Uncommon Userinit Child Process

Status: test
Severity: high
Log source: product windows, category process_creation
Author: Tom Ueltschi (@c_APT_ure), Tim Shelton
Source: github.com/SigmaHQ/sigma

Detects uncommon "userinit.exe" child processes, which could be a sign of uncommon shells or login scripts used for persistence.

MITRE ATT&CK coverage

Tactic	Techniques
Persistence	`T1037.001` Boot or Logon Initialization Scripts: Logon Script (Windows)
Privilege Escalation	`T1037.001` Boot or Logon Initialization Scripts: Logon Script (Windows)

Event coverage

Provider	Event	Title
Sysmon	Event ID 1	Process creation
Security-Auditing	Event ID 4688	A new process has been created.

Rule body yaml

title: Uncommon Userinit Child Process
id: 0a98a10c-685d-4ab0-bddc-b6bdd1d48458
related:
    - id: 21d856f9-9281-4ded-9377-51a1a6e2a432
      type: similar
status: test
description: Detects uncommon "userinit.exe" child processes, which could be a sign of uncommon shells or login scripts used for persistence.
references:
    - https://cocomelonc.github.io/persistence/2022/12/09/malware-pers-20.html
    - https://learn.microsoft.com/en-us/windows-server/administration/server-core/server-core-sconfig#powershell-is-the-default-shell-on-server-core
author: Tom Ueltschi (@c_APT_ure), Tim Shelton
date: 2019-01-12
modified: 2023-11-14
tags:
    - attack.privilege-escalation
    - attack.t1037.001
    - attack.persistence
logsource:
    category: process_creation
    product: windows
detection:
    selection:
        ParentImage|endswith: '\userinit.exe'
    filter_main_explorer:
        Image|endswith: ':\WINDOWS\explorer.exe'
    filter_optional_logonscripts:
        CommandLine|contains:
            - 'netlogon.bat'
            - 'UsrLogon.cmd'
    filter_optional_windows_core:
        # Note: This filter is mandatory on Windows Core machines as the default shell spawned by "userinit" is "powershell.exe".
        # https://learn.microsoft.com/en-us/windows-server/administration/server-core/server-core-sconfig#powershell-is-the-default-shell-on-server-core
        CommandLine: 'PowerShell.exe'
    filter_optional_proquota:
        Image|endswith:
            - ':\Windows\System32\proquota.exe'
            - ':\Windows\SysWOW64\proquota.exe'
    filter_optional_citrix:
        Image|endswith:
            # As reported by https://github.com/SigmaHQ/sigma/issues/4569
            - ':\Program Files (x86)\Citrix\HDX\bin\cmstart.exe' # https://support.citrix.com/article/CTX983798/purpose-of-cmstart-command
            - ':\Program Files (x86)\Citrix\HDX\bin\icast.exe' # https://support.citrix.com/article/CTX983798/purpose-of-cmstart-command
            - ':\Program Files (x86)\Citrix\System32\icast.exe'
            - ':\Program Files\Citrix\HDX\bin\cmstart.exe' # https://support.citrix.com/article/CTX983798/purpose-of-cmstart-command
            - ':\Program Files\Citrix\HDX\bin\icast.exe' # https://support.citrix.com/article/CTX983798/purpose-of-cmstart-command
            - ':\Program Files\Citrix\System32\icast.exe'
    filter_optional_image_null:
        Image: null
    condition: selection and not 1 of filter_main_* and not 1 of filter_optional_*
falsepositives:
    - Legitimate logon scripts or custom shells may trigger false positives. Apply additional filters accordingly.
level: high

Stages and Predicates

Stage 0: `condition`

selection and not 1 of filter_main_* and not 1 of filter_optional_*

Stage 1: `selection`

selection:
    ParentImage|endswith: '\userinit.exe'

Stage 2: `not filter_main_explorer`

filter_main_explorer:
    Image|endswith: ':\WINDOWS\explorer.exe'

Stage 3: `not filter_optional_*`

filter_optional_logonscripts:
    CommandLine|contains:
        - 'netlogon.bat'
        - 'UsrLogon.cmd'
filter_optional_windows_core:
    CommandLine: 'PowerShell.exe'
filter_optional_proquota:
    Image|endswith:
        - ':\Windows\System32\proquota.exe'
        - ':\Windows\SysWOW64\proquota.exe'
filter_optional_citrix:
    Image|endswith:
        - ':\Program Files (x86)\Citrix\HDX\bin\cmstart.exe'
        - ':\Program Files (x86)\Citrix\HDX\bin\icast.exe'
        - ':\Program Files (x86)\Citrix\System32\icast.exe'
        - ':\Program Files\Citrix\HDX\bin\cmstart.exe'
        - ':\Program Files\Citrix\HDX\bin\icast.exe'
        - ':\Program Files\Citrix\System32\icast.exe'
filter_optional_image_null:
    Image: null

Exclusions

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

Field	Kind	Excluded values
`Image`	ends_with	`:\WINDOWS\explorer.exe`
`CommandLine`	eq	`PowerShell.exe`
`CommandLine`	match	`UsrLogon.cmd`
`CommandLine`	match	`netlogon.bat`
`Image`	ends_with	`:\Program Files (x86)\Citrix\HDX\bin\cmstart.exe`
`Image`	ends_with	`:\Program Files (x86)\Citrix\HDX\bin\icast.exe`
`Image`	ends_with	`:\Program Files (x86)\Citrix\System32\icast.exe`
`Image`	ends_with	`:\Program Files\Citrix\HDX\bin\cmstart.exe`
`Image`	ends_with	`:\Program Files\Citrix\HDX\bin\icast.exe`
`Image`	ends_with	`:\Program Files\Citrix\System32\icast.exe`
`Image`	ends_with	`:\Windows\SysWOW64\proquota.exe`
`Image`	ends_with	`:\Windows\System32\proquota.exe`
`Image`	is_null	(no value, null check)

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.

