W32.Donut

SUMMARY

W32.Donut is a virus that targets .exe files that were created for the Microsoft .NET framework. W32.Donut is a concept virus. It does not have any significant chance of becoming widespread. However, it shows that virus writers are paying close attention to the new .NET architecture and are attempting to learn how to exploit it before the .NET framework is available on most systems.

For additional information about Microsoft .NET, go to http://www.microsoft.com/net/.

Protection

• Virus Definitions (Intelligent Updater) January 10, 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

On January 9, 2002, the author of this virus sent copies to several antivirus companies. The virus was named "dotNET" by its creator; Norton AntiVirus detects it as W32.Donut.

The virus targets .exe files that were created for the Microsoft .NET framework.

.NET files do not have any platform-dependent code, but they do have a small 5-byte stub. This stub executes the Mscoree.dll _CorExeMain() function, and thus the .NET MISL (intermediate language) gets control if the .NET framework is installed.

Currently a .NET application executes native code before it will execute the platform-independent code. According to Microsoft, this native code will be removed, and the operating system itself will recognize and execute .NET images.

The virus infects .NET executables by attacking the 5-byte jump to the _CorExeMain() function. It replaces this jump with another one to point into the last section of the executable, and it overwrites its .reloc section with itself and nullifies the relocation directory.

When an infected file is executed, the virus code gets control as a 386 application. The virus checks the platform and only infects Windows 2000 and later. It attempts to infect all .exe files that exist in the same folder as the virus and in up to 20 folders above it. It must be noted that there are many assumptions made about the .NET file structure which will not be the case with most executables. Nonetheless, many C# complied files would have similar structure. The virus author worked with the Beta 2 .NET framework and it checks files for the new header signature "BSJB". The virus would therefore ignore the .NET Beta 1 file format. The virus injects itself into the file by using standard virus techniques to get access to the API addresses that it needs to call. Most APIs are referenced in the code as CRCs. The virus also modifies the checksum field of PE headers to make the image look valid.

W32.Donut also injects a small piece of MSIL code and metadata into the infected file. These was designed to execute the payload of the virus and display the following message box with a 1-in-10 chance:



However, due to a bug in the virus code, the payload does not execute as designed, and the message is not displayed. Instead, the virus code will be executed mutiple times instead of the payload being exectued. This is because the entry point call was not fixed up to the original 5 bytes from the copied infected program and will not call the _CorExeMain() function.

Infected files look like regular program files. The virus first drops a file with a fixed .NET header pointer in the Data folder, as well as the jump to the _CorExeMain() function so the application can run as a .NET file whenever the framework is installed. In this case, the MSIL code of the virus gets control and displays the message in the previous paragraph. When the host application returns, the virus creates another copy of the file, and in this case the original MSIL code is executed and the file runs normally. During this process the virus creates a temporary file with the name of the host executable and a space. For example, Runme.exe will have the temporary file "Runme .exe".

W32.Donut is a concept virus, and it has little chance of becoming widespread. It does demonstrate that virus writers are looking at the new Microsoft .NET architecture and that they will attempt to learn it before the framework is available on most systems.

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.