Suspicious Csi.exe Usage — Sigma Rule

Rule metadata and linked tags

Author: Konstantin Grishchenko, oscd.community. Source status: test; severity: medium. Source dates: 2020-10-17 / 2022-07-11.

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

Pinned original Sigma rule · Detection Rule License 1.1

Source SHA-256: 0836505a9dc5c390e21a2f2005bdaec06d5c755659274b171394280621be5ab8

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: Suspicious Csi.exe Usage
id: 40b95d31-1afc-469e-8d34-9a3a667d058e
status: test
description: Csi.exe is a signed binary from Microsoft that comes with Visual Studio and provides C# interactive capabilities. It can be used to run C# code from a file passed as a parameter in command line. Early version of this utility provided with Microsoft “Roslyn” Community Technology Preview was named 'rcsi.exe'
references:
    - https://lolbas-project.github.io/lolbas/OtherMSBinaries/Csi/
    - https://lolbas-project.github.io/lolbas/OtherMSBinaries/Rcsi/
    - https://enigma0x3.net/2016/11/21/bypassing-application-whitelisting-by-using-rcsi-exe/
    - https://twitter.com/Z3Jpa29z/status/1317545798981324801
author: Konstantin Grishchenko, oscd.community
date: 2020-10-17
modified: 2022-07-11
tags:
    - attack.lateral-movement
    - attack.execution
    - attack.stealth
    - attack.t1072
    - attack.t1218
logsource:
    category: process_creation
    product: windows
detection:
    selection_img:
        - Image|endswith:
              - '\csi.exe'
              - '\rcsi.exe'
        - OriginalFileName:
              - 'csi.exe'
              - 'rcsi.exe'
    selection_cli:
        Company: 'Microsoft Corporation'
    condition: all of selection*
falsepositives:
    - Legitimate usage by software developers
level: medium

Original YAML and metadata in JSON

False positives

• Legitimate usage by software developers

Source references

• https://lolbas-project.github.io/lolbas/OtherMSBinaries/Csi/
• https://lolbas-project.github.io/lolbas/OtherMSBinaries/Rcsi/
• https://enigma0x3.net/2016/11/21/bypassing-application-whitelisting-by-using-rcsi-exe/
• https://twitter.com/Z3Jpa29z/status/1317545798981324801