Change Default File Association Via Assoc

Status: test
Severity: low
Log source: category process_creation, product windows
Author: Timur Zinniatullin, oscd.community
Source: github.com/SigmaHQ/sigma

Detects file association changes using the builtin "assoc" command. 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.

Known false positives

Admin activity

MITRE ATT&CK coverage

Tactic	Techniques
Persistence	`T1546.001` Event Triggered Execution: Change Default File Association
Privilege Escalation	`T1546.001` Event Triggered Execution: Change Default File Association

Telemetry coverage

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

Rule body

title: Change Default File Association Via Assoc
id: 3d3aa6cd-6272-44d6-8afc-7e88dfef7061
related:
    - id: ae6f14e6-14de-45b0-9f44-c0986f50dc89
      type: similar
status: test
description: |
    Detects file association changes using the builtin "assoc" command.
    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://github.com/redcanaryco/atomic-red-team/blob/f339e7da7d05f6057fdfcdd3742bfcf365fee2a9/atomics/T1546.001/T1546.001.md
author: Timur Zinniatullin, oscd.community
date: 2019-10-21
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: 'assoc'
    condition: all of selection_*
falsepositives:
    - Admin activity
level: low
regression_tests_path: regression_data/rules/windows/process_creation/proc_creation_win_cmd_assoc_execution/info.yml

Stages and Predicates

Stage 0: `condition`

all of selection_*

Stage 1: `selection_img`

selection_img:
    - Image|endswith: '\cmd.exe'
    - OriginalFileName: 'Cmd.Exe'

Stage 2: `selection_cli`

selection_cli:
    CommandLine|contains: 'assoc'

Indicators

These rows show field, operator, and value matches.

Field	Kind	Values	Search
`CommandLine`	match	`assoc`	`field:"CommandLine" kind:match value:"assoc"`
`Image`	ends_with	`\cmd.exe` corpus 130 (sigma 130)	`field:"Image" kind:ends_with value:"\cmd.exe"`
`OriginalFileName`	eq	`Cmd.Exe` corpus 81 (sigma 43, elastic 21, splunk 17)	`field:"OriginalFileName" kind:eq value:"Cmd.Exe"`