Suspicious Calculator Usage

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

Detects suspicious use of 'calc.exe' with command line parameters or in a suspicious directory, which is likely caused by some PoC or detection evasion.

MITRE ATT&CK coverage

Tactic	Techniques
Stealth	`T1036` Masquerading

Telemetry coverage

Provider	Record / event type
Sysmon	Event ID 1: Process creation
Security-Auditing	Event ID 4688: A new process has been created.

Rule body

title: Suspicious Calculator Usage
id: 737e618a-a410-49b5-bec3-9e55ff7fbc15
status: test
description: |
    Detects suspicious use of 'calc.exe' with command line parameters or in a suspicious directory, which is likely caused by some PoC or detection evasion.
references:
    - https://twitter.com/ItsReallyNick/status/1094080242686312448
author: Florian Roth (Nextron Systems)
date: 2019-02-09
modified: 2023-11-09
tags:
    - attack.stealth
    - attack.t1036
logsource:
    category: process_creation
    product: windows
detection:
    selection_1:
        CommandLine|contains: '\calc.exe '
    selection_2:
        Image|endswith: '\calc.exe'
    filter_main_known_locations:
        Image|contains:
            - ':\Windows\System32\'
            - ':\Windows\SysWOW64\'
            - ':\Windows\WinSxS\'
    condition: selection_1 or ( selection_2 and not filter_main_known_locations )
falsepositives:
    - Unknown
level: high

Stages and Predicates

Stage 0: `condition`

selection_1 or ( selection_2 and not filter_main_known_locations )

Stage 1: `selection_1`

selection_1:
    CommandLine|contains: '\calc.exe '

Stage 2: `selection_2`

selection_2:
    Image|endswith: '\calc.exe'

Stage 3: `not filter_main_known_locations`

filter_main_known_locations:
    Image|contains:
        - ':\Windows\System32\'
        - ':\Windows\SysWOW64\'
        - ':\Windows\WinSxS\'

Exclusions

The rule actively suppresses these predicates.

Field	Kind	Excluded values	Search
`Image`	match	`:\Windows\SysWOW64\`	`excludes:Image` `field:"Image" value:":\Windows\SysWOW64\"`
`Image`	match	`:\Windows\System32\`	`excludes:Image` `field:"Image" value:":\Windows\System32\"`
`Image`	match	`:\Windows\WinSxS\`	`excludes:Image` `field:"Image" value:":\Windows\WinSxS\"`

Indicators

These rows show field, operator, and value matches.

Field	Kind	Values	Search
`CommandLine`	match	`\calc.exe`	`field:"CommandLine" kind:match value:"\calc.exe "`
`Image`	ends_with	`\calc.exe` corpus 14 (sigma 14)	`field:"Image" kind:ends_with value:"\calc.exe"`