MMC Executing Files with Reversed Extensions Using RTLO Abuse — Sigma Rule

Rule metadata and linked tags

Author: Swachchhanda Shrawan Poudel (Nextron Systems). Source status: experimental; severity: high. Source dates: 2025-02-05 / not supplied.

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

Pinned original Sigma rule · Detection Rule License 1.1

Source SHA-256: 6917f7c0e6039b4f5fd16f7a643f87e66488451279f6e6543902d7036c03277a

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: MMC Executing Files with Reversed Extensions Using RTLO Abuse
id: 9cfe4b27-1e56-48b4-b7a8-d46851c91a44
status: experimental
description: Detects malicious behavior where the MMC utility (`mmc.exe`) executes files with reversed extensions caused by Right-to-Left Override (RLO) abuse, disguising them as document formats.
references:
    - https://www.unicode.org/versions/Unicode5.2.0/ch02.pdf
    - https://en.wikipedia.org/wiki/Right-to-left_override
    - https://tria.ge/241015-l98snsyeje/behavioral2
author: Swachchhanda Shrawan Poudel (Nextron Systems)
date: 2025-02-05
tags:
    - attack.execution
    - attack.stealth
    - attack.t1204.002
    - attack.t1218.014
    - attack.t1036.002
logsource:
    category: process_creation
    product: windows
detection:
    selection_image:
        - Image|endswith: '\mmc.exe'
        - OriginalFileName: 'MMC.exe'
    selection_commandline:
        CommandLine|contains: # While looking at these files the prefix of their name will look something like csm.pdf, but in reality it is msc file
            - 'cod.msc'  # Reversed `.doc`
            - 'fdp.msc'  # Reversed `.pdf`
            - 'ftr.msc'  # Reversed `.rtf`
            - 'lmth.msc'  # Reversed `.html`
            - 'slx.msc'  # Reversed `.xls`
            - 'tdo.msc'  # Reversed `.odt`
            - 'xcod.msc'  # Reversed `.docx`
            - 'xslx.msc'  # Reversed `.xlsx`
            - 'xtpp.msc'  # Reversed `.pptx`
    condition: all of selection_*
falsepositives:
    - Legitimate administrative actions using MMC to execute misnamed `.msc` files.
    - Unconventional but non-malicious usage of RLO or reversed extensions.
level: high

Original YAML and metadata in JSON

False positives

• Legitimate administrative actions using MMC to execute misnamed `.msc` files.
• Unconventional but non-malicious usage of RLO or reversed extensions.

Source references

• https://www.unicode.org/versions/Unicode5.2.0/ch02.pdf
• https://en.wikipedia.org/wiki/Right-to-left_override
• https://tria.ge/241015-l98snsyeje/behavioral2