Field	Kind	Values
`ParentImage`	ends_with	`\userinit.exe` corpus 3 (sigma 3)

`j` / `k`	Scroll down / up
`d` / `u`	Half-page down / up
`gg` / `G`	Top / bottom
`h` / `l`	History back / forward
`f`	Follow link (`Shift` = new tab)
`/`	Focus search
`?`	Toggle this help
`↑` / `↓`	Navigate search results
`Enter`	Open highlighted result
`Esc`	Close results / dialog

`type:`	`events` / `rules` / `providers`
`vendor:`	`sigma` / `elastic` / `splunk` / `kusto` / `chronicle` (vendor name alone also works: `sigma:`, `kql:`, `secops:`…)
`tactic:`	TA-id, slug, or name: `credential_access`, `TA0006`
`technique:`	technique or sub-technique ID: `T1003`, `T1003.001` (alias `tech:`)
`severity:`	`critical` / `high` / `medium` / `low` / `informational` (alias `sev:`)
`risk_score`	Numeric comparison on the Elastic risk score (0 to 100): `risk_score>50`, `risk_score<=20`, `risk_score=99` (alias `risk`; Elastic rules only)
`stages:`	Rules with exactly N pipeline stages
`correlation:`	`single_event` / `sequence` / `alternatives` / `alternatives_cross_log` / `all_required` / `correlated`
`with:`	Co-occurrence event-id; stacks (`with:4624 with:4769`) to require all, while a comma list in one occurrence (`with:4624,4769`) is an either-or group. Implies multi-event
`like:`	Structural neighbors of a rule slug (equivalents + subsumption stricter / broader): `like:comsvcs_lsass_memory_dump-splunk-sysmon`
`groupby:`	Entity-grouping substring match against `group_by_keys`: `groupby:user`, `groupby:host`
`uses:`	Rules whose predicate tree touches the field (any kind, any value): `uses:CommandLine`
`excludes:`	Rules with top-level `not()` clauses on the field (FP whitelists): `excludes:ParentImage`
`field:` / `value:`	Predicate search; narrows rule cards to those with a matching leaf and drives the indicator tier. Unquoted = substring, wildcards allowed (`value:mimikatz`)
`indicator:`	Shorthand for `field:F value:V`: `indicator:Image=*\powershell.exe`
`kind:`	Filter by predicate kind. Narrows rule cards to those carrying a matching predicate leaf (`vendor:elastic kind:cidr_match`) and drives the indicator tier: `contains` / `starts_with` / `ends_with` / `regex` / `cidr` / `eq` / `in` … (operator aliases `op:`/`match:`)
`has:` / `no:`	`sample`, `field`, `notes`, `refs`, `trace`, `thirdparty`, `rule`, `pattern`, `timewindow`, `threshold`, `newterms`, `sigma`/`elastic`/`splunk`/`kusto`/`chronicle`
`-op:val`	Exclude matches; works on most operators but not `type:`/`like:`/`has:`/`no:` (use `no:<flag>` to exclude a rule flag): `tactic:execution -vendor:splunk`. Standalone `-kind:`/`-field:`/`-value:` drop every rule carrying a matching predicate leaf (`type:rules -kind:is_null`)
`field:"…"` / `value:"…"`	Quoted value = anchored exact match (also allows spaces): `value:"net user"`
`a,b`	Comma = OR inside one operator (`vendor:sigma,elastic`, `severity:high,critical`); repeating a facet merges the same way. `field:`/`value:` never split (literal commas)
`vendors:` / `stage:`	Singular and plural spellings fold to the canonical operator and value: `tactics:` = `tactic:`, `type:event` = `type:events`, `correlation:sequences` = `correlation:sequence`, `has:thresholds` = `has:threshold`
`"quoted phrase"`	Exact-match a multi-word phrase (free text)

Uncommon Userinit Child Process

MITRE ATT&CK coverage

Event coverage

Rule body yaml

Stages and Predicates

Stage 0: `condition`

Stage 1: `selection`

Stage 2: `not filter_main_explorer`

Stage 3: `not filter_optional_*`

Exclusions

Indicators

Keyboard shortcuts

Search operators

Uncommon Userinit Child Process

MITRE ATT&CK coverage

Event coverage

Rule body yaml

Stages and Predicates

Stage 0: condition

Stage 1: selection

Stage 2: not filter_main_explorer

Stage 3: not filter_optional_*

Exclusions

Indicators

Stage 0: `condition`

Stage 1: `selection`

Stage 2: `not filter_main_explorer`

Stage 3: `not filter_optional_*`