Summary

Fu is one of the most widely utilized rootkits in the wild. Other malware, such as rbot and sdbot variants, have used its features to hide themselves. Fu is a kernel-mode rootkit that modifies kernel data structures, which allows it to hide e.g. processes.

Detection

Summary

Fu rootkit does not try to hide itself on the compromised machine. Since it is not able to hide files, the hidden malware can still be detected and removed with signature-based AV scanners. F-Secure BlackLight engine is able to detect all processes that have been hidden by Fu rootkit.

F-Secure BlackLight engine is able to disable the processes hidden by Fu rootkit. To proceed, follow the information below based on which version you are using:

F-Secure BlackLight Beta:

http://www.f-secure.com/blacklight/help/

F-Secure Internet Security 2006:

http://www.f-secure.com/security-risks/defaulthiddenitemdescription.shtml

Additional Details

Summary

Fu rootkit is publicly available in binary and source code format. The technical information below is based on the public version.

Installation

The public version of Fu rootkit consists of two files: one executable file (.exe) and one kernel-mode driver file (.sys). The driver does the actual hiding where as the executable file is used to configure and command the driver.

Fu requires administrative privileges to install successfully. The executable file installs and loads the driver. This results in one registry key:

• HKLM\SYSTEM\CurrentControlSet\Services\[driver_name]

where [driver_name] represents the name of the driver file without the .sys extension.

Hiding Technique

Fu allows the intruder to hide information from user-mode applications and even from kernel-mode modules. Following items can be hidden:

• Processes
• Kernel-mode modules

Fu hides information by directly modifying certain kernel data structures used by the operating system. Specifically, it removes to-be-hidden entries from two linked lists with symbolic names: PsActiveProcessHead and PsLoadedModuleList.

In addition, Fu is able to modify a process' token to change its security context. This has two impacts on the compromised system. First, it can modify privileges and access rights of any running process. Second, it can fool security auditing by replacing the owner SID of any running process.

Write-up: Kimmo Kasslin

Technical Details: Kimmo Kasslin, January 23, 2006

Description Updated: Kimmo Kasslin, January 23, 2006

F-Secure Corporation