Fsi.exe

Commands

1. AWL BypassUser T1059

   Execute payload with Microsoft signed binary to bypass WDAC policies

   fsi.exe {PATH:.fsscript}

   Execute F# code via script file

   os: Windows 10 2004 (likely previous and newer versions as well)

   Execute: FSharp

2. AWL BypassUser T1059

   Execute payload with Microsoft signed binary to bypass WDAC policies

   fsi.exe

   Execute F# code via interactive command line

   os: Windows 10 2004 (likely previous and newer versions as well)

   Execute: FSharp

Detection

• Elastic https://github.com/elastic/detection-rules/blob/414d32027632a49fb239abb8fbbb55d3fa8dd861/rules/windows/defense_evasion_unusual_process_network_connection.toml
• Elastic https://github.com/elastic/detection-rules/blob/414d32027632a49fb239abb8fbbb55d3fa8dd861/rules/windows/defense_evasion_network_connection_from_windows_binary.toml
• BlockRule https://docs.microsoft.com/en-us/windows/security/threat-protection/windows-defender-application-control/microsoft-recommended-block-rules
• IOCFsi.exe execution may be suspicious on non-developer machines
• Sigma https://github.com/SigmaHQ/sigma/blob/6b34764215b0e97e32cbc4c6325fc933d2695c3a/rules/windows/process_creation/proc_creation_win_lolbin_fsharp_interpreters.yml

Code samples

• https://gist.github.com/NickTyrer/51eb8c774a909634fa69b4d06fc79ae1

Resources

• https://twitter.com/NickTyrer/status/904273264385589248
• https://bohops.com/2020/11/02/exploring-the-wdac-microsoft-recommended-block-rules-part-ii-wfc-fsi/