Fake excel spreadsheet installs a Backdoor Trojan

November 1, 2013

The Dell SonicWALL Threats Research team has received reports of a backdoor Trojan posing as an Excel spreadsheet. This Trojan targets the Windows platform and is being distributed through compromised legitimate websites. It overwrites a legitimate windows system file and connects to a remote server waiting for further instructions.

Infection Cycle:

Upon execution it creates the following files:

  • %TEMP%*chinese characters*.xls
  • %TEMP%InstallServer.exe [Detected as GAV: Laproy_2 (Trojan)]

It opens the .xls file to display its content:

InstallServer.exe then creates the following files:

  • %SYSTEM%netman.dll [Detected as GAV: Laproy_3 (Trojan)]
  • %SYSTEM%dllcachenetman.dll [Detected as GAV: Laproy_3 (Trojan)]

These copies of netman.dll will overwrite the legitimate Network Connections Manager library (netman.dll) from the user's system. The malicious copy even shares the same export functions as the real windows DLL with the addition of two extra functions to install and uninstall the malware.

The malware then communicates to a remote server, sending identifiable information that marks its presence in the victim machine. Upon confirmation, it receives the address of another remote server:

During our analysis, we have only observed this trojan constantly sending SYN to the server awaiting for further instructions:

Dell SonicWALL Gateway AntiVirus provides protection against this threat via the following signatures:

  • GAV: Laproy (Trojan)
  • GAV: Laproy_2 (Trojan)
  • GAV: Laproy_3 (Trojan)