Rundll32.EXE Calling DllRegisterServer Export Function Explicitly

Status: test
Severity: medium
Log source: product windows, category process_creation
Author: Andreas Braathen (mnemonic.io)
Source: github.com/SigmaHQ/sigma

Detects when the DLL export function 'DllRegisterServer' is called in the commandline by Rundll32 explicitly where the DLL is located in a non-standard path.

MITRE ATT&CK coverage

Tactic	Techniques
Stealth	`T1218` System Binary Proxy Execution

Event coverage

Provider	Event	Title
Sysmon	Event ID 1	Process creation

Rule body yaml

title: Rundll32.EXE Calling DllRegisterServer Export Function Explicitly
id: d81a9fc6-55db-4461-b962-0e78fea5b0ad
related:
    - id: 2569ed8c-1147-498a-9b8c-2ad3656b10ed # Renamed rundll32
      type: similar
status: test
description: |
    Detects when the DLL export function 'DllRegisterServer' is called in the commandline by Rundll32 explicitly where the DLL is located in a non-standard path.
references:
    - https://thedfirreport.com/2023/08/28/html-smuggling-leads-to-domain-wide-ransomware/
    - https://www.virustotal.com/gui/file/94816439312563db982cd038cf77cbc5ef4c7003e3edee86e2b0f99e675ed4ed/behavior
    - https://learn.microsoft.com/en-us/windows/win32/api/olectl/nf-olectl-dllregisterserver
author: Andreas Braathen (mnemonic.io)
date: 2023-10-17
tags:
    - attack.stealth
    - attack.t1218
    - detection.threat-hunting
logsource:
    category: process_creation
    product: windows
detection:
    selection_image:
        - Image|endswith: '\rundll32.exe'
        - OriginalFileName: 'RUNDLL32.EXE'
    selection_cmdline:
        CommandLine|contains: 'DllRegisterServer'
    filter_main_legit_paths:
        CommandLine|contains:
            - ':\Program Files (x86)'
            - ':\Program Files\'
            - ':\Windows\System32\'
            - ':\Windows\SysWOW64\'
    condition: all of selection_* and not 1 of filter_main_*
falsepositives:
    - Legitimate usage as part of application installation, but less likely from e.g. temporary paths.
    - Not every instance is considered malicious, but this rule will capture the malicious usages.
level: medium

Stages and Predicates

Stage 0: `condition`

all of selection_* and not 1 of filter_main_*

Stage 1: `selection_image`

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

Stage 2: `selection_cmdline`

selection_cmdline:
    CommandLine|contains: 'DllRegisterServer'

Stage 3: `not filter_main_legit_paths`

filter_main_legit_paths:
    CommandLine|contains:
        - ':\Program Files (x86)'
        - ':\Program Files\'
        - ':\Windows\System32\'
        - ':\Windows\SysWOW64\'

Exclusions

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

Field	Kind	Excluded values
`CommandLine`	match	`:\Program Files (x86)`
`CommandLine`	match	`:\Program Files\`
`CommandLine`	match	`:\Windows\SysWOW64\`
`CommandLine`	match	`:\Windows\System32\`

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	`DllRegisterServer` corpus 3 (sigma 2, splunk 1)
`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.EXE Calling DllRegisterServer Export Function Explicitly

MITRE ATT&CK coverage

Event coverage

Rule body yaml

Stages and Predicates

Stage 0: `condition`

Stage 1: `selection_image`

Stage 2: `selection_cmdline`

Stage 3: `not filter_main_legit_paths`

Exclusions

Indicators

Keyboard shortcuts

Search operators

Rundll32.EXE Calling DllRegisterServer Export Function Explicitly

MITRE ATT&CK coverage

Event coverage

Rule body yaml

Stages and Predicates

Stage 0: condition

Stage 1: selection_image

Stage 2: selection_cmdline

Stage 3: not filter_main_legit_paths

Exclusions

Indicators

Stage 0: `condition`

Stage 1: `selection_image`

Stage 2: `selection_cmdline`

Stage 3: `not filter_main_legit_paths`