>Fsi.exe

64-bit FSharp (F#) Interpreter included with Visual Studio and DotNet Core SDK.

Default Executable Paths

C:\Program Files\dotnet\sdk\<version>\FSharp\fsi.exe

C:\Program Files (x86)\Microsoft Visual Studio\2019\Professional\Common7\IDE\CommonExtensions\Microsoft\FSharp\fsi.exe

Integrity & Metadata

Category Count: 2 weaponized patterns

Forensics & Triage Insight

Documented in LOLBAS project by Jimmy (@bohops).

Detection Engineering Notes

Monitor process creation events where FileName is "Fsi.exe" or command line references known attack arguments.

Executive Summary & AI Quick Reference

High-Fidelity Telemetry Profile

Fsi.exe is a native utility pre-installed on Windows systems. Threat actors and red teams abuse Fsi.exe as a Living-off-the-Land Binary (LOLBin) to achieve Defense Evasion without triggering traditional antivirus file-hash alerts. Legitimate system administrators use it for routine administration, making behavioral command-line telemetry essential for differentiation.

Primary Abused Tactics

Defense Evasion

Minimum Privilege

User

Key Telemetry Source

Sysmon 1 / 4688

ATT&CK Mapping

T1059

Weaponized Parameters & Proof-of-Concepts

Documented attack commands, parameter flags, and privilege prerequisites.

2 documented methods

Filter Purpose:

Defense EvasionPrivileges: User

T1059: Command and Scripting Interpreter

Execute F# code via script file

fsi.exe {PATH:.fsscript}

Defense EvasionPrivileges: User

T1059: Command and Scripting Interpreter

Execute F# code via interactive command line

fsi.exe

SIEM & EDR Detection Rules

Production telemetry rules configured for Microsoft Defender, Sigma, and Splunk.

Telemetry & SIEM Detection Suite (Fsi.exe)

Validated against MITRE T1059

Fsi.exe Microsoft Defender Querykql

1// Microsoft Defender XDR / Microsoft Sentinel
2// Tactic: Defense Evasion
3DeviceProcessEvents
4| where FileName =~ "Fsi.exe"
5| where ProcessCommandLine has_any ("Fsi")
6| project
7    Timestamp,
8    DeviceName,
9    AccountName,
10    InitiatingProcessParentFileName,
11    InitiatingProcessFileName,
12    InitiatingProcessCommandLine,
13    FileName,
14    FolderPath,
15    ProcessCommandLine,
16    MD5,
17    SHA256
18| sort by Timestamp desc

Mitigation, Hardening & Prevention

Recommended defensive controls to block or constrain Fsi.exe abuse.

Application Whitelisting

Enforce Windows Defender Application Control (WDAC) or AppLocker publisher rules to prevent execution outside approved enterprise administration scripts.

Attack Surface Reduction

Enable ASR rules: Block executable content from email client and webmail and Block Office applications from creating child processes spawning Fsi.exe.

Behavioral Alerting

Deploy the KQL, Sigma, and Splunk detection queries below. Alert on anomalous child processes, web requests to unrated external IPs, or encoded payload strings.

Frequently Asked Questions (FAQ) • Fsi.exe

Key operational questions and defensive answers about Fsi.exe LOLBin tradecraft.

What is Fsi.exe and why is it classified as a LOLBin?

Fsi.exe is a legitimate, pre-installed or trusted utility on Windows. It is classified as a Living-off-the-Land Binary (LOLBin) because adversaries abuse its built-in functionality to perform malicious operations—such as Defense Evasion—without dropping custom malware binaries on disk.

How do threat actors abuse Fsi.exe in cyber intrusions?

Adversaries leverage Fsi.exe to execute stealthy actions while evading signature-based security controls. For example, attackers execute commands like "fsi.exe {PATH:.fsscript}" to achieve Defense Evasion, masquerading behind legitimate system process lineage.

How can SOC analysts detect suspicious Fsi.exe activity?

Detection engineers monitor process creation events (Windows Event ID 4688 or Sysmon Event ID 1; Linux auditd EXECVE; macOS Endpoint Security) inspecting anomalous command line arguments, unusual parent-child process relationships (e.g. Office apps or web servers spawning Fsi.exe), and unexpected network connections.

What MITRE ATT&CK techniques does Fsi.exe map to?

Fsi.exe maps to MITRE ATT&CK technique(s): T1059. These techniques cover adversary actions across tactics such as Defense Evasion, Execution, Persistence, and Command and Control.

How can organizations mitigate and restrict Fsi.exe?

Mitigation strategies include implementing Application Control policies (AppLocker or Windows Defender Application Control - WDAC), enforcing Attack Surface Reduction (ASR) rules, configuring strict privilege separation (disallowing standard users from running sensitive switches), and alerting on execution from non-standard directories.

Curated & Verified by

Sharon Ben Moshe

Cybersecurity Researcher & Detection Engineer • Check Point

LinkedIn Profile Methodology & Papers

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