Use of FSharp Interpreters

Status: test
Severity: medium
Log source: category process_creation, product windows
Author: Christopher Peacock @SecurePeacock, SCYTHE @scythe_io
Source: github.com/SigmaHQ/sigma

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.

Known false positives

Legitimate use by a software developer.

MITRE ATT&CK coverage

Tactic	Techniques
Execution	`T1059` Command and Scripting Interpreter

Telemetry coverage

Provider	Record / event type
Sysmon	Event ID 1: Process creation

Rule body

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

Stages and Predicates

Stage 0: `condition`

selection

Stage 1: `selection`

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

Indicators

These rows show field, operator, and value matches.

Field	Kind	Values	Search
`Image`	ends_with	`\fsi.exe` `\fsianycpu.exe`	`field:"Image" kind:ends_with`
`OriginalFileName`	eq	`fsi.exe` `fsianycpu.exe`	`field:"OriginalFileName" kind:eq`