Rundll32 Execution With Uncommon DLL Extension

Severity: medium
Author: Tim Shelton, Florian Roth (Nextron Systems), Yassine Oukessou
Source: upstream

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

MITRE ATT&CK coverage

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

Event coverage

Provider	Event ID	Title
Sysmon	1	Process creation

Stages and Predicates

Stage 1: `selection`

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

Stage 2: `not 1 of filter_main_*`

or:
CommandLine|contains: .tmp
CommandLine|contains: ':\Windows\Installer\'
CommandLine|contains: 'zzzzInvokeManagedCustomActionOutOfProc'
ParentImage|endswith: '\msiexec.exe'
CommandLine|endswith: .cpl
CommandLine|endswith: .dll
CommandLine|endswith: .inf
CommandLine: ''
CommandLine: null
CommandLine|contains: ' -localserver '
CommandLine|contains: '.cpl '
CommandLine|contains: '.cpl'''
CommandLine|contains: '.cpl,'
CommandLine|contains: '.cpl"'
CommandLine|contains: '.dll '
CommandLine|contains: '.dll'''
CommandLine|contains: '.dll,'
CommandLine|contains: '.dll"'
CommandLine|contains: '.inf '
CommandLine|contains: '.inf'''
CommandLine|contains: '.inf,'
CommandLine|contains: '.inf"'

Stage 3: `not 1 of filter_optional_EdgeUpdate`

ParentCommandLine|contains: '--install-archive='
ParentCommandLine|contains: '--msedgewebview --verbose-logging --do-not-launch-msedge --user-level'
ParentCommandLine|contains: '--previous-version='
ParentCommandLine|contains: '.tmp\setup.exe'
ParentCommandLine|contains: ':\Users\'
ParentCommandLine|contains: '\AppData\Local\Microsoft\EdgeUpdate\Install\{'
ParentCommandLine|contains: '\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
`CommandLine`	ends_with	`.cpl` corpus 2 (sigma 2) `.dll` corpus 9 (sigma 9) `.inf`
`CommandLine`	match	`-localserver` `.cpl` `.cpl"` `.cpl'` `.cpl,` `.dll` corpus 2 (sigma 2) `.dll"` corpus 2 (sigma 2) `.dll'` `.dll,` `.inf` `.inf"` `.inf'` `.inf,` `.tmp` corpus 3 (sigma 3) `:\Windows\Installer\` `zzzzInvokeManagedCustomActionOutOfProc` corpus 2 (sigma 2)
`Image`	ends_with	`\rundll32.exe` corpus 76 (sigma 76)
`OriginalFileName`	eq	`RUNDLL32.EXE` corpus 28 (sigma 25, splunk 3)
`ParentCommandLine`	match	`--install-archive=` `--msedgewebview --verbose-logging --do-not-launch-msedge --user-level` `--previous-version=` `.tmp\setup.exe` `:\Users\` corpus 3 (sigma 3) `\AppData\Local\Microsoft\EdgeUpdate\Install\{` `\EDGEMITMP_`
`ParentImage`	ends_with	`\msiexec.exe` corpus 2 (sigma 2)