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Detection rules › Sigma

Change Default File Association To Executable Via Assoc

Status
test
Severity
high
Log source
product windows, category process_creation
Author
Nasreddine Bencherchali (Nextron Systems)
Source
github.com/SigmaHQ/sigma

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.

MITRE ATT&CK coverage

TacticTechniques
PersistenceT1546.001 Event Triggered Execution: Change Default File Association
Privilege EscalationT1546.001 Event Triggered Execution: Change Default File Association

Event coverage

ProviderEventTitle
SysmonEvent ID 1Process creation

Rule body yaml

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

Stages and Predicates

Stage 0: condition

all of selection_* and not filter

Stage 1: selection_img

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

Stage 2: selection_cli

selection_cli:
    CommandLine|contains|all:
        - 'assoc '
        - 'exefile'

Stage 3: not filter

filter:
    CommandLine|contains: '.exe=exefile'

Exclusions

Top-level NOT(...) conjuncts: predicates this rule actively suppresses.

FieldKindExcluded values
CommandLinematch.exe=exefile

Indicators

Each row is a field, operator, and value that the rule matches. The corpus column counts how many other rules in the catalog look for the same combination: high numbers point to widely-used, community-vetted indicators. Blank or 1 shows that the indicator is specific to this rule.

FieldKindValues
CommandLinematch
  • assoc
  • exefile
Imageends_with
  • \cmd.exe corpus 130 (sigma 130)
OriginalFileNameeq
  • Cmd.Exe corpus 65 (sigma 43, splunk 17, elastic 5)