Use of FSharp Interpreters — Sigma Rule

Rule metadata and linked tags

Author: Christopher Peacock @SecurePeacock, SCYTHE @scythe_io. Source status: test; severity: medium. Source dates: 2022-06-02 / 2024-04-23.

{
  "category": "process_creation",
  "product": "windows"
}

Pinned original Sigma rule · Detection Rule License 1.1

Source SHA-256: bef3302765f6b2bb6e29b89aecb9a2279ae2f3c9e3587d692db7650a4f5c1f18

Detection logic

Original source YAML. Source-tag agreement is not proof of complete semantic coverage. This rule has not been compiled for a SIEM backend or validated against live telemetry here.

title: Use of FSharp Interpreters
id: b96b2031-7c17-4473-afe7-a30ce714db29
status: test
description: |
    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.
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/
author: Christopher Peacock @SecurePeacock, SCYTHE @scythe_io
date: 2022-06-02
modified: 2024-04-23
tags:
    - attack.execution
    - attack.t1059
logsource:
    category: process_creation
    product: windows
detection:
    selection:
        - Image|endswith:
              - '\fsi.exe'
              - '\fsianycpu.exe'
        - OriginalFileName:
              - 'fsi.exe'
              - 'fsianycpu.exe'
    condition: selection
falsepositives:
    - Legitimate use by a software developer.
level: medium

Original YAML and metadata in JSON

False positives

• Legitimate use by a software developer.

Source 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/