MMC Executing Files with Reversed Extensions Using RTLO Abuse

Status: experimental
Severity: high
Log source: category process_creation, product windows
Author: Swachchhanda Shrawan Poudel (Nextron Systems)
Source: github.com/SigmaHQ/sigma

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.

Known false positives

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

MITRE ATT&CK coverage

Tactic	Techniques
Execution	`T1204.002` User Execution: Malicious File
Stealth	`T1036.002` Masquerading: Right-to-Left Override `T1218.014` System Binary Proxy Execution: MMC

Telemetry coverage

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

Rule body

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

Stages and Predicates

Stage 0: `condition`

all of selection_*

Stage 1: `selection_image`

selection_image:
    - Image|endswith: '\mmc.exe'
    - OriginalFileName: 'MMC.exe'

Stage 2: `selection_commandline`

selection_commandline:
    CommandLine|contains:
        - 'cod.msc'
        - 'fdp.msc'
        - 'ftr.msc'
        - 'lmth.msc'
        - 'slx.msc'
        - 'tdo.msc'
        - 'xcod.msc'
        - 'xslx.msc'
        - 'xtpp.msc'

Indicators

These rows show field, operator, and value matches.

Field	Kind	Values	Search
`CommandLine`	match	`cod.msc` `fdp.msc` `ftr.msc` `lmth.msc` `slx.msc` `tdo.msc` `xcod.msc` `xslx.msc` `xtpp.msc`	`field:"CommandLine" kind:match`
`Image`	ends_with	`\mmc.exe` corpus 5 (sigma 5)	`field:"Image" kind:ends_with value:"\mmc.exe"`
`OriginalFileName`	eq	`MMC.exe` corpus 2 (sigma 2)	`field:"OriginalFileName" kind:eq value:"MMC.exe"`