Suspicious PowerShell Invocations - Specific - ProcessCreation

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

Detects suspicious PowerShell invocation command parameters

MITRE ATT&CK coverage

Tactic	Techniques
Stealth	No specific technique

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: Suspicious PowerShell Invocations - Specific - ProcessCreation
id: 536e2947-3729-478c-9903-745aaffe60d2
related:
    - id: fce5f582-cc00-41e1-941a-c6fabf0fdb8c
      type: obsolete
    - id: ae7fbf8e-f3cb-49fd-8db4-5f3bed522c71
      type: similar
    - id: 8ff28fdd-e2fa-4dfa-aeda-ef3d61c62090
      type: similar
status: test
description: Detects suspicious PowerShell invocation command parameters
references:
    - Internal Research
author: Nasreddine Bencherchali (Nextron Systems)
date: 2023-01-05
tags:
    - attack.stealth
logsource:
    category: process_creation
    product: windows
detection:
    selection_convert_b64:
        CommandLine|contains|all:
            - '-nop'
            - ' -w '
            - 'hidden'
            - ' -c '
            - '[Convert]::FromBase64String'
    selection_iex:
        CommandLine|contains|all:
            - ' -w '
            - 'hidden'
            - '-noni'
            - '-nop'
            - ' -c '
            - 'iex'
            - 'New-Object'
    selection_enc:
        CommandLine|contains|all:
            - ' -w '
            - 'hidden'
            - '-ep'
            - 'bypass'
            - '-Enc'
    selection_reg:
        CommandLine|contains|all:
            - 'powershell'
            - 'reg'
            - 'add'
            - '\software\'
    selection_webclient:
        CommandLine|contains|all:
            - 'bypass'
            - '-noprofile'
            - '-windowstyle'
            - 'hidden'
            - 'new-object'
            - 'system.net.webclient'
            - '.download'
    selection_iex_webclient:
        CommandLine|contains|all:
            - 'iex'
            - 'New-Object'
            - 'Net.WebClient'
            - '.Download'
    filter_chocolatey:
        CommandLine|contains:
            - "(New-Object System.Net.WebClient).DownloadString('https://community.chocolatey.org/install.ps1"
            - 'Write-ChocolateyWarning'
    condition: 1 of selection_* and not 1 of filter_*
falsepositives:
    - Unknown
level: medium

Stages and Predicates

Stage 0: `condition`

1 of selection_* and not 1 of filter_*

Stage 1: `selection_convert_b64`

selection_convert_b64:
    CommandLine|contains|all:
        - '-nop'
        - ' -w '
        - 'hidden'
        - ' -c '
        - '[Convert]::FromBase64String'

Stage 2: `selection_iex`

selection_iex:
    CommandLine|contains|all:
        - ' -w '
        - 'hidden'
        - '-noni'
        - '-nop'
        - ' -c '
        - 'iex'
        - 'New-Object'

Stage 3: `selection_enc`

selection_enc:
    CommandLine|contains|all:
        - ' -w '
        - 'hidden'
        - '-ep'
        - 'bypass'
        - '-Enc'

Stage 4: `selection_reg`

selection_reg:
    CommandLine|contains|all:
        - 'powershell'
        - 'reg'
        - 'add'
        - '\software\'

Stage 5: `selection_webclient`

selection_webclient:
    CommandLine|contains|all:
        - 'bypass'
        - '-noprofile'
        - '-windowstyle'
        - 'hidden'
        - 'new-object'
        - 'system.net.webclient'
        - '.download'

Stage 6: `selection_iex_webclient`

selection_iex_webclient:
    CommandLine|contains|all:
        - 'iex'
        - 'New-Object'
        - 'Net.WebClient'
        - '.Download'

Stage 7: `not filter_chocolatey`

filter_chocolatey:
    CommandLine|contains:
        - "(New-Object System.Net.WebClient).DownloadString('https://community.chocolatey.org/install.ps1"
        - 'Write-ChocolateyWarning'

Exclusions

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

Field	Kind	Excluded values
`CommandLine`	match	`(New-Object System.Net.WebClient).DownloadString('https://community.chocolatey.org/install.ps1`
`CommandLine`	match	`Write-ChocolateyWarning`

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
`CommandLine`	match	`-c` corpus 14 (sigma 13, elastic 1) `-w` `-Enc` corpus 2 (sigma 2) `-ep` `-noni` `-nop` corpus 2 (sigma 2) `-noprofile` `-windowstyle` `.Download` corpus 2 (sigma 2) `.download` corpus 2 (sigma 2) `Net.WebClient` corpus 2 (sigma 2) `New-Object` corpus 3 (sigma 3) `[Convert]::FromBase64String` `\software\` corpus 2 (sigma 2) `add` corpus 34 (sigma 26, splunk 4, chronicle 2, kusto 2) `bypass` corpus 6 (sigma 6) `hidden` corpus 4 (sigma 4) `iex` corpus 6 (sigma 5, splunk 1) `new-object` corpus 3 (sigma 3) `powershell` corpus 25 (sigma 24, chronicle 1) `reg` corpus 7 (sigma 5, splunk 1, kusto 1) `system.net.webclient` corpus 2 (sigma 1, chronicle 1)

`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)

Suspicious PowerShell Invocations - Specific - ProcessCreation

MITRE ATT&CK coverage

Event coverage

Rule body yaml

Stages and Predicates

Stage 0: `condition`

Stage 1: `selection_convert_b64`

Stage 2: `selection_iex`

Stage 3: `selection_enc`

Stage 4: `selection_reg`

Stage 5: `selection_webclient`

Stage 6: `selection_iex_webclient`

Stage 7: `not filter_chocolatey`

Exclusions

Indicators

Keyboard shortcuts

Search operators

Suspicious PowerShell Invocations - Specific - ProcessCreation

MITRE ATT&CK coverage

Event coverage

Rule body yaml

Stages and Predicates

Stage 0: condition

Stage 1: selection_convert_b64

Stage 2: selection_iex

Stage 3: selection_enc

Stage 4: selection_reg

Stage 5: selection_webclient

Stage 6: selection_iex_webclient

Stage 7: not filter_chocolatey

Exclusions

Indicators

Stage 0: `condition`

Stage 1: `selection_convert_b64`

Stage 2: `selection_iex`

Stage 3: `selection_enc`

Stage 4: `selection_reg`

Stage 5: `selection_webclient`

Stage 6: `selection_iex_webclient`

Stage 7: `not filter_chocolatey`