LOLBAS/yml/OtherMSBinaries/Fsi.yml

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---
Name: Fsi.exe
Description: 64-bit FSharp (F#) Interpreter included with Visual Studio and DotNet Core SDK.
Author: Jimmy (@bohops)
Created: 2021-09-26
Commands:
- Command: fsi.exe c:\path\to\test.fsscript
Description: Execute F# code via script file
Usecase: Execute payload with Microsoft signed binary to bypass WDAC policies
Category: AWL Bypass
Privileges: User
MitreID: T1059
OperatingSystem: Windows 10 2004 (likely previous and newer versions as well)
- Command: fsi.exe
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Description: Execute F# code via interactive command line
Usecase: Execute payload with Microsoft signed binary to bypass WDAC policies
Category: AWL Bypass
Privileges: User
MitreID: T1059
OperatingSystem: Windows 10 2004 (likely previous and newer versions as well)
Full_Path:
- Path: C:\Program Files\dotnet\sdk\[sdk version]\FSharp\fsi.exe
- Path: C:\Program Files (x86)\Microsoft Visual Studio\2019\Professional\Common7\IDE\CommonExtensions\Microsoft\FSharp\fsi.exe
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Code_Sample:
- Code: https://gist.github.com/NickTyrer/51eb8c774a909634fa69b4d06fc79ae1
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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
- IOC: Fsi.exe execution may be suspicious on non-developer machines
Resources:
- Link: https://twitter.com/NickTyrer/status/904273264385589248
- Link: https://bohops.com/2020/11/02/exploring-the-wdac-microsoft-recommended-block-rules-part-ii-wfc-fsi/
Acknowledgement:
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- Person: Nick Tyrer
Handle: '@NickTyrer'
- Person: Jimmy
Handle: '@bohops'
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---