MUI Poisoning in practice

In my old post I discussed the idea of MUI poisoning. Today I want to show a practical example of this technique – one that has an interesting impact on incident response efforts.

Some security solutions rely on running local, native OS binaries to collect information from the system. Tool like netstat, ipconfig, etc. are executed on regular basis and data is collected and aggregated in some log repository.

These local tools often rely on MUI files and this is where we step in. By modifying the MUI files of selected tools one could force these tools to return complete garbage. For instance, the following example shows netstat.exe where its MUI was modified to always return a source IP where the destination IP would be listed. The change can be made using the old tool Resource Hacker:

Once we replace the MUI file, netstat.exe will return stuff like this:

This anti-forensic technique could be potentially expanded to cover every single piece of software that relies on external language files (let it be MUI, or anything else). As long as these format string patterns can be manipulated security software could be forced to present garbage output; for instance – malware alerts reporting wrong paths (e.g. hardcoded, non existing paths), or Windows Event logs reporting misleading information.