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.lnk file is the format of the Windows’ shortcuts. The vulnerability recently found in this format actually lies in the way Windows processes the Control Panel shortcuts. Normally, these shortcuts are processed as below:

Control Panel

Each Control Panel shortcut is linked to an executable file. For example, shortcut “Automatic Update” is linked to Windows’ update utility. Windows, specifically Windows Shell, will load a PE file with .cpl extension to get icon from its resource to display this shortcut’s icon. In this case, the PE file loaded is “C:\Windows\System32\wuaucpl.cpl”.

Taking advantage of Windows Shell’s loading PE file to display the shortcut’s icon, hacker is able to create a Control Panel shortcut file with a path to a  malicious file. When Windows Shell performs the abovementioned steps to display shortcut’s icon, the malicious file will be loaded. The figure below describes the parsing process of crafted Control Panel shortcut to load malicious file:

The parsing process of crafted Control Panel shortcut

Below is the description of the Control Panel shortcut format which is used to exploit the vulnerability:

Crafted shortcut file format

So, to execute an arbitrary malicious file (in this case, it is DLL file), which may be located in a USB drive just like Autorun feature, hacker only needs to create the lnk format with the path in “fake cpl path file” linking to the malicious file.

Bui Quang Minh

Senior Security Researcher

Bkis Global Taskforce

 

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