New or Renamed User Account with '$' Character

Status: test
Severity: medium
Log source: product windows, service security
Author: Ilyas Ochkov, oscd.community
Source: github.com/SigmaHQ/sigma

Detects the creation of a user with the "$" character. This can be used by attackers to hide a user or trick detection systems that lack the parsing mechanisms.

MITRE ATT&CK coverage

Tactic	Techniques
Stealth	`T1036` Masquerading

Telemetry coverage

Provider	Record / event type
Security-Auditing	Event ID 4720: A user account was created.
Security-Auditing	Event ID 4781: The name of an account was changed.

Rule body

title: New or Renamed User Account with '$' Character
id: cfeed607-6aa4-4bbd-9627-b637deb723c8
status: test
description: |
    Detects the creation of a user with the "$" character. This can be used by attackers to hide a user or trick detection systems that lack the parsing mechanisms.
references:
    - https://twitter.com/SBousseaden/status/1387743867663958021
author: Ilyas Ochkov, oscd.community
date: 2019-10-25
modified: 2024-01-16
tags:
    - attack.stealth
    - attack.t1036
logsource:
    product: windows
    service: security
detection:
    selection_create:
        EventID: 4720 # create user
        SamAccountName|contains: '$'
    selection_rename:
        EventID: 4781 # rename user
        NewTargetUserName|contains: '$'
    filter_main_homegroup:
        EventID: 4720
        TargetUserName: 'HomeGroupUser$'
    condition: 1 of selection_* and not 1 of filter_main_*
falsepositives:
    - Unknown
level: medium

Stages and Predicates

Stage 0: `condition`

1 of selection_* and not 1 of filter_main_*

Stage 1: `selection_create`

selection_create:
    EventID: 4720
    SamAccountName|contains: '$'

Stage 2: `selection_rename`

selection_rename:
    EventID: 4781
    NewTargetUserName|contains: '$'

Stage 3: `not filter_main_homegroup`

filter_main_homegroup:
    EventID: 4720
    TargetUserName: 'HomeGroupUser$'

Exclusions

The rule actively suppresses these predicates.

Field	Kind	Excluded values	Search
`TargetUserName`	eq	`HomeGroupUser$`	`excludes:TargetUserName` `field:"TargetUserName" value:"HomeGroupUser$"`

Indicators

These rows show field, operator, and value matches.

Field	Kind	Values	Search
`NewTargetUserName`	match	`$`	`field:"NewTargetUserName" kind:match value:"$"`
`SamAccountName`	match	`$`	`field:"SamAccountName" kind:match value:"$"`