Creation of a Hidden Local User Account

Identifies the creation of a hidden local user account by appending the dollar sign to the account name. This is sometimes done by attackers to increase access to a system and avoid appearing in the results of accounts listing using the net users command.

Rule type: eql

Rule indices:

winlogbeat-*
logs-endpoint.events.registry-*
logs-windows.sysmon_operational-*
endgame-*
logs-m365_defender.event-*
logs-sentinel_one_cloud_funnel.*

Severity: high

Risk score: 73

Runs every: 5m

Searches indices from: now-9m (Date Math format, see also Additional look-back time)

Maximum alerts per execution: 100

References:

Tags:

Domain: Endpoint
OS: Windows
Use Case: Threat Detection
Tactic: Persistence
Resources: Investigation Guide
Data Source: Elastic Endgame
Data Source: Elastic Defend
Data Source: Sysmon
Data Source: Microsoft Defender for Endpoint
Data Source: SentinelOne

Version: 311

Rule authors:

Elastic

Rule license: Elastic License v2

Investigation guide

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Triage and analysis

Investigating Creation of a Hidden Local User Account

Attackers can create accounts ending with a $ symbol to make the account hidden to user enumeration utilities and bypass detections that identify computer accounts by this pattern to apply filters.

This rule uses registry events to identify the creation of local hidden accounts.

Possible investigation steps

Identify the user account that performed the action and whether it should perform this kind of action.
Investigate the process execution chain (parent process tree) for unknown processes. Examine their executable files for prevalence, whether they are located in expected locations, and if they are signed with valid digital signatures.
Investigate other alerts associated with the user/host during the past 48 hours.

False positive analysis

This activity is unlikely to happen legitimately. Benign true positive (B-TPs) can be added as exceptions if necessary.

Response and remediation

Initiate the incident response process based on the outcome of the triage.
Isolate the involved hosts to prevent further post-compromise behavior.
Delete the hidden account.
Review the privileges assigned to the involved users to ensure that the least privilege principle is being followed.
Determine the initial vector abused by the attacker and take action to prevent reinfection through the same vector.
Using the incident response data, update logging and audit policies to improve the mean time to detect (MTTD) and the mean time to respond (MTTR).

Rule query

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registry where host.os.type == "windows" and event.type == "change" and
  registry.path : (
    "HKLM\\SAM\\SAM\\Domains\\Account\\Users\\Names\\*$\\",
    "\\REGISTRY\\MACHINE\\SAM\\SAM\\Domains\\Account\\Users\\Names\\*$\\",
    "MACHINE\\SAM\\SAM\\Domains\\Account\\Users\\Names\\*$\\"
)

Framework: MITRE ATT&CK^TM

Tactic:
- Name: Persistence
- ID: TA0003
- Reference URL: https://attack.mitre.org/tactics/TA0003/
Technique:
- Name: Create Account
- ID: T1136
- Reference URL: https://attack.mitre.org/techniques/T1136/
Sub-technique:
- Name: Local Account
- ID: T1136.001
- Reference URL: https://attack.mitre.org/techniques/T1136/001/

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