CISA received three files for analysis. The files included three webshells written in PHP: Hypertext Preprocessor (PHP), Active Server Pages Extended (ASPX), and .NET Dynamic-Link Library (DLL). The sample “sd.php” is highly obfuscated and uses rot13 algorithm, zlib for compression and base64 encoding for obfuscation. The “osker.aspx” webshell code was padded with junk code. The .NET DLL webshell is a .NET compiled version of osker.aspx. The samples are interactive webshells and have the ability to upload and manage files, create directories and files, and execute commands on the target machine.


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