries to create a Mutex named "MAIN". If a Mutex with this name already exists, then Trojan:Win32/Yidvar.A terminates.
Trojan:Win32/Yidvar.A then assigns itself the SeDebugPrivilege privilege. It gets the process ID of “explorer.exe” using NtQuerySystemInformation; if it fails, it will keep looping until it can get the process ID in this manner. Because of this, it will just run in a loop and won’t do anything malicious on non-NT-based systems (Win95/98/ME).
Trojan:Win32/Yidvar.A obtains the MAC address of the local computer’s network card, the Windows version, the hostname of local machine, and the ANSI code-page identifier and Locale Info (possibly in an attempt to identify the infected user's language).
The trojan then injects 131072 bytes of code (8 functions) and 10643 bytes of decrypted text (along with the local information gathered above) into the explorer.exe process, creating 5 remote threads in the explorer.exe process to start 5 of the 8 injected functions.
Trojan:Win32/Yidvar.A copies its original trojan file to the following locations:
<system folder>\inbackup.exe
<system fodler>\service.exe
The trojan sets the timestamps on those two files to match the file timestamp found on <sysdir>\kernel32.dll and then deletes the original trojan file.
Trojan:Win32/Yidvar.A sets registry value "Userinit" = "<sysdir>\userinit.exe,<sysdir>\service.exe" under registry key "HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon"
It will then keep sending HTTP GET requests to a Web site specified in the trojan until it gets returned text from the webserver with first character 'y'. The trojan also downloads up to 4K of text from http://winpcap.diyserver.net/httpdocs/mm/:AA-BB-CC-DD-EE-FF/Cmwhite. It will keep trying to download from this URL until it succeeds.
It then uses HTTP POST to send data to a remote Web site.
Prevention
Take the following steps to help prevent infection on your system:
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Enable a firewall on your computer.
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Get the latest computer updates.
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Use up-to-date antivirus software.
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Use caution with attachments and file transfers.
Enable a firewall on your computer
Use a third-party firewall product or turn on the Microsoft Windows XP Internet Connection Firewall.
To turn on the Internet Connection Firewall in Windows XP
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Click Start, and click Control Panel.
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Click Network and Internet Connections. If you do not see Network and Internet Connections, click Switch to Category View.
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Click Change Windows Firewall Settings.
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Select On.
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Click OK.
Get the latest computer updates
Updates help protect your computer from viruses, worms, and other threats as they are discovered. You can use the Automatic Updates feature in Windows XP to automatically download future Microsoft security updates while your computer is on and connected to the Internet.
To turn on Automatic Updates in Windows XP
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Click Start, and click Control Panel.
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Click System.
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Click Automatic Updates.
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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/athome/security/downloads/default.mspx
Use caution with attachments and 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.
