Sigma rule: Use of FSharp Interpreters

Rule

Title

Use of FSharp Interpreters

Status

test

Level

medium

Author

Christopher Peacock @SecurePeacock, SCYTHE @scythe_io

Date

2022-06-02

Modified

2024-04-23

File

rules/windows/process_creation/proc_creation_win_fsi_fsharp_code_execution.yml

Commit

ccbb0369507f796db0b0ae6854c8a2be471b9d84

Detects the execution of FSharp Interpreters "FsiAnyCpu.exe" and "FSi.exe"
Both can be used for AWL bypass and to execute F# code via scripts or inline.

Log source

product

windows

category

process_creation

Detection

selection:
    - Image|endswith:
          - '\fsi.exe'
          - '\fsianycpu.exe'
    - OriginalFileName:
          - 'fsi.exe'
          - 'fsianycpu.exe'
condition: selection

False positives (as the rule lists them)

• Legitimate use by a software developer.

Tags

• attack.execution
• attack.t1059

MITRE ATT&CK ids this rule names

• T1059

References

• https://learn.microsoft.com/en-us/windows/security/application-security/application-control/windows-defender-application-control/design/applications-that-can-bypass-wdac
• https://bohops.com/2020/11/02/exploring-the-wdac-microsoft-recommended-block-rules-part-ii-wfc-fsi/
• https://lolbas-project.github.io/lolbas/OtherMSBinaries/FsiAnyCpu/
• https://lolbas-project.github.io/lolbas/OtherMSBinaries/Fsi/

Read on 2026-11-03T00:58:21.399Z under licence sigma-drl-1.1@d5f4c36a from https://github.com/SigmaHQ/sigma/blob/ccbb0369507f796db0b0ae6854c8a2be471b9d84/rules/windows/process_creation/proc_creation_win_fsi_fsharp_code_execution.yml