huntrule

Windows Process Execution of F# Interpreters (Fsi.exe, FsiAnyCpu.exe)

Flags execution of F# interpreter binaries fsi.exe and fsianycpu.exe on Windows.

FreeReviewedSigma · Medium · v2
Product
windows
Category
process_creation
Author
Christopher Peacock @SecurePeacock, SCYTHE @scythe_io (SigmaHQ), DRL 1.1
Published
2022-06-02
Updated
2026-07-31
AnalysisDetection

ATT&CK techniques

Execution

  1. Recon

  2. Resource Dev

  3. Initial Access

  4. Execution

  5. Persistence

  6. Priv Esc

  7. Defense Evasion

  8. Cred Access

  9. Discovery

  10. Lateral Movement

  11. Collection

  12. C2

  13. Exfiltration

  14. Impact

What it detects

This rule flags process creation events where the executed binary is the F# interpreter, specifically Fsi.exe (Fsi.exe / fsianycpu.exe) or FsiAnyCpu.exe (fsianycpu.exe). Such interpreters can enable execution of F# code from scripts or inline content, which attackers may use to run code within a Windows environment. It relies on process creation telemetry, matching on the executable path ending with the interpreter names and the OriginalFileName values.

Reporting behind it

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Malicious IIS Worker Process Spawning Command Shell after ASP.NET ViewState Injection (via process_creation)
Suspicious MSSQL xp_cmdshell Enablement via sp_configure (via process_creation)
Windows Process Execution of F# Interpreters (Fsi.exe, FsiAnyCpu.exe)
Pivot detection · T1059 · 9 related

Changelog

v2
  1. v2
    Candidate ingested via manual entry.2026-07-31
  2. v1
    No changelog recorded for this version.2026-07-30

Detection content is published as a reviewed draft. Tune thresholds and exclusions against your own telemetry before enabling this rule for alerting.