Rundll32 Execution With Uncommon DLL Extension

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

Detects the execution of rundll32 with a command line that doesn't contain a common extension

MITRE ATT&CK coverage

Tactic	Techniques
Stealth	`T1218.011` System Binary Proxy Execution: Rundll32

Event coverage

Provider	Event	Title
Sysmon	Event ID 1	Process creation

Rule body yaml

title: Rundll32 Execution With Uncommon DLL Extension
id: c3a99af4-35a9-4668-879e-c09aeb4f2bdf
status: test
description: Detects the execution of rundll32 with a command line that doesn't contain a common extension
references:
    - https://twitter.com/mrd0x/status/1481630810495139841?s=12
author: Tim Shelton, Florian Roth (Nextron Systems), Yassine Oukessou
date: 2022-01-13
modified: 2024-04-04
tags:
    - attack.stealth
    - attack.t1218.011
logsource:
    category: process_creation
    product: windows
detection:
    selection:
        - Image|endswith: '\rundll32.exe'
        - OriginalFileName: 'RUNDLL32.EXE'
    filter_main_null:
        CommandLine: null
    filter_main_empty:
        CommandLine: ''
    filter_main_known_extension:
        - CommandLine|contains:
              # Note: This aims to cover: single and double quotes in addition to spaces and comma "," usage.
              - '.cpl '
              - '.cpl,'
              - '.cpl"'
              - ".cpl'"
              - '.dll '
              - '.dll,'
              - '.dll"'
              - ".dll'"
              - '.inf '
              - '.inf,'
              - '.inf"'
              - ".inf'"
        - CommandLine|endswith:
              # Note: This aims to cover: single and double quotes in addition to spaces and comma "," usage.
              - '.cpl'
              - '.dll'
              - '.inf'
    filter_main_localserver:
        CommandLine|contains: ' -localserver '
    filter_main_zzzzInvokeManagedCustomActionOutOfProc:
        ParentImage|endswith: '\msiexec.exe'
        CommandLine|contains|all:
            - ':\Windows\Installer\'
            - '.tmp'
            - 'zzzzInvokeManagedCustomActionOutOfProc'
    filter_optional_EdgeUpdate:
        ParentCommandLine|contains|all:
            - ':\Users\'
            - '\AppData\Local\Microsoft\EdgeUpdate\Install\{'
            - '\EDGEMITMP_'
            - '.tmp\setup.exe'
            - '--install-archive='
            - '--previous-version='
            - '--msedgewebview --verbose-logging --do-not-launch-msedge --user-level'
    condition: selection and not 1 of filter_main_* and not 1 of filter_optional_*
falsepositives:
    - Unknown
level: medium

Stages and Predicates

Stage 0: `condition`

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

Stage 1: `selection`

selection:
    - Image|endswith: '\rundll32.exe'
    - OriginalFileName: 'RUNDLL32.EXE'

Stage 2: `not filter_main_*`

filter_main_null:
    CommandLine: null
filter_main_empty:
    CommandLine: ''
filter_main_known_extension:
    - CommandLine|contains:
          - '.cpl '
          - '.cpl,'
          - '.cpl"'
          - ".cpl'"
          - '.dll '
          - '.dll,'
          - '.dll"'
          - ".dll'"
          - '.inf '
          - '.inf,'
          - '.inf"'
          - ".inf'"
    - CommandLine|endswith:
          - '.cpl'
          - '.dll'
          - '.inf'
filter_main_localserver:
    CommandLine|contains: ' -localserver '
filter_main_zzzzInvokeManagedCustomActionOutOfProc:
    ParentImage|endswith: '\msiexec.exe'
    CommandLine|contains|all:
        - ':\Windows\Installer\'
        - '.tmp'
        - 'zzzzInvokeManagedCustomActionOutOfProc'

Stage 3: `not filter_optional_EdgeUpdate`

filter_optional_EdgeUpdate:
    ParentCommandLine|contains|all:
        - ':\Users\'
        - '\AppData\Local\Microsoft\EdgeUpdate\Install\{'
        - '\EDGEMITMP_'
        - '.tmp\setup.exe'
        - '--install-archive='
        - '--previous-version='
        - '--msedgewebview --verbose-logging --do-not-launch-msedge --user-level'

Exclusions

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

Field	Kind	Excluded values
`CommandLine`	match	`.tmp`
`CommandLine`	match	`:\Windows\Installer\`
`CommandLine`	match	`zzzzInvokeManagedCustomActionOutOfProc`
`ParentImage`	ends_with	`\msiexec.exe`
`CommandLine`	ends_with	`.cpl`
`CommandLine`	ends_with	`.dll`
`CommandLine`	ends_with	`.inf`
`CommandLine`	is_null	(no value, null check)
`CommandLine`	match	`-localserver`
`CommandLine`	match	`.cpl`
`CommandLine`	match	`.cpl'`
`CommandLine`	match	`.cpl,`
`CommandLine`	match	`.cpl"`
`CommandLine`	match	`.dll`
`CommandLine`	match	`.dll'`
`CommandLine`	match	`.dll,`
`CommandLine`	match	`.dll"`
`CommandLine`	match	`.inf`
`CommandLine`	match	`.inf'`
`CommandLine`	match	`.inf,`
`CommandLine`	match	`.inf"`
`ParentCommandLine`	match	`--install-archive=`
`ParentCommandLine`	match	`--msedgewebview --verbose-logging --do-not-launch-msedge --user-level`
`ParentCommandLine`	match	`--previous-version=`
`ParentCommandLine`	match	`.tmp\setup.exe`
`ParentCommandLine`	match	`:\Users\`
`ParentCommandLine`	match	`\AppData\Local\Microsoft\EdgeUpdate\Install\{`
`ParentCommandLine`	match	`\EDGEMITMP_`

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
`Image`	ends_with	`\rundll32.exe` corpus 95 (sigma 95)
`OriginalFileName`	eq	`RUNDLL32.EXE` corpus 62 (sigma 35, splunk 21, elastic 6)

`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)

Rundll32 Execution With Uncommon DLL Extension

MITRE ATT&CK coverage

Event coverage

Rule body yaml

Stages and Predicates

Stage 0: `condition`

Stage 1: `selection`

Stage 2: `not filter_main_*`

Stage 3: `not filter_optional_EdgeUpdate`

Exclusions

Indicators

Keyboard shortcuts

Search operators

Rundll32 Execution With Uncommon DLL Extension

MITRE ATT&CK coverage

Event coverage

Rule body yaml

Stages and Predicates

Stage 0: condition

Stage 1: selection

Stage 2: not filter_main_*

Stage 3: not filter_optional_EdgeUpdate

Exclusions

Indicators

Stage 0: `condition`

Stage 1: `selection`

Stage 2: `not filter_main_*`

Stage 3: `not filter_optional_EdgeUpdate`