Change Default File Association To Executable Via Assoc

SIGMA detection rule by Nasreddine Bencherchali (Nextron Systems) · NOASSERTION

Detects when a program changes the default file association of any extension to an executable. When a file is opened, the default program used to open the file (also called the file association or handler) is checked. File association selections are stored in the Windows Registry and can be edited by users, administrators, or programs that have Registry access or by administrators using the built-in assoc utility. Applications can modify the file association for a given file extension to call an arbitrary program when a file with the given extension is opened.

title: Change Default File Association To Executable Via Assoc
id: ae6f14e6-14de-45b0-9f44-c0986f50dc89
related:
    - id: 3d3aa6cd-6272-44d6-8afc-7e88dfef7061
      type: derived
status: test
description: |
    Detects when a program changes the default file association of any extension to an executable.
    When a file is opened, the default program used to open the file (also called the file association or handler) is checked. File association selections are stored in the Windows Registry and can be edited by users, administrators, or programs that have Registry access or by administrators using the built-in assoc utility. Applications can modify the file association for a given file extension to call an arbitrary program when a file with the given extension is opened.
references:
    - https://learn.microsoft.com/en-us/windows-server/administration/windows-commands/assoc
author: Nasreddine Bencherchali (Nextron Systems)
date: 2022-06-28
modified: 2023-03-06
tags:
    - attack.privilege-escalation
    - attack.persistence
    - attack.t1546.001
logsource:
    category: process_creation
    product: windows
detection:
    selection_img:
        - Image|endswith: '\cmd.exe'
        - OriginalFileName: 'Cmd.Exe'
    selection_cli:
        CommandLine|contains|all:
            - 'assoc '
            - 'exefile'
    filter:
        CommandLine|contains: '.exe=exefile'
    condition: all of selection_* and not filter
falsepositives:
    - Unknown
level: high

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dc504148-61fe-49e6-b1ec-ec446755ab62

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Executable code, not a declarative rule

This submission is arbitrary [[ currentRule.format ? currentRule.format.toUpperCase() : '' ]] code, not a declarative detection rule — review it as you would any script before running it, and note that it cannot be safely aggregated or auto-executed alongside YARA/Sigma/Suricata/CRS rules.

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Format & documentation suggestions

